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Animal Diversity: Chordata
Anamniotes
Amphibia
Dr. Monisha Khanna
Acharya Narendra Dev College
University of Delhi
Kalkaji
New Delhi – 110 019
[email protected]
List Of Contents
I.
Introduction
II.
Classification
A. Order 1. Apoda (Gymnophiona / Caecilia)
B. Order 2. Urodela (Caudata)
C. Order 3. Anura
III.
Origin
A. From Early Chordates To The First Land Vertebrates
a. Origin of Chordates
b. Origin of Vertebrates
c. Origin of Tetrapods
B. Acquisition of Adaptations For Life On Land
IV.
Amphibia: General Organization
a. External Appearance
b. Integument
c. Alimentary Canal
d. Respiratory Organs And Voice Apparatus
e. Blood Vascular System
f. Endoskeleton
g. Nervous System And Sensory Organs
h. Urinogenital System And Osmoregulation
Parental Care In Amphibia
V.
A. Order Anura
B. Order Urodela
C. Order Apoda
I. INTRODUCTION
Members of the phylum Chordata are commonly referred to as chordates. Four characters, of
prime diagnostic importance, are possessed by all chordates: 1) A primitive endoskeletal
structure called the notochord is present during early embryonic life. This pliant, rod-like
structure, composed of a peculiar type of connective tissue, is located along the mid-dorsal
line, where it forms the axis of support for the body. In some animals it persists as such
throughout life, but in most chordates it serves as a foundation around which the vertebral
column is built. 2) A hollow, dorsal nerve tube is present sometime during life. The central
nervous system, made up of the brain and the spinal cord, is located in a dorsal position just
above the notochord. It is a hollow canal from one end to the other. 3) Gill slits or traces of
them connecting to the pharynx are present at some stage of life. Most aquatic chordates
respire by gills made up of vascular lamellae or filaments lining the borders of the gill slits,
which connect to the pharynx and open directly or indirectly to the outside. Even terrestrial
chordates, which never breathe by gills, nevertheless have traces of gill slits present as
transient structures, during early embryonic life. No vascular lamellae line these temporary
structures, nor do they open to the outside, but the fact that they are present in all chordates is
of prime importance in denoting close relationship. 4) Chordates possess a post-anal tail in
some stage of their life that represents a posterior elongation of the body extending beyond
the anus. The tail is primarily an extension of the chordate locomotor apparatus, the
segmental musculature and notochord. Apart from the above four features, chordates also
have certain characteristics common to some other phyla as well. 5) They are bilaterally
symmetrical; 6) are metameric; 7) have a true body cavity or coelom, lined with mesoderm;
8) show cephalization or the concentration of nervous tissue and specialized sense organs in
or towards the head; 9) the blood is pumped anteriorly from the ventrally located heart and
forced to the dorsal side. It then moves posteriorly and returns to the heart by veins.
The phylum Chordata is usually subdivided into four main groups or subphyla. The first three
of these include a few relatively simple animals, which lack a cranium and brain. These
organisms are sometimes collectively referred to as the Acrania. The animals included in this
category are believed to show similarities to the chordate ancestors, hence are frequently
known as the protochordates. These are the subphyla Hemichordata (acorn worms),
Urochordata (tunicates), and the Cephalochordata (amphioxus). Vertebrata (Craniata) is a
large group, embracing chordates having a brain; endoskeleton; notochord not extending
forward under the brain; paired eyes; presence of red blood cells; a ventrally placed heart;
presence of a sympathetic nervous system; and presence of a hepatic portal system.
Vertebrates include the jawless forms, lacking vertebrae (Super class Agnatha), and the jawed
vertebrates (Super class Gnathostomata). Furthermore, the latter include the series, Pisces
embracing the lower forms commonly known as fishes. The remaining vertebrates are
included in the group Tetrapoda, which are basically four-footed animals, although in some
the limbs have been lost or modified secondarily.
Tetrapods are those members of the subphylum Vertebrata having paired appendages in the
form of limbs rather than fins, though in some forms the limbs either degenerate completely
or show modifications. Among other characteristics which distinguish tetrapods from fishes
are a cornified outer layer of skin; nasal passages which communicate with the mouth cavity
and which transport air; lungs used in respiration; and a bony skeleton along with a reduction
in the number of skull bones.
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Tetrapoda
The class Amphibia is composed of tetrapods in which the transition from aquatic to
terrestrial life is clearly indicated. Amphibians are the first vertebrates to live on land,
although they lay their eggs in water or in moist situations.
The first tetrapods evolved from rhipidistian crossopterygian fishes. The fossil remains of
primitive tetrapods have been found in the eastern parts of Greenland in Devonian deposits.
These specimens have features intermediate between late crossopterygians and early
amphibians.
The group Tetrapoda is divided into four classes made up of amphibians, reptiles, birds and
mammals. The living representatives of the class Amphibia include salamanders, newts,
frogs, toads, and the caecilians. The amphibians lead a double life, that is, first in the water,
and then on the land. The result of this ambitious attempt is that they present a medley of
makeshift adaptations, which leave them still a long way from vertebrate perfection.
Among the dual adjustments that they make, are those associated with locomotion and
protection against desiccation. In water, an elongated fishlike body, propelled by a muscular
tail, has proved to be the most efficient mechanism for locomotion. However on land, the
weight of the body is no longer supported by the surrounding aqueous medium, so that the
two pairs of appendages become modified into legs, which act as levers to lift the body away
from the ground. Such levers are equipped with adequate muscles without adding excessively
to the body weight. However the amphibians are not particularly successful at locomotion on
land. Even in frogs and toads, where amphibian legs reach their highest development, such
locomotor appendages are so inefficiently anchored to a single vertebra of the supporting
backbone that these animals cannot bear their weight upon them in the sustained manner
necessary for standing or walking, and can progress only by the momentary exertion of
hopping or jumping.
The problem of dessication arises from the fact that the surrounding air, takes up moisture
rapidly from any moist surface. Amphibians not only utilize gills and primitive lungs in
respiration, but also exchange gases to a very large extent directly through the skin.
Consequently, these animals can live only in moist places. In comparison, the higher land
animals, in which an efficient pulmonary system is formed, are not restricted because they
develop a thick, relatively dry integument, which is resistant to dessication. Thus, relatively
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inefficient respiratory organs, together with other anatomical handicaps prevent amphibians
from maintaining a body temperature independent of that of the surroundings.
The difficulty of avoiding dessication is also involved in the breeding habits of amphibians
because they have not made the changes required of true land vertebrates. No amnion (liquidfilled sac) is produced by the embryos of lower vertebrates including the Amphibia. The
latter must therefore go back to the water to breed in most cases. Furthermore, the
metamorphosis of such an amphibian as a frog or a toad, necessitated by its emergence from
water to land, works profound changes both in its structure and in its feeding habits. For
instance, during its lifetime a toad changes its diet six times. While in the egg it absorbs the
yolk; upon hatching it develops a temporary mouth and eats the jelly of the egg envelopes;
next it becomes the free swimming tadpole feeding mainly upon the aquatic vegetation; the
juvenile stage has fat bodies provided to meet the intervening demands of hibernation; with
the warmth of spring the young toad catches slugs and insects for a living.
The distinct features of amphibians can be summarized as follows:
1. Amphibians are ectothermal vertebrates.
2. They have varied body forms – ranging from elongated forms, with a distinct head,
trunk and tail; to a compact, depressed body with a fused head and trunk and no
intervening neck.
3. Limbs are usually four in number, although some forms are limbless.
4. Skin is smooth and moist with many glands including pigment cells. Poison glands are
sometimes present but scales are mostly absent.
5. Mouth is usually large, with small teeth in either upper or both jaws. Teeth are bicuspid
and pedicellate. In some forms, teeth are completely absent. The nostrils open into the
anterior part of the mouth cavity.
6. Skeleton is mostly bony, with varying number of vertebrae; ribs are present in some
forms but absent in others. Ribs if present do not encircle the body. Centra of vertebrae
are cylindrical. Similar type of vertebra is also found among several groups of early
tetrapods. There is the presence of double or paired occipital condyle. The posterior
skull bones have been lost. Small, widely separated pterygoids are found. A small bone
in the skull called operculum is present and is fused to the ear bones in most anurans; it
is perhaps involved in hearing and balancing.
7. Ability to elevate the eye with specially developed levitator bulbi muscle. There is also
the presence of a special type of visual cell in the retina known as the green rod. (This
however is absent in Apoda).
8. Respiration occurs by lungs, skin and gills, either separately or in combination. A
forced pump respiratory mechanism exists. The larval forms have the external gills that
may persist throughout life in some forms.
9. Presence of a three-chambered heart having two atria and one ventricle. A double
circulation takes place through the heart.
10. The excretory system consists of paired mesonephric kidneys and urea is the main
nitrogenous waste.
11. Sexes are separate; fertilization is mostly internal in salamanders and caecilians but
generally external in frogs and toads. Amphibians are predominantly oviparous, rarely
ovoviviparous. Eggs are moderately yolky with jelly-like membrane coverings.
Metamorphosis is usually present. Fat bodies are associated with gonads.
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II. CLASSIFICATION
Basal tetrapods have been variably subdivided although the relationships among these groups
remain unclear. The most primitive amphibians known from fossil remains are the
Labyrinthodonts dating back to the late Devonian period, the name being based upon the
complex folding of the enamel layer of the teeth. These animals are sometimes called the
Stegocephalians because of the solid roofing of the skull. In certain features they resembled
the rhipidistian crossopterygian fishes. Many labyrinthodonts had an armor of overlapping
bony plates. By the Carboniferous, many groups are recognized including Temnospondyli,
Anthracosauria, and Microsauria. It is not clear as to what is the relationship of living
amphibians known as Lissamphibia to these groups. One hypothesis suggests that
lissamphibians are the sister group to Temnospondyli. Alternatively, lissamphibians may
have evolved from a temnospondyl ancestor. It is hypothesized that lissamphibians are either
monophyletic (a common temnospondyl ancestor) or diphyletic (apodans descended from a
microsaurian ancestor). What we can say from the knowledge of amphibian relationships is
that the class Amphibia, as traditionally defined, is a paraphyletic group that omits its
amniote descendants. Successive mutations and natural selection increasingly adapted basal
amphibian descendants for terrestrial life culminating with the origin of the amniotes.
The following classification is as given by Young:
Table 1: Classification of amphibia
Subclass 1:
Labyrinthodontia (folded teeth)
Subclass 2:
Lepospondyli (scale
vertebrae)
eg.: Diplocaulus,
Ophiderpeton,
Microbrachis, Sauropleura
Subclass 3:
Lissamphibia (smooth
amphibia)
Order 1:
Urodela / Caudata (tails)
eg.: Molge, Salamandra,
Triton, Ambystoma,
Necturus
Order 2:
Apoda / Caecilia /
Gymnophiona (no limbs).
eg.:Ichthyophis,
Typhlonectes
Order 1:
Ichthyostegalia (fish vertebrae)
eg.: Ichthyostega, Elpistostege
Order 2:
Temnospondyli (divided
vertebrae).
Suborder 1:
Rhachitomi (stem animals)
eg.: Loxomma, Eryops, Cacops,
Archegosaurus
Suborder 2:
Stereospondyli (ring vertebrae)
eg.: Capitosaurus, Buettnaria,
Mastodonsaurus
Order 3:
Anthracosauria (coal lizards).
Eg.: Palaeogyrinus, Seymouria,
Pteroplax
Order 3:
Anura (no tails)
Eg.:Protobatrachus,
Leiopelma, Rana, Bufo,
Hyla, Pipa
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The following alternate classification of amphibians is as given by Parker and Haswell:
Table 2: Classification of amphibia
Subclass 1:
Apsidospondyli
Super order 1:
Labyrinthodontia
Order 1:
Ichthyostegalia
Order 2:
Rhachitomi
Order 3:
Stereospondyli
Subclass 2:
Lepospondyli
Order 1:
Aistopoda
Order 2:
Nectridia
Order 3:
Microsauria
Order 4:
Urodela
Order 5:
Apoda
Order 4:
Embolomeri
Order 5:
Seymouriamorpha
Super order 2:
Salientia
Order 1:
Eoanura
Order 2:
Proanura
Order 3:
Anura
Labyrinthodonts The oldest amphibians were the swamp-dwelling labyrinthodonts.
Ichthyostega was the earliest specimen appearing in the Devonian. Labyrinthodonts were a
large, widely dispersed and diverse assemblage. On the basis of the morphology of their
vertebrae, paleontologists have been of the opinion that fossil amphibians with
stereospondylous and embolomerous vertebrae were not in the amniote line. Labyrinthodonts
had many features seldom seen in modern amphibians. These included minute bony scales in
the skin dermis; a fishlike tail supported by dermal fin rays; and skull similar to those of
rhipidistian fishes. Labyrinthodonts, like their aquatic ancestors, had a sensory canal system
of neuromast organs. One or another of the labyrinthodonts was ancestral to the first amniote.
Temnospondyls was a group that was common in the Permian with its fossil record
extending back to the Mississippian. Members of the temnospondyls have achieved skeletal
similarities to modern frogs and salamanders, suggestive of their close relationship. A
number of lissamphibian skeletal features and their relatively smaller size can be explained as
the retention of juvenile ancestral temnospondyl features. The condition in caecilians does
not fit easily into this scenario, possibly suggesting an independent origin from microsaurs.
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Microsaurs represent a diverse group of fossil forms known from the Pennsylvanian to the
lower Permian. They share a number of skeletal features with caecilians, which may suggest
either a close relationship or convergence on an elongate body form specialized for
burrowing.
Anthracosaurs Anthracosauria is a small Paleozoic group, thought to be in direct line to the
amniotes. Their fossil record extends from the Mississippian to the Triassic.
Lissamphibians Living amphibians of approximately 2000 species may be grouped in three
orders: Apoda, Urodela and Anura.
a. ORDER 1. APODA (GYMNOPHIONA / CAECILIA).
Members of the order are pantropical in distribution. The caecilians are burrowing forms,
with worm like bodies, lacking limbs. The tail is very short suited to their mostly
terrestrial habits and the anus is almost terminal. The skull is solid and bony, again suited
for a burrowing lifestyle. The animals are blind, but carry special sensory tentacles.
Unlike other amphibians, some caecilians have dermal scales. Adults lack gills and gill
slits. The very small eyes are buried beneath the skin or under the skull bones. Because of
the presence of an intromittent organ in males, internal fertilization is assumed. In some
caecilians, eggs are laid, which hatch into free-living larvae. The eggs are large, yolky
and cleavage is meroblastic; they are laid on land in Ichthyophis, and the embryos
develop around the yolk sac, but often have long, plumed gills. The female guards the
eggs until the larvae hatch and move to the aquatic habitat. Other genera skip over the
aquatic larval stage and a few have specialized external gills. In still other genera, the
eggs are retained within the female, metamorphosis occurring before birth. Viviparity is
common in the aquatic form, Typhlonectes. Important Apoda families are as follows:
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Table 3: Classification of Apoda up to families
Family
Character
Eocaecilia
Distribution
Example
Fossil from Early
Jurassic of N.
America
Eocaecilia
Rhinotrematidae
Small (up to 30 cm)
Terrestrial with aquatic
larvae
9 Species in S.
America
Epicrionops,
Rhinatrema
Ichthyophidae
Moderately large (up to 50
cm) Terrestrial with aquatic
larvae
36 Species in Asia
Caudacaecilia,
Ichthyophis
Uraeotyphlidae
Small terrestrial, oviparous
forms with possibly direct
development
4 Species in India
Uraeotyphlus
Scoleocomorphidae
Moderately large terrestrial
forms, possibly viviparous
5 Species in Africa
Crotaphatrema,
Scoleocomorphus
Caeciliidae
Very small (10 cm) to very
large (1.5 cm) terrestrial
and aquatic forms,
oviparous and viviparous
species, no aquatic larval
stage
~ 90 Species in
Central and S.
America, Africa,
India and the
Seychelles Islands
Boulengerula,
Brasilotyphlus,
Caecilia,
Dermophis,
Gegeneophis,
Geotrypetes,
Gymnopis
Typhlonectidae
Small to large (75 cm)
aquatic and semi-aquatic
forms, viviparous with
aquatic larvae
13 Species in S.
America
Typhlonectes,
Atretochoana,
Chthonerpeton,
Nectocaecilia,
Potomotyphlus
b. ORDER 2. URODELA (CAUDATA).
These include the salamanders and newts, the latter being small, semi-aquatic forms.
Urodeles are found in temperate and subtropical climates in the Northern Hemisphere but
do not reach the tropics in the New World. The elongated body consists of head, trunk,
and a well developed tail, the latter being retained throughout life. Two pairs of limbs
occur in most species. Larvae resemble adults except for the presence of gills, and like
adults, have teeth in both the upper and lower jaws. The urodeles have a greater tendency
to show generalized characters of the class amphibia, in comparison to the much more
specialized Anura. The group shows different types of forms, varying from the terrestrial
salamanders, such as Salamandra maculosa, which is viviparous, to the fully aquatic
forms, such as Necturus. Furthermore, there is a tendency to retain larval characters in the
adults of certain aquatic forms, the process known as paedomorphosis / neoteny.
Examples include Megalobatrachus, which has no eyelids but loses its gills in the adult;
In Cryptobranchus, the spiracle remains open being used for expulsion of water during
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respiration. Amphiuma is an elongated form with very small legs, no eyelids and four
branchial arches. An extreme example of neotenous forms is Necturus, which has external
gills but has such a reduced lung that the animal can live as a permanently aquatic form.
Similarly, Siren shows all larval characters and has no hind limbs. The terrestrial newts
are of different types: some are definitely terrestrial like Triturus vulgaris, although it is
not able to live in very dry habitats. The limbs support the body weight, their soles being
applied to the ground and turned forwards. The tail shows reduction to form a rod-like
organ but when the animal returns to the water for breeding purposes, the tail develops a
large fin. On the other hand, is the genus Ambystoma, which has eleven species, in which
some races become mature without metamorphosis, because of lack of iodine in water,
whereas others are genetically neotenous. The important families of urodeles include:
Table 4: Classification of Urodela up to families
Family
Character
Karaurus
Distribution
Example Species
Fossil from Jurassic of
Kazakhstan
Karaurus sharovi
4 Species in N.
America
Siren, Habrosaurus,
Pseudobranchus
Sirenidae
Small (15 cm) to large
(75 cm) elongate
aquatic forms, with
external gills, pelvic
girdles and hind-limbs
absent
Cryptobranchidae
Very large (1 m) to
1 Species in N.
huge (> 1.5 m) aquatic America and 2 Species
forms, paedomorphic
in Asia
with external
fertilization of the eggs
Cryptobranchus,
Megalobatrachus
Hynobiidae
Small to medium size
(30 cm) aquatic or
terrestrial forms,
external fertilization of
eggs, aquatic larvae
~ 36 Species in Asia
Amphiumidae
Very large (1m)
elongate, aquatic
forms, lacking gills
3 Species in N.
America
Batrachuperus,
Hynobius,
Onychodactylus,
Pachynynobius,
Ranodon,
Salamendrella
Amphiuma
Plethodontidae
Tiny (3 cm) to large
(30 cm) aquatic or
terrestrial forms, direct
development or some
with aquatic forms
~ 265 Species in N., C.
and S. America, 1
Species in Europe
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Gyrinophilus, Eurycea,
Pseudotriton,
Manculus
Family
Character
Distribution
Example Species
RHYACOTRITONIDAE Very small (<10 cm)
semi-aquatic forms,
aquatic larvae
4 Species in N.
America
Rhyacotriton
PROTEIDAE
Small to large (30 cm)
aquatic forms,
paedomorphic with
external gills
5 Species in N.
America, 1 Species in
Europe
Necturus, Proteus
Salamandridae
Small to medium size
(20 cm), terrestrial and
aquatic forms
49 Species in Europe
and Asia, 6 in N.
America
Tylototriton,
Pleurodeles, Triturus,
Euproctus, Salamandra
Ambystomatidae
Small to large (30 cm)
terrestrial forms,
aquatic larvae
32 Species in N.
America
Ambystoma
Dicamptodontidae
Small to large (10-35
cm) semi aquatic
forms, aquatic larvae
4 Species in N.
America
Dicamptodon
c. ORDER 3. ANURA.
Frogs and toads, which come under this category, show fused head and trunk, and no
neck region. Two pairs of well-developed limbs occur, the hind pair being particularly
adapted for leaping. The feet may be webbed and adapted for swimming or resemble long
fingers with suction pads for climbing. Frogs and toads are the first vertebrates to have
vocal cords for sound production. The anuran larva or the tadpole has head and body
fused into a single, egg-shaped mass and a long tail with a median fin. Metamorphosis is
clearly defined, involving the loss of gills as the lungs develop; resorption of tail and the
appearance of legs.
There are many distinctive features of living frogs. Frogs have not more than nine
vertebrae in front of the sacrum, and the 3-4 vertebrae behind the sacrum are fused into a
rod-like structure called the urostyle. In comparison are the urodeles and the apoda that
have many more vertebrae and lack the urostyle. Furthermore, anurans lack a tail in the
adult stage, unlike the other two groups. Frogs also have a radio-ulna, which represents a
fused radius and ulna (bones of the forearm), and a tibio-fibula, the fused tibia and fibula
(bones of the shank). The ankle bones of frogs: the tibiale and fibulare, also known as the
astragalus and calcaneum respectively, are greatly elongate. Thus there is an additional
lever system that frogs can utilize in jumping. Frogs also have a distinct life phase called
as the tadpole: a highly specialized feeding form. Although urodeles and the apoda do
have a larval stage in their life cycle, but these larval forms do not have the diverse
specializations that the frog tadpole has. Even the most primitive frogs have the
beginnings of a unique method of tongue projection, associated with extreme
modification of the gill arches into a fused hyobranchial plate.
Although there is no scientific difference between frogs and toads, the former live in
water, are mostly smooth-skinned and possess long hind limbs for leaping; while the
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latter living on land have a warty dry skin, and shorter hind limbs for hopping. Anurans
inhabit a wide variety of habitats, ranging from arid deserts to mountainous regions to
swampy areas to tropical rain forests. Temperature and water regulation are critical to
amphibians generally, and the anurans particularly. Being ectothermal, frogs and toads
depend on the ambient temperature for body temperature regulation. In winters, frogs in
temperate zones hibernate or enter into a state of extremely reduced activity. On the other
hand, they avoid the extreme heat of summer months in the tropics, by remaining
underground during daytime and being active at night. Anurans are also susceptible to the
loss of body moisture due to extremely hot or dry conditions. Those in temperate climates
maintain moist skin to assist in evaporative cooling. In addition, their permeable skin,
gives the frog an ability to absorb water simply by jumping into water. In contrast are the
frogs in arid regions, which have the skin impermeable to water so as to prevent rapid
evaporation and dehydration. Instead, they cover their body with a mucus film, or burrow
to avoid the heat altogether.
Breeding in frogs is triggered by temperature change and rainfall. During the breeding
season, thousands of frogs may congregate. The males attract their mates by calling. The
latter usually occurs near a water body, where the eggs can be laid and fertilized. Parental
care is variable; some species lay many smaller eggs and show no parental care, while
others lay a few larger eggs and remain with them till the young ones develop.
Among the frogs and toads, many genera are suited for special modes of life. Ascaphus
and Leiopelma, for example live in mountain streams and have reduced lungs. These
show a combination of specialized and primitive features. Internal fertilization occurs by
a penis-like extension of the cloaca. The primitive characters include: presence of tail
muscles, amphicoelous vertebrae, free ribs, abdominal ribs, and persistent posterior
cardinal veins. In Alytes, the males carry the eggs wrapped around the legs. The related
aquatic frog, Pipa, is still more specialized, having no tongue and having developed an
elaborate arrangement by which the young are carried in pits on the back. Xenopus is
related to Pipa, but without the habit of carrying its young. The bufonid toads are among
the most successful of all amphibian groups and well adapted for a terrestrial life, though
always returning to the water to breed. Bufo and related genera are cosmopolitan in
distribution. Only one genus: Nectophrynoides is viviparous. Hyla and other tree frogs are
similar to the bufonids, but show many arboreal adaptations including the presence of
pads on the toes for climbing. Many tropical frogs have devised methods of avoiding
having to return to water for breeding. In Nototrema, for example, the young develop in a
sac on the back of the female, this sac sometimes being protected by special calcareous
plates. Rana and its allies, the true frogs, are also cosmopolitan. A number of its related
genera have got adapted to an arboreal existence. An example is Polypedates, a
widespread genus and several others, each independently derived from ranids. Burrowing
forms have also developed among the anurans, as Breviceps, which digs for ants and has
a snout, as in anteaters. The important anuran families are as follows:
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Table 5: Classification of Anura up to families
Character
Distribution
Small (10 cm)
Fossil from Early
Triassic of Madagascar
Example
Family
Triadobatrachus
Triadobatrachus
massinoti
Small (3 cm) aquatic
forms, found in cold
springs and
mountain streams,
fertilization internal
1 Species in N.
America
Leiopelmatidae
Small semi-aquatic
or terrestrial forms
3 Species in New
Zealand
Bombinatoridae
Small to medium
size semi-aquatic
forms
8 Species in Europe
and Asia
DISCOGLOSSIDAE
Small to medium
size terrestrial and
semi-aquatic forms
5 Species in Western
Europe and North
Africa
Discoglossus
Pipidae
Specialized aquatic
forms, direct
development or with
aquatic larvae
30 Species in S.
America and Africa
Hymenochirus,
Pipa
Rhinophrynidae
Burrowing form
with aquatic larvae
1 Species, extreme
southern Texas to
Costa Rica
Rhinophrynus
Megophryidae
Small to medium
size forest-floor
forms
~ 80 Species from
Pakistan, N. India
through Southeast Asia
and the Philippines to
Indonesia
Small terrestrial
frogs with aquatic
larvae
2 Species in Europe
and Asia
Pelodytes
Short-legged
terrestrial forms,
with aquatic larvae
11 Species in N.
America
Pelobates
Ascaphidae
Pelodytidae
Pelobatidae
Allophrynidae
Ascaphus
Leiopelma
Bombina
Megophrys
Small arboreal forms 1 Species in S. America
Allophryne
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Family
BRACHYCEPHALIDAE
Bufonidae
CENTROLENIDAE
Heleophrynidae
Hylidae
Leptodactylidae
Myobatrachidae
Sooglossidae
Character
Distribution
Example
Very small (<16
mm)
terrestrial frogs,
direct
development
5 Species in Southeast
Brazil
Brachycephalus
Small (20 mm) to
large (25 cm)
terrestrial forms,
most with
aquatic larvae, some
viviparous
~ 400 Species in N. C.
and S. America, Africa,
Europe and Asia
Small arboreal frogs, > 130 Species in
aquatic larvae
Central and South
America
Medium size frogs,
5 Species in extreme
with aquatic larvae
southern Africa
Bufo
Hyalinobatrachium
Heleophryne
Arboreal, aquatic or
terrestrial,
aquatic larvae
mostly,
marsupial forms
show direct
development of
juvenile frogs
in the female’s skin
~ 760 Species in N. C.
and S. America,
Europe, Asia and
Australia
Very small (12 mm)
to huge
(25 cm) frogs,
diverse habitats
and modes of
reproduction
Small (20 mm) to
large (12 cm)
aquatic and
terrestrial frogs,
diverse modes of
reproduction
>900 Species in
southern N. America,
Central and S. America
and the West Indies
Eleutherodactylus
~ 120 Species in
Australia, Tasmania
and New Guinea
Mixophyes
Small terrestrial
frogs, Eggs
hatch into juvenile
frogs or
into non-feeding
tadpoles
3 Species in the
Seychelles Islands
Sooglossus
13
Litoria, Hyla
Character
Distribution
Example
Family
PSEUDIDAE
Rhinodermatidae
Arthroleptidae
DENDROBATIDAE
Hemisotidae
Hyperoliidae
Microhylidae
Ranidae
carried on the back
of the adult
Aquatic frogs with
huge
tadpoles that
metamorphose
into medium-size
adults
Small terrestrial
frogs,
tadpoles either
transported to
water or
development
completed in the
vocal sacs of the
male
Small or medium
size terrestrial frogs
Small terrestrial
frogs, brightly
colored and highly
toxic, tadpoles are
transported to water
by the adult, direct
development
Small burrowing
forms
Small to medium
sized, mostly
arboreal frogs with
aquatic larvae
Small to medium
sized, terrestrial or
arboreal frogs;
aquatic larvae
mostly, some have
non feeding
tadpoles, others with
direct development
Medium sized to
enormous, aquatic or
terrestrial frogs,
aquatic tadpoles
mostly, some with
direct development
14
4 Species in S. America
Lysapsus, Pseudis
2 Species in southern
Chile and Argentina
Rhinoderma
75 Species from subSaharan Africa
~185 Species in C. and
S. America
Arthroleptis
8 Species from subSaharan Africa
Hemisus
~ 230 Species in
Africa, Madagascar,
and the Seychelles
Islands
~315 Species in N. C.
and S. America, Asia,
Africa and Madagascar
Leptopelis
> 700 Species in N. C.
and S. America,
Europe, Asia and
Africa
Lithobates, Rana
Dendrobates
Hypopachus
Family
Rhacophoridae
Scaphiopodidae
Character
Distribution
Example
Very small to large,
mostly arboreal
frogs, filter feeding
aquatic larvae, some
lay eggs in tree holes
and have non
feeding larvae
Round with short
legs, Terrestrial
>900 Species in
southern N. America,
Central and S. America
and the West Indies
Rhacophorus
Amphignathodontidae
Mantellidae
Terrestrial,
arboreal or
aquatic. Body size
ranges from 3 to 10
cm in length
Native to Southern
Canada and U.S.A
South to Southern
Mexico, comprising of
seven families
Native to Neotropical
America (=Central
America and South
America)
Found only in
Madagascar and
Mayotte
Spea
Gastrotheca,
Flectonotus,
Amphignathodon
Mantella,
Laliostoma,
Aglyptodactylus,
Wakea,
Blommersia,
Guibemantis
III. ORIGIN
A. From early chordates to the first land vertebrates
The vertebrate story unfolds over a span of almost 544 million years, during which time;
some of the largest and most complex animals ever known have evolved among the
vertebrates (Figure 1). They show all the four defining chordate characters: notochord,
pharyngeal slits, tubular dorsal nerve cord and a post-anal tail. Vertebrates occupy marine,
freshwater, terrestrial, and aerial environments and exhibit a vast array of lifestyles. The
oldest craniates include the vertebrate fossils from the Lower Cambrian of China. These are
the ostracoderms. These strange fishes, 2 cm to 2 m long and of diverse appearances, had no
jaws, most were without paired fins, and were filter feeders. Broad bony plates in the skin
formed a protective shield over the head and trunk. Fossils that can be clearly identified as
ostracoderms date back to the beginning of the Ordovician. The jawless ostracoderms were
succeeded in the seas by jawed fishes, and amphibians eventually became established on
land. The perplexing problem is who were the ancestors of ostracoderms?
15
a.
Origin of Chordates
Ostracoderms were chordates, therefore we can look for clues in the protochordates that are
with us today and in the fossil record that immediately preceded ostracoderms to trace the
ancestry of ostracoderms. Cephalochordates have a notochord; pharyngeal slits; a dorsal
hollow central nervous system with brain and cord; a metameric body wall musculature; a
two-layered skin; and arterial and venous channels similar to those of fishes and to the
embryonic vessels of tetrapods. Cephalochordates are deuterostomous, coelomate, and filter
feeders, as were many early ostracoderms. These similarities bespeak close genetic ties
between the ancestors of cephalochordates and those of vertebrates. Although we know on
the basis of the current data, that protochordates preceded craniates in the course of natural
history, we have to speculate concerning the lineages that might have led from prechordate
invertebrates, to protochordates on the one hand, and to craniates on the other.
Cephalochordates as we know them today were not the genetic ancestors of the first
craniates. We must consider the observation that echinoderms, like vertebrates have
16
mineralized tissue in their mesoderm; that echinoderms, like cephalochordates, form their
mesoderm and coelom as outpocketings of their archenteron; that echinoderms,
enteropneusts, cephalochordates, and craniates are all deuterostomes a trait found in only one
other invertebrate taxon, the Chaetognatha; and that all have larvae in their history.
Ongoing phylogenetic research and the availability of new molecular methods provide an
improved, although certainly incomplete view of protochordate evolution. Vertebrates arise
within the deuterostome radiation, part of the chordate clade (Figure 2). The other clade
includes the echinoderms along with the hemichordates, which are more closely related to
each other than to the chordates. Some fossil echinoderms preserved a bilateral symmetry,
but most, including all living groups, diverged dramatically, becoming pentaradial losing
pharyngeal slits and a distinct neurulated nerve cord. Hemichordates are monophyletic, with
pterobranchs arising within the enteropneusts, and retain some chordate characters
(pharyngeal slits, neurulated nerve cord, and endostyle). Urochordates are also monophyletic,
a sister group to the rest of the chordates (cephalochordates plus vertebrates).
Cephalochordates are the immediate relatives of the vertebrates.
This phylogenetic view suggests that a wormlike ancestor, perhaps similar to an enteropneust
worm, evolved into the hemichordates / echinoderms on one side of the deuterostomes and
into a chordate on the other. Strictly speaking, this means that chordates did not evolve from
echinoderms and certainly not from annelids / arthropods. Although unsettled and
controversial in its specifics, the origin of chordates lies certainly somewhere among the
invertebrates, a transition occurring in the remote Proterozoic times. Within the early
chordates the basic body plan was established: namely, pharyngeal slits, notochord, dorsal
hollow nerve cord, and the post anal tail. Feeding depended on the separation of suspended
food particles from the water and involved the pharynx, a specialized area of the gut with
walls lined by cilia to conduct the flow of food-bearing water. Pharyngeal slits allowed a one-
17
way flow of water. Locomotor equipment included a notochord and segmentally arranged
muscles extending from the body into a post anal tail. Subsequent evolutionary modifications
were centered on feeding and locomotion and led to the wealth of adaptations found within
the later vertebrates.
b.
Origin of Vertebrates
The origin and early evolution of vertebrates took place in marine waters. Evolution of early
vertebrates was characterized by increasingly active lifestyles hypothesized to proceed in
three major steps: Step 1 comprised a suspension-feeding Prevertebrate, which deployed
only cilia to produce the food-bearing current. Step 2 comprised an Agnathan, an early
vertebrate lacking jaws but possessing a muscular pump to generate a food-bearing current.
Step 3 comprised a Gnathostome, a vertebrate with jaws. It fed on larger food items with a
muscularized mouth that rapidly snatched prey from the water.
Conodonts are fossils extremely common in rocks from the late Cambrian to the end of the
Triassic. The fossils bore evidence that the conodonts are vertebrates. The trunk showed
evidence of V-shaped myomeres, a notochord down the midline, and caudal fin rays on a
post-anal tail. There was also the presence of mineralized dental tissues: cellular bone,
calcium phosphate crystals, calcified cartilage, enamel and dentine. The conodont feeding
apparatus consisted of tongue-like or cartilaginous plates that moved in and out of the mouth,
catching and delivering the crushed food. Thus this mechanism is very similar to the lingual
feeding mechanism of hagfishes. Following the conodonts, Ostracoderms appeared in the
very late Cambrian and radiated in the Silurian and early Devonian. Like the conodonts, they
had complex eye muscles and dentine-like tissues. They were the first vertebrates to possess
paired appendages, a lateral line system, an inner ear with two semicircular canals, and bone
although the latter is located in the outer exoskeleton that encases the body in a bony armor
just beneath the epidermis.
One of the most significant changes during early vertebrate evolution was the development of
jaws in primitive fishes, derived from the anterior pharyngeal arches. Two early groups of
jawed fishes are known: The Acanthodians and the Placodermi. This adaptation opened up
an expanded predatory way of life. Early gnathostomes also had two sets of paired fins, the
pectorals and the pelvics, that were articulated with supportive bony or cartilaginous girdles
within the body wall. This radiation of gnathostomes proceeded along two major lines of
evolution: one produced the Chondrichthyes, the other the Teleostomi. The modern
chondrichthyans consist of two groups: the sharks and rays (elasmobranchs) and the
chimaeras (holocephalans). Both groups have similar fin structures, cartilaginous skeleton
and pelvic claspers. The Teleostomi is a large group embracing the acanthodians, the bony
fishes, and their tetrapod derivatives. Most living vertebrates are bony fishes, members of the
Osteichthyes. Bony fishes have a set of characters including an adjustable, gas-filled swim
bladder (possibly modified from lungs) to provide buoyancy; and an extensive ossification of
the endoskeleton. Bony fishes consist of two unequal-sized groups, the actinopterygians that
compose the vast majority of bony fishes; and the sarcopterygians. The latter group is
important as they gave rise to the very first terrestrial vertebrates. The group called
rhipidistians includes the sarcopterygians that are most closely related to tetrapods. Ossified
neural and hemal arches accompany the notochord. The braincase had a hinge-like joint
running transversely across its middle so that the front of the braincase swiveled on the back
of the braincase. Simultaneously, there were modifications in skull bones and jaw
musculature, bringing about a specialized feeding style involving a powerful bite. The jaws
had labyrinthodont teeth characterized by complex infolding of the tooth wall around a
central pulp cavity. Rhipidistians gave rise to tetrapods during the Devonian but they became
extinct in the early Permian. The demands of terrestrial life and the new opportunities
18
available led to an extensive remodeling of the fish design as tetrapods diversified into
terrestrial and eventually aerial modes of life. It however, also includes some derived groups
with secondary loss of limbs, such as snakes.
c. Origin of Tetrapods
The development of vertebrates which lived on land, started about 350 million years ago in
the Devonian period. At this time, some fish began to crawl out of the water and started
walking on land and breathing air. The climate of the world at the end of Devonian became
hot and arid. This would have caused the water in shallow pools and lakes to become warmer,
and many small water bodies may have evaporated during the seasonal droughts. The fish
ancestors of the first land vertebrates must have had two important features i.e. firstly: the
presence of lungs as simple pouches leading from the throat, which developed a rich supply
of blood vessels. And secondly: the development of limbs from the bony supports of the fins.
The most likely ancestors of the amphibians were the rhipidistians that were common in the
Permian. Unfortunately, the fossil record of the origin of amphibians is very poor. Rock
deposits from the middle Devonian period contain typical rhipidistian fish; while early
amphibian ancestors appear in the late Devonian. However no fossil species, which directly
link the two groups, have been found during the intervening period of about 30 million years.
Until more fossil species are found, which show the transitional forms between fishes and
amphibians, this important period of vertebrate evolution will remain uncertain. The earliest
fossil amphibians that have been found had already solved the problems of living on land.
They were the Labyrinthodontia, and Ichthyostega is a typical example. The earliest
amphibians were all carnivores and must have been feeding on other animals. Modern
amphibians are specialized animals, which do not resemble primitive amphibians very
closely. They are so specialized that it is not clear when they separated from the primitive
amphibia, from which group they derived, or how closely related the modern forms are to
each other. Three distinct groups of modern amphibia remain, which as adults feed on insects
or other small invertebrates. These groups clubbed together as Lissamphibia include:
Urodela (newts and salamanders); Anura (frogs and toads); and Gymnophiona (caecilians).
B. Acquisition of adaptations for life on land
Amphibia, the first vertebrates to become adapted to a terrestrial mode of life, may be
differentiated from their fish predecessors, mainly on the basis of their pentadactyl limbs; the
absence of fin rays in the unpaired fins, if present; and by the presence of a middle ear.
Amphibians breathe by gills in the larval stages and by lungs when adult. The skin, which is
usually naked, often plays an important role in respiration. The skull is autostylic and the free
hyomandibular has got converted into a columella auris that stretches between the inner ear
and the tympanic membrane. An opening called fenestra ovalis is present through which the
columella transmits sound vibrations to the inner ear. This is another important modification
with respect to terrestrial mode of life.
Important changes have also occurred in the skeleton and musculature: The skull has become
movably attached to the vertebral column by one or two occipital condyles. The head no
longer supported by water, required a more powerful musculature and corresponding
elaboration of articular surfaces in the skull and adjacent endoskeleton. The lower jaws too,
required an elaborate musculature for their support and operation. The girdles had not only to
provide support in locomotion but also to protect the internal organs from injury. With the
advent of heavy upward pressures during walking, there arose anteriorly a powerful scapula
(shoulder blade) bound to the front ribs of the thorax; and posteriorly a triradiated pelvic
apparatus. In each girdle, there also arose endoskeletal processes for the firmer attachment of
muscles and the developed specialized limb bones, including digits and other refinements.
19
The complex system of tetrapod limb muscles is arranged in two series that are derived from
the simpler musculature of the upper and lower aspects of fins. A comparison of the bony and
skeletal structures of crossopterygians and early amphibians shows that their limbs are very
closely allied.
Ancient tetrapods, the Labyrinthodonts retained bony scales in the abdominal region.
Grooves in the skull of some juveniles carried the lateral line system, which however was
absent in the adults of the same species. Thus, many ancient tetrapods, like modern
amphibians, were probably aquatic as juveniles and terrestrial as adults. These
labyrinthodonts / stegocephalia originated no later than the Upper Devonian. Their important
features include: 1) the loss of bones rigidly linking the skull to the shoulder girdle. This was
accompanied by the appearance of a mobile neck allowing the head to be moved relative to
the trunk. 2) The operculum was lost, as it was no longer needed in these early choanates
since they had lost the internal gills of their early ancestors. 3) A reduction of the notochord
and a rigid spine. The thick centra constricted the notochord. Special articulatory surfaces
called as zygapophyses linked the neural arches to each other. Also the notochord was shorter
and did not extend into the braincase. A sacral rib connected the axial skeleton to the pelvic
girdle, allowing the weight of the tetrapod body to be transmitted to the hind limb. The
dermal fin rays that were no longer needed on land were lost. 4) There was the development
of four muscular limbs with discreet digits.
The conquest of land also depended on some means of aerial respiration. The primitive lung
is a characteristic of ancient fishes; it came first and from it evolved the swim bladder, an
organ of specialized hydrostatic and other functions found only in bony fishes. In the
Devonian, the development of a respiratory sac, capable of absorbing atmospheric oxygen,
would be of immense help to early fishes that were compelled to live in water that became
periodically low on oxygen level and clogged with rotting vegetation. The amphibians
suffered a loss of true biting teeth. They were forced by virtue of their imperfect adaptation,
to confine their feeding to slow moving prey and later to insects that could be reached with a
sudden flick of the muscular, sticky and protrusible tongue that they later developed. The
lateral line system, although retained in the larval forms, was soon lost in the land-living
adults.
Terrestrial life depended not only on the development of efficient lungs and walking legs, but
every part of the animal body was involved. Not only did animals require to breathe
atmospheric air and to walk; they had to withstand desiccation, rid themselves of the lateral
line system, detect air-borne substances of a much greater dilution, see and hear predators and
prey at much greater distances. However, no new organs were formed.
The Devonian was an age of great climatic instability. The fresh water streams and lagoons of
that time were alternately filled and dried out. Such conditions of periodic desiccation would
have resulted in the extinction of numerous species, and resulted in the survival of those
possessing the physiological and structural adaptations suitable to function in the new
conditions. Ancient bony freshwater fishes had developed a bony supporting skeleton,
osmotic homeostasis, internal nares, and lungs. In the crossopterygians, the appearance of
two pairs of highly mobile, muscular, lobe-like lateral fins supported by bones, gave an
indication of later development of walking legs of the modern tetrapods. This is a classic
example of Pre-Adaptation: the possession by an organism of characters that are conducive to
its survival under altered conditions. Thus those types that had locomotory, respiratory,
integumentary, excretory and sensory specializations related to drought survival, would
prosper and reproduce. Those whose adaptations were directed towards purely aquatic
efficiency would fail. A comparison of the earliest Amphibia with Palaeozoic fishes shows
many similarities between the embolomerous labyrinthodonts and the osteolepids of the
20
Devonian. This is particularly marked in the general structure of the skulls, in the similar
labyrinthodont pattern of the teeth, in the possession of the large palatal tusks, and in the
structures of the girdles.
The amphibian skin became heavily keratinized. This assisted water retention, but at the same
time allowed cutaneous respiration. Yet, amphibians in general cannot live away from moist
situations. The permeability of the amphibian skin has thus imposed serious limitations on the
choice of habitat as well as upon geographical distribution. Secondly, the inevitable failure of
the amphibia to develop temperature control further limits their chances of land colonization.
From the time of their emergence they have remained imperfectly adapted to terrestrial life:
most land-going species remained dependent upon fresh water for reproduction. Majority of
amphibians require a damp environment in which to breed because their eggs and embryos
must extract oxygen and food from the surrounding water and at the same time excrete waste
material directly into it. They have developed no protective shell, as have reptiles, birds and
primitive mammals. They also lay down little yolk for the nourishment of the growing young.
The earliest known anuran ancestor, Protobatrachus, did not appear until the early Mesozoic.
Although it had a tail in the adult, but the skull was similar to those of modern frogs. The ribs
were short, the presacral vertebrae were reduced in number and there were free caudal
vertebrae. There was an elongate ilium and a fairly long femur. However, the radius and ulna,
and the tibia and fibula were separate. The earliest true frog remains occur in the Upper
Jurassic. The earliest known salientia is Triadobatrachus massinoti, from early Triassic. This
“proto-frog” is around 250 million years old and does not have the combination of characters
normally associated with frogs. The earliest true frog is Vieraella herbsti from early Jurassic,
while the fossil from middle Jurassic is Notobatrachus degiustoi, which is around 155-170
million years old. Another well-preserved Jurassic fossil known as Eodiscoglossus santonje,
has 8 pre-sacral vertebrae and thus is clearly within Anura. The Urodeles, too, have not been
found before the Jurassic period. No fossil Apoda has been discovered. Today, the most
terrestrial amphibia are the Anura. The Urodela have retreated once more to the water and the
degenerate Apoda into moist holes on the ground. Although amphibians were the dominant
land fauna in the Carboniferous, little more than 2000 species live today, although some
remain plentiful in appropriate areas. Of the three surviving amphibian groups, the Anura are
abundant in all the greater zoogeographical regions but are absent from most oceanic islands.
The Urodela are almost exclusively Palaearctic and Nearctic forms, occurring in North
America, Europe, Asia, and North Africa. A few species extend southwards into the
Neotropical and Oriental regions. The Apoda, on the other hand, are mainly tropical,
occurring in the Neotropical, Ethiopian and Oriental regions. They are absent from
Madagascar, Australasia, and the Pacific Islands.
IV. AMPHIBIA: GENERAL ORGANIZATION
a. External Appearance- In the Anura, the head is large and depressed with a wide
mouth and large tympanic membranes in most genera. The eyes are large and
prominent, provided with an upper eyelid and a nictitating membrane. The trunk is
short, the tail absent and the cloacal aperture is terminal. The hind limbs are much
longer than the forelimbs (Figure 3a). The forelimb consists of an upper arm or
brachium, a forearm or antebrachium, and a hand or manus, ending in four tapering
digits. The hind limb consists of the thigh or femur, the shank or crus, and the foot,
which consists of a tarsal region and five slender digits. The arboreal forms have
plate-like adhesive discs at the termination of the digits of all four legs. The discs hold
on to the substratum with the help of capillary action and adhesion by means of
21
mucus. In certain tree frogs, like Rhacophorus, the ventral surface and webbing of the
elongated digits together produce a volplane mechanism functionally comparable with
that of the reptile Draco, and certain marsupials and rodents. In the Apoda, the body is
elongated and snake-like, the head is small and not depressed, and limbs are absent. Tail
is absent. These tropical animals live in burrows, are blind although they possess sensory
tentacles lodged in pits. In burrowing forms the snout is sharp and may have dermal
ossifications. The Urodela are of three types: the perennibranchiate or persistent-gilled
have an elongate trunk separated by a slight constriction from the depressed head and at
the other end passes into a compressed tail having a continuous median fin. Limbs are
small and weak and eyes are small and lidless. Tympanic membrane is absent. On each
side of the neck are two-gill slits leading into the pharynx. From the dorsal end of each of
the three branchial arches arises a branched external gill. To this group belong genera
such as Siren, Necturus and Proteus. The remaining urodeles are called caducibranchiate
or deciduous-gilled and are further of two types: the derotrematous forms, that loose the
gills but retain the gill clefts in the adult, an example being Amphiuma whose body is eel
like and the limbs are extremely small. To the second group belong the salamandrine
forms, in which all trace of branchiate organization disappears in the adult as seen in the
Spotted Salamander and the common British newts. The limbs in land salamanders stand
out from the trunk and are plantigrade.
22
The median fin is completely lost and the tail is cylindrical (Figure 3b).
b. Integument- The epidermis of amphibians is composed of several layers of cells and
is the first to have a dead stratum corneum. The epidermis is six to eight celled thick
and divisible into three layers: stratum corneum, stratum germinativum, and a basal
part in contact with the basement membrane. Shedding of dead skin fragments occurs
periodically and consists of removing a unicellular sheet of stratum corneum. A dead
corneal layer is an adaptation to terrestrial life, protecting the body and preventing
excessive loss of moisture. The dermis is relatively thin in amphibians. It is composed
of two layers: an outer stratum spongiosum; and an inner stratum compactum. A great
number of gland types are recognized. Mucus glands are numerous; their secretion
helps to maintain a moist skin. A second group of glands, the poison glands, secrete a
number of substances that are distasteful and / or poisonous in some tropical frogs.
The amphibian skin is an important organ of respiration. Blood vessels, lymph spaces,
glands, and nerves are abundant in the stratum spongiosum while the inner stratum
compactum is rich in muscle fibers. Chromatophores of complex structure lie between
the epidermis and dermis (Figure 4).
The skin is soft and usually slimy due to the secretion of the cutaneous glands. Many
toads and a few salamanders have small poison glands aggregated into prominent
swellings called the parotoid glands located on each side of the head. Furthermore, the
large and conspicuous warts of toads are each perforated by a pore that leads to a
23
poison gland beneath. The exudate from the gland contains toxins known as bufotalin
and bufogin. Some specialized integumentary glands in amphibians have a tubular
structure. They include those on the feet of certain tree-dwelling frogs and toads.
Suctorial discs on the toes aid in climbing. Other examples include the glandular
thumb pads of male frogs and toads and the mental glands of the male salamanders.
Unicellular glands are present on the snouts of tadpoles and urodele larvae. Their
secretion has digestive properties, thus helping in freeing the larva from the egg
capsule during early development. Large glands of Leydig are unicellular glands of
uncertain function in the epidermis of some larval urodeles.
The color of the skin is often brilliant in many salamanders and frogs however certain
tree frogs have a protective green coloration. The beautiful and strongly contrasted
hues of the Spotted Salamander and of certain frogs are examples of warning colors;
their conspicuous colors serve to warn off the predators that would otherwise devour
them. The sudden flash and sudden disappearance of color in Phyllomedusa may be
meant for confusing the possible predators. Apart from camouflage and warning,
some colors have a sexual significance. The ground color of the skin of frogs can
change as a result of environmental stimuli. The skin contains the deeply situated
black melanophores, layers of guanophores, and yellow lipophores, which lie close
below the epidermis. The various colors of the frog skin are produced both by
pigments and purely physical phenomena and are under a neuro-endocrine control.
The skin of modern amphibians lacks scales except in a few toads and in some of the
burrowing, limbless caecilians. The skin is usually smooth and moist. Many
burrowing Apoda have an exoskeleton made of small dermal scales. Similarly, bony
exoskeletal plates occur in some Anura, beneath the skin of the back. In the anuran
Xenopus and also in the urodele Onychodactylus, small, horny claws are found on the
digits. The frog Leptodactylus pentadactylus, has horny chest grapples. However,
apart from these few examples, the amphibian skin generally lacks hard parts.
c. Alimentary canal- The wide mouth of anurans leads into a capacious buccal cavity,
having internal nares in its roof, the eye bulges and the openings of the Eustachian
tubes. The lips are little developed but the tongue is a characteristic organ for catching
food and is a special feature required for terrestrial life. In Rana, it is attached to the
floor of the mouth anteriorly and flicked outwards by its muscles. To keep it moist
and sticky, a special intermaxillary gland is present. The saliva contains a weak
amylase, which may serve to release sufficient substances for tasting. Special tracts of
cilia carry the secretion from the intermaxillary glands to the vomeronasal organ and
to the palatal taste buds. Behind the tongue is a slit like glottis. The teeth are small,
fused to the bones and singly or doubly pointed. The teeth are without pulp or nervetissue. There are teeth only on the upper jaw of frogs, on the premaxillae, maxillae
and vomers. They are used basically to prevent the escape of prey.
Behind the buccal cavity is the pharynx, which leads by a short oesophagus or gullet into
a stomach. Buccal cavity and oesophagus of anurans have mucus-producing goblet cells.
The stomach consists of a wide cardiac and a narrow pyloric division (Figure 5). Its
epithelium consists of mucus secreting cells and is folded, with simple tubular glands
opening at the base of these folds. These glands, unlike those of mammals, are composed
of only a single type of cell, which secretes both the acid and the pepsin found in the
stomach. A pyloric sphincter guards the entrance into the duodenum, which is richly
supplied with goblet cells. Here, the digested food is absorbed into the hepatic portal
system. The remaining ileum is coiled and dilates into a large intestine or rectum that is
24
special development for water absorption by terrestrial vertebrates. The rectum passes
into the cloaca, the latter terminating into a cloacal aperture.
The liver has two large lateral lobes and one small median lobe. Between its right and left
lobes, occurs the gall bladder. Bile passes from the liver into the gall bladder via cystic
ducts. Liver along with the fat bodies plays an important role in fat absorption. Pancreas,
having both exocrine and endocrine functions, is held by mesenteries between stomach
and duodenum. Digestive juices from the acinar cells flow down the pancreatic duct,
which is bound to the bile duct and together they empty into the duodenum through a
common opening. Paired thyroids are present below the floor of the mouth in front of the
glottis and lateral to the hyoid apparatus. Removal of thyroids in tadpoles prevents
metamorphosis. Furthermore, the periodic ecdysis / molting of the keratinous epidermal
layers of the skin is under the control of anterior pituitary and thyroid, so that removal of
either causes the cornified layers to remain unshed as a dark thick covering. On the other
hand, administration of thyroid extracts will cause retardation of growth and bring about
sudden metamorphosis in various anurans. Parathyroids occur as paired ovoid bodies,
associated with calcium metabolism. The paired thymus gland situated behind and below
the tympanic membrane is of doubtful function. On the ventral face of each kidney there
occurs the elongated, yellow, compound adrenal / suprarenal gland. Spleen is a
haematopoetic organ in anura, found attached to the anterior end of the rectum via a
mesentery.
Modifications in the alimentary canal of various amphibian groups are as follows: Few
amphibians bite, though biting teeth are found in the adult Ceratophrys ornata. The South
American tree frog Amphignathodon has teeth on lower and upper jaws and presumably
has redeveloped them, a remarkable case of reversal of evolution. In many anurans, such
as common toad, teeth are entirely absent. Most urodeles have teeth both on the upper and
lower jaws. A deciduous fetal dentition occurs in ovoviviparous caecilians. These teeth
are replaced by adult teeth at the time of parturition. Tongue in most urodeles is fixed and
immovable while in several anurans it is free behind and attached in front. However, in
Xenopus and Pipa, tongue is absent. The adhesive power of tongue in many frogs and
some urodeles is enhanced by secretions of the lingual gland and the intermaxillary gland
between the premaxillae and nasal capsule. Anurans are additionally equipped with a
pharyngeal gland that discharges into the internal nares. The oesophagus may be ciliated
and in the anurans, both buccal cavity and oesophagus possess mucus-producing goblet
cells. Stomach glands of anurans secrete both pepsin and acid. The stomach is
25
enormously distensible. The small intestine is a tube of almost uniform width in all
amphibians. It is almost straight in Apoda, and is extremely long in certain tadpoles. A
spiral valve does not occur, an increased absorption surface is obtained by great increase
in length. There is some evidence that the liver is of unusual importance in fat storage.
Certainly there is much evidence that amphibians can live for long periods without food.
Axolotls, for instance, have remained alive for 650 days under conditions of starvation,
losing about 80% of their former body weight.
d. Respiratory organs and voice apparatus- A majority of amphibians have external
gills in the larval state but these disappear in the adults of terrestrial forms. The
perennibranchiate urodeles retain the external gills throughout life. These are
branched cutaneous structures, richly supplied with blood vessels but not homologous
to pharyngeal gills. On the other hand, internal gills are developed only in the larvae
of Anura. They appear along the branchial arches below the external gills.
In frog, the respiratory tract begins from the external nares, which are a pair of small
apertures located on the snout. These lead into small nasal chambers lying in the skull.
Vomeronasal / Jacobson’s organ is a pair of blind diverticula extending from the ventromedial part of the nasal chambers and acts as the accessory olfactory organ. The nasal
chambers open into a buccopharyngeal cavity via small apertures called the internal
nares / choanae. The buccopharyngeal cavity communicates with the small median
laryngo-tracheal chamber with the help of a small slit like aperture known as the glottis.
The laryngo-tracheal chamber leads into two very short tubes, the bronchi that terminate
into the lungs. The latter are paired sac-like structures located in the anterior part of the
body cavity on the sides of the heart. Lungs are pinkish in color, with thin, highly
vascular, elastic walls. The inner surface possesses a network of low ridges called the
septa. These enclose shallow depressions called the alveoli. Together, the septa and
alveoli increase the respiratory area of the lungs (Figure 6 a). Histologically, the lung
wall consists of the outermost layer called the peritoneum; the middle connective tissue
layer and the inner epithelial layer.
26
27
Respiratory movements involve the alternate raising and lowering of the floor of the
buccal cavity, and are brought about by two sets of muscles, the sternohyal and the
petrohyal muscles. Contraction of the sternohyal muscles causes a lowering of the
buccal cavity. As a result, the buccopharyngeal cavity gets enlarged and air pressure
inside it is reduced. Consequently, the outside high-pressured air immediately rushes
into the buccopharyngeal cavity through the external nares, nasal chambers and
internal nares. Thereafter, the premaxillae are raised due to the up-pushing of the
lower jaw so that the external nares are closed. The petrohyal muscles now contract
raising the floor of the buccal cavity. The reduced volume of the buccopharyngeal
cavity forces the air into the lungs through the laryngo-tracheal chamber and the
bronchi. In the lungs, exchange of gases occurs between the air in the alveoli and the
blood flowing in the peripheral blood vessels. Air in the alveoli thus gets depleted of
oxygen, becoming loaded with excess carbon dioxide. During expiration, floor of the
buccopharyngeal cavity is lowered, the lungs contract and the glottis permits air from
the lungs to enter the buccopharyngeal cavity. Finally the external nares open,
followed by the raising of the floor of the buccopharyngeal cavity. The high-pressured
air from within is expelled to the outside. Mouth and oesophagus are kept closed
during pulmonary respiration.
Modifications in the structure of lungs among the various amphibian groups are as
follows: In frogs and toads, trachea is non-existent, an exception being the members
of the family Pipidae where a definite trachea is present and the lungs function as
hydrostatic organs. In urodeles, the trachea is usually short, though in Amphiuma and
Siren, it is 4-5 cm long. In all amphibians, the tracheal cartilages are small and
irregular in distribution. Trachea, whenever present, divides into two bronchi that lead
directly into the lungs. Amphibians generally have simple lungs. In urodeles, the left
lung is often larger than the right one. Alveoli may or may not be present or they may
be restricted to the basal part of the lungs. However, in some salamanders, lungs are
completely lacking and respiration is cutaneous and pharyngeal. In those species
where it is present, the salamander lung also acts as a hydrostatic organ. In the
perennibranchiate urodeles like Necturus, lungs as well as gills are simultaneously
found in the adult. In Apoda, the left lung is very short, whereas, the right lung has
alveoli all over the surface. The left lung is rudimentary in some Apoda, which
instead have a tracheal lung. A larger alveolar respiratory surface is found in those
amphibians, which are adapted to a terrestrial mode of life.
Pulmonary respiration is used by a frog only during times of great oxygen need.
Normally the oxygen requirement is met satisfactorily by cutaneous and
buccopharyngeal respiration. It is believed that respiration through the skin accounts
for almost 70% expulsion of carbon dioxide from the body. The characteristic feature
of the moist amphibian skin is that it is highly vascular so as to function as an
effective respiratory organ. Many amphibians show supplementary adaptations to
cutaneous respiration. For instance, there is presence of highly vascular folds in
Cryptobranchus. Similarly, the African Hairy frog, Astylosternus has extensive tracts
of vascular papillae associated with the hind limbs of the male during the breeding
season. These compensate for its reduced lungs and the greater need for oxygen
during the reproductive period. The aquatic Typhlonectes has a richly vascular skin
through which it respires.
Amphibians are the first vertebrates that show evolution of a true voice apparatus. In
frogs, larynx is represented by the laryngo-tracheal chamber (Figure 6 b) and is
located near the posterior corner of the hyoid apparatus. At one end it communicates
28
with the buccopharyngeal cavity through the glottis, while at the other end, it leads to
the lungs by a short pair of bronchi. A framework provided by a pair of arytenoids
and a single cricoid cartilage supports the glottis and walls of the laryngo-tracheal
chamber. The cricoid cartilage is oval in shape while the arytenoids are semi lunar
and lie inside the ring formed by the ovoid cricoid cartilage. The mucus membrane of
the laryngeal chambers is raised into a pair of horizontal folds known as the vocal
cords. These occur in both sexes, but are much better developed in the male anurans.
A narrow space exists between the inner free edges of the vocal cords. It is through
this space that the air has to pass on its way to lungs and back. The vocal cords vibrate
as air is forced back and forth between lungs and the voice box resulting in the
production of sound. The pitch of the sound is controlled by the level of the tension
generated in the vocal cords. Vocal sacs are additional structures found in male frogs
only. These are buccal diverticula connected with the mouth by small slit-like
apertures and extend ventrally and laterally so as to lie under the outer skin and
muscles of the throat region. The vocal sacs when fully dilated act as resonators. Male
frogs are capable of producing sound under water as well as outside water. Voice is
produced with the mouth closed.
Among amphibians, the simplest type of larynx is found in certain urodeles like
Necturus where only a pair of lateral cartilages encircles the glottis. Urodeles merely
produce a hissing or squeaking sound. Other amphibians including frog show
modification of these lateral cartilages by acquiring an anterior pair of arytenoids and
the posterior cricoid cartilage.
e. Blood vascular system- Double circulation, first introduced in lung fishes, also
becomes a characteristic feature of amphibians (Figure 7). The auricle becomes
completely divided into the left and right chambers by an interauricular septum. As a
result, the two blood streams are kept separate in the auricles. The system of vessels
draining the lungs to the heart and those going from the heart to the lungs is referred
to as the pulmonary circulation. Other system of vessels through which blood is
circulated to different body parts is known as the systemic circulation. Such a double
circulation helps to cope with the terrestrial mode of life since the gills have been
replaced by the lungs as respiratory organs. Secondly, the sinus venosus has shifted its
position and opens into the right auricle. The third advancement in the amphibian
heart is that the ventricle lining is thrown into many pocket-like structures formed by
muscular bands. The incorporation of a complicated system of valves inside the
auriculo-ventricular aperture is yet another improvement in the amphibian heart over
that of fishes. All these
features help to separate the
aerated and the non aerated
blood streams.
The heart of frog is a muscular,
reddish-colored conical organ that lies
in the pericardial cavity ventral to the
oesophagus and in front of a septum
tranversum that completely separates
the pericardial and coelomic cavities.
The heart is ensheathed by the
pericardium, a double-walled sac. The
inner wall (epicardium) is applied to
the heart surface. The two walls unite at the base of the arterial arches but around the
29
heart they are separated by a pericardial space containing serous fluid. The pericardial
fluid not only protects the heart from mechanical shock, but also allows easy
contraction and relaxation movements. The anuran heart consists of a sinus venosus,
right and left auricles, a single ventricle, and a conus arteriosus (Figure 8). The sinus
venosus is a triangular, thin-walled chamber located in the dorsal heart surface with
its apex directed backwards. It receives impure blood via the right and left pre-cavals
and the post-caval veins. Blood is then delivered into the right auricle via the aperture
guarded by two lip-like sinu-auricular valves. These valves guide the blood from the
sinus venosus to the right auricle and prevent its backflow. The left auricle receives
pure blood brought from the lungs via the pulmonary veins through a small aperture
present in the left wall of the left auricle towards the anterior side. No valves however
guard this aperture. Backflow of blood from the left auricle to the pulmonary veins is
prevented by the oblique orientation of the pulmonary vein inside the wall of the
auricle, which gets closed down whenever the wall of the auricle contracts. The two
auricles are separated by an inter-auricular septum.
The auricles contract
and push their blood
into the single ventricle
via an aperture called
the auriculo-ventricular
aperture. It is guarded
by
four
flap-like
auriculo-ventricular
valves (also known as
the
atrio-ventricular
valves). Of the two
larger valves, one arises
from the dorsal border
and the other from the
ventral border of the
auriculo-ventricular
aperture. Fine thread-like structures called the chordae tendinae connect the free edges
of the valves to the inner surface of the ventricle. The two smaller valves arise from
the lateral walls of the auriculo-ventricular aperture. No chordae tendinae are attached
to these. All these valves can open only towards the ventricle. The ventricle is a
triangular, thick-walled, muscular structure. Its walls are raised up into muscular
ridges or columnar carneae with interstices between them to prevent the mixing of
blood of the two types of blood streams. Blood is thereafter driven into the conus and
is expelled into the systemic arches.
The conus arteriosus is a tubular chamber oriented obliquely on the ventral side of the
right auricle. It originates from the right anterior border of the ventricle from the
ventral side. At its base, the conus has three pocket-like semi lunar valves. These
valves are arranged in a ring-like manner with their openings facing the conus. Blood
can flow freely from the ventricle into the conus arteriosus. The main cavity of conus
is divided internally into two unequal parts by a second ring of semi lunar valves.
These parts include the long proximal pylangium and the short distal synangium.
There is present a well-developed longitudinal spiral valve inside the cavity of the
pylangium. Thus the internal cavity of the pylangium is incompletely divided into two
30
parts: the cavum pulmocutaneum and the cavum aorticum. One part carrying pure
blood leads into the carotid and systemic arches; the other carrying impure blood
drains into the pulmo-cutaneous arches. The synangium is completely divided into
two chambers: a dorsal chamber and a ventral chamber. The dorsal chamber sets up
connection in front with the pulmo-cutaneous arches and behind with the cavum
pulmocutaneum via an aperture located just in front of the spiral valve. The ventral
chamber of the synangium also establishes connections in front with the carotid and
systemic arches and behind with the cavum aorticum.
Two sets of valves are present to
prevent the backflow of blood:
one at the junction of the conus
and ventricle, the other between
the conus arteriosus and the
ventral aorta. Each ventral aortae
is divided into three vessels: the
anterior, carotid trunk; the
middle, systemic / aortic trunk;
and the posterior, pulmocutaneous trunk. The carotid
divides into an external and
internal branch, which supply
the head. The systemic trunks
curve around the oesophagus,
the right arch becomes the dorsal
aorta, the left continues as the
coeliaco-mesenteric artery. The pulmo-cutaneous trunk divides into the pulmonary
artery going to the lungs and the cutaneous artery going to the skin. In the tadpole,
there are four pairs of aortic arches, associated with the gill capillaries. In the adult,
the gills disappear. The 3rd aortic arch loses its connection with the dorsal aorta and
becomes the carotid trunk. The 4th retains its connection with the dorsal aorta and
becomes the systemic trunk. The 5th disappears. The 6th becomes the pulmo-cutaneous
trunk (Figures 9 a, b).
The deoxygenated blood is
returned from the head via the
internal and external jugular
veins that drain into the precaval vein. The latter also
receives the brachial vein from
the forelimb, and the musculocutaneous veins from the skin
and muscles. Two portal systems
occur. The blood from the hind
leg is brought back by the
femoral, that divides and its
dorsal branch called as the renal
portal receives blood from the
sciatic vein and passes into the
kidneys, breaking up into
31
capillaries. The ventral branch called as the pelvic vein joins with its opposite partner
into the anterior abdominal vein, which divides into capillaries in the liver, where it is
joined by the hepatic portal vein, bringing the blood from the stomach, intestine,
spleen and pancreas. The renal veins that unite collect blood from the kidneys into the
large post-caval vein. This passes through the liver, receiving the hepatic veins and
finally opening into the sinus venosus (Figure 10).
In the perennibranchiate urodeles, circulation is like that of a fish. The bulbus / ventral
aorta gives off four afferent branchial arteries: 3 to the external gills and the 4th that
curves around the oesophagus and joins the dorsal aorta. From each gill arises the
efferent branchial artery, all of which then unite into the dorsal aorta. Each afferent
with the corresponding efferent artery constitutes an aortic arch. Carotids arise from
the 1st efferent artery and when the lungs arise, a pulmonary artery is given off from
the 4th aortic arch. When the gills atrophy, the 3rd aortic arch becomes the carotid, the
4th becomes the systemic, the 5th undergoes variable degrees of reduction, and the 6th
becomes the pulmonary artery.
The urodeles show a transition from the fish-type to the frog type of venous system.
The caudal vein brings the blood from the tail and it divides into the two renal portals
that enter the kidneys; from the kidney blood drains into the paired cardinals. The
anterior portions of the cardinals degenerate into two small azygous veins, receiving
the blood from the back. Their posterior portions unite into the post-cava vein. The
latter along with the hepatics, drains into the sinus venosus. Likewise, the iliac from
the hind limb, divides into two branches: one joins the renal portal while the other
forms the anterior abdominal and joins the hepatic portal.
Amphibian RBCs are oval and nucleated. These arise either from the kidney or from
the spleen or may arise from the bone marrow. White cells consist of large phagocytic
macrophages, monocytes, phagocytic polymorpho-nuclear granulocytes and
lymphocytes.
In the closed system of vessels, as of vertebrates, the capillaries are separated from
other body tissues by fluid-filled tissue spaces. These are filled with tissue fluid,
which is essentially blood plasma that has seeped out of the capillaries. The tissue
spaces communicate with minute lymph vessels that continue into larger vessels to
32
make up the highly developed lymphatic system. Frogs are remarkable for the
dilatation of many of its lymph vessels into large lymph sinuses. Between the skin and
muscle are various spacious subcutaneous sinuses, separated from each other by
fibrous partitions. The sinuses allow the frog’s skin to slide back and forth across the
underlying structures. The dorsal aorta is surrounded by a large sub vertebral sinus.
Lymph is driven through the system of lymph vessels into the venous system by
means of lymph-hearts. The anterior pair is located beneath the supra-scapulae. The
posterior pair is near the urostyle. The lymph hearts open into veins so that the lymph
fluid can be again mixed with the general circulation.
f. Endoskeleton- The axial skeleton of frog includes the skull, vertebral column, ribs
and sternum while the appendicular skeleton consists of the pectoral and pelvic
girdles along with bones
of the forelimbs and the
hind limbs (Figure 11).
The skulls of many early
tetrapods
retained
certain
features of their piscine ancestors
but those of modern amphibians
show considerable deviation.
There has been a reduction in the
number of bones as well as a
general flattening of the skull.
The auditory capsule bears a
ventral opening, the fenestra
ovalis, into which fits the bony / cartilaginous stapedial plate of the columella, derived
from the hyomandibular cartilage. The columella has developed in connection with
the evolution of the sense of hearing and with the change from the hyostylic to the
autostylic method of jaw suspension in which the hyomandibular loses its significance
as a suspensorium. The chondrocranium persists to a considerable extent in
amphibians, but some of it has been replaced by cartilage bones. Basioccipital and
supraoccipital regions are not ossified. The atlas is articulated with the skull by a pair
of occipital condyles, projections of the exoccipitals. Basisphenoid and presphenoids
also are not ossified. Prootics, and sometimes the opisthotics are ossified and fused to
the exoccipitals. Membrane bones form the greater part of the roof of the skull. They
are no longer closely related to the integument and occupy a deeper position in the
head than in the fishes. A large membrane bone, the parasphenoid, covers much of the
ventral part of the chondrocranium. The quadrate in amphibians is fused to the
auditory region of the skull. Palatine and pterygoid membrane bones, which form
about the anterior part of the palatoquadrate, are well developed. The outer arch of
membrane bones is represented by pre-maxillaries and maxillaries. Anurans also have
a quadratojugal. The lower jaw consists of a core of Meckel’s cartilage surrounded by
membrane bones. The remaining part of the visceral skeleton is reduced in
comparison with fishes. The main features of a frog’s skull are as follows:
33
Skull of frog (Figure 12)
1. Skull is triangular, dorso-ventrally flattened and broad.
2. It is dicondylic, with two occipital condyles that articulate with the atlas vertebra.
3. Occipital region is greatly reduced.
4. The cranium is small and narrow.
5. The skull is platybasic lacking an inter-orbital septum and the cranium extends
beyond orbits.
6. The fronto-parietal is present on the roof of the cranium.
7. The nasals are large triangular bones covering the olfactory capsules.
8. The sphenethmoid extends forward into the region of olfactory capsule and is partly
covered by the fronto-parietal and nasals above and the parasphenoid below.
9. Parasphenoid is a dagger-shaped bone forming the floor of the cranium.
10. Vomers lie beneath the nasals and bear vomerine teeth.
11. Upper jaw consists of premaxillae, maxillae and quadratojugals.
12. Lower jaw consists of dentaries and angulosplenials.
13. The suspensorium is autostylic, with the lower jaw attached to the skull through rodlike quadrate cartilage.
14. Basisphenoids, alisphenoids, presphenoids and supra-and basioccipitals are absent.
15. Prootic bones are present on the sides of the exoccipitals.
16. Squamosals are T-shaped bones present on the dorsal side.
17. Pterygoids lie opposite to the squamosals on the ventral side.
18. Palatines are rod-shaped bones present on the ventral side with one end touching the
maxilla and the other in contact with the sphenethmoid.
34
Bones seen on the dorsal surface:
Premaxillae, maxillae, quadratojugals, squamosals, septomaxillaries, nasals, fronto-parietals,
prootics, exoccipitals and occipital condyles
Bones seen on the ventral surface:
Premaxillae, maxillae, quadratojugals, vomers, palatines, sphenethmoids, parasphenoid,
pterygoids and exoccipitals
The skull of urodela differs from that of the frog in many ways: The trabeculae do not meet
either below the brain to form a basis cranii or above it to form a cranial roof. There is,
above, a huge superior cranial fontanelle, and below an equally large basicranial fontanelle.
The former is covered, in the complete skull, by the parietals and frontals, the latter by the
parasphenoid. The parietals and frontals are separate. The parasphenoid is not T-shaped. A
single bone, the vomeropalatine bearing teeth, represents the palatine and vomer. The hyoid
arch is large and its dorsal end may be separated as a hyomandibular. There are three or four
branchial arches. The stapes has no extra-columella and no tympanic cavity or membrane. In
some anuran species, there is the presence of small supra and basi-occipitals. In others,
investing bones of the roof are very strongly developed. In the apoda, the investing bones are
very large and form a substantial structure.
The vertebral column is divisible into: the cervical region, an abdominal / thoracolumbar
region, a sacral region and the caudal region. The total number of vertebrae in urodeles and
apoda may be as much as 250, while there are only 9 vertebrae and a single rod shaped
caudal bone called the urostyle in anurans. In the lower urodela the centra are biconcave as in
fishes. However, the neural arches are much better developed than in any fish and have well
developed zygapophyses. The apoda also have biconcave vertebrae but in the higher
urodeles, the anterior surface of the centrum has a convexity while the posterior surface
retains its concavity, thus forming a ball and socket joint and the condition is known as
opisthocoelous. In the anura the condition gets reversed with respect to the anterior and
posterior surfaces and this condition is called as procoelous. A frog’s vertebral column
includes the following bones:
Vertebrae of frog (Figure 13)
Atlas vertebra
1. The first vertebra is called the atlas.
2. It is small and ring-like in form.
3. Centrum
reduced.
and
neural
spine
4. Transverse
processes
prezygapophysis are absent.
are
and
5. The neural arch is large.
6. The anterior face of centrum
possesses a pair of concave facets for
the articulation with the occipital
condyles of the skull.
7. The posterior margin of the neural
arch
bears
a
pair
of
postzygapophyses.
35
Typical second vertebra
1. In frog the 2nd vertebra is typical in structure.
2. The centrum is procoelous (concave on the anterior face and convex on the
posterior face).
3. It is ring-like having a large hole called the neural canal.
4. The solid arch on the dorsal side of the ring is called the neural arch.
5. The neural arch bears a small, mid-dorsal neural spine, which is directed
backwards.
6. Transverse processes are broad and wing-like.
7. A pair of small upwardly and inwardly directed articular facets called the
prezygapophyses is present on the anterior margin of the neural arch.
8. A pair of small downwardly and outwardly directed postzygapophyses is present
on the posterior margin of the neural arch.
Third and Fourth vertebrae
These vertebrae resemble the typical vertebra in structure except slight variations.
1.
The transverse processes are stout and elongated.
Fifth, Sixth and Seventh vertebrae
These vertebrae also resemble the typical vertebra in structure except slight difference.
1. The transverse processes are pointed.
Eighth vertebra
1. The centrum is amphicoelous (biconcave on both the sides).
2. The anterior concavity receives the posterior convexity of seventh vertebra.
3. The posterior concavity receives the anterior convexity of ninth vertebra.
4. Transverse processes are pointed and upwardly directed.
5. Prezygapophyses and postzygapophyses are present on the anterior and
posterior margins respectively.
Ninth vertebra
1. Ninth vertebra is also known as sacral vertebra.
2. The centrum is biconvex, i.e., convex on both the sides (bearing one convexity
anteriorly and two convexities posteriorly).
3. The anterior convexity fits into the posterior concavity of the eighth vertebra.
4. The posterior convexities fit into the anterior concavities of urostyle.
5. Transverse processes are cylindrical, stout and backwardly directed.
6. An iliac facet is present at the tip of each transverse process for the articulation of
ilium bone of pelvic girdle.
7. Neural spine is inconspicuous, i.e., greatly reduced.
8. Prezygapophyses are well developed along the anterior end of neural arch, while
the postzygapophyses are entirely absent.
36
Urostyle
1. Urostyle is the Xth vertebra representing the caudal region in the frog.
2. It is long and triangular with a pointed apex directed backwards.
3. Centrum is long, rod-like with a broad anterior end bearing two concavities to
receive the convexities of IXth vertebra.
4. Dorsally it is raised into a vertical ridge gradually tapering posteriorly.
5. Anteriorly the vertical ridge contains a short, narrow canal for spinal cord.
6. Transverse processes, pre-and postzygapophyses are entirely absent.
The pectoral girdle has basically the same structure, as found in the other pentadactyl
craniates. The scapula is ossified, and is connected by its dorsal edge with a suprascapula,
formed partly of bone and partly of cartilage. The coracoid is also ossified, but the
precoracoid is cartilaginous and has an investing bone called clavicle associated with it. The
cartilaginous epicoracoid connects the coracoid and precoracoid ventrally. The rounded head
of the humerus fits into the glenoid cavity. Passing forwards from the anterior ends of the
united epicoracoids, is a bony structure, the omosternum, which expands into a cartilage plate
called the episternum; and passing backwards from the epicoracoids is another bony rod, the
mesosternum, which is also associated with a cartilage structure known as the xiphisternum.
This is the first indication of a sternum in the terrestrial vertebrates; however, the sternal
apparatus of the amphibians differs developmentally from the sternum of higher vertebrates.
Pectoral girdle and sternum of frog (Figure 14)
1. Pectoral girdle is present in the
thoracic region (shoulder region)
and provides attachment to the
fore-limbs and their muscles.
2. It protects the inner softer parts of
the thorax.
3. It consists of two similar halves
united mid-ventrally and separated
dorsally.
4. Each half is divided into a dorsal
scapular portion and a ventral
coracoid portion.
5. The scapular portion comprises the supra-scapula and scapula.
6. Supra-scapula is a thin cartilaginous plate on the dorsal side.
7. Scapula is a bony plate having a glenoid cavity into which articulates the head of
humerus.
8. The coracoid portion comprises the clavicle, coracoid, precoracoid and epicoracoid.
9. Clavicle and coracoid meet mid-ventrally with the sternum and their counterparts of other
side by a strip of cartilage-the epicoracoid.
37
10. The sternum lies in the mid-ventral line. It consists of episternum, omosternum, and
xiphisternum.
11. The episternum is a flat, almost circular plate of cartilage.
12. The omosternum is a bony rod connected to the episternum on the anterior side and
clavicle on the posterior side.
13. The mesosternum is a cartilaginous rod lying opposite the omosternum.
14. The xiphisternum is the terminal broad cartilaginous plate lying at the tip of the
mesosternum.
The forelimbs show the fusion of the radius and ulna into a single radius-ulna. There are only
four complete digits with a vestigial one, the prepollex. Only six carpals are present.
Forelimb bones of frog (Figure 15)
Humerus
1. It is the bone of fore-limb and is the
component of upper-arm.
2. It is a short, stout and cylindrical
bone with a slightly curved shaft.
3. Its proximal end is known as the
head which fits into the glenoid
cavity of pectoral girdle.
4. The head is covered with calcified
cartilage.
5. The ridge below the head is known
as deltoid ridge.
6. The distal end forms a rounded trochlea with a condylar ridge on either side.
7. The trochlea articulates with the groove of radius-ulna.
Radius-ulna
1. It is a compound bone of fore-limb and is the component of fore-arm.
2. It is formed by the fusion of radius and ulna bones.
3. Its proximal end has a concavity to receive the trochlea of humerus.
4. The ulna projects into an olecranon process.
5. The distal portion of radius-ulna is somewhat flat having a groove.
6. Distal portion has an articular surface for the metacarpals.
Carpus-metacarpus and digits
1. The bones of the wrist are called carpals.
2. The carpal bones are six in number and arranged in two rows of three each.
3. The bones of the proximal rows are called ulnare, intermedium and radiale. These
bones articulate with the radius-ulna.
38
4. The bones of distal row are called capitato-hematum, trapezoid and trapezium. These
bones articulate with the metacarpals.
5. The hand is provided with five slender metacarpals. The first metacarpal is
rudimentary.
6. The digit corresponding to the thumb is absent.
7. The remaining four metacarpals are supported by phalanges.
8. The second digit bears 2 phalanges.
9. The third and fourth digits bear 3 phalanges each.
The pelvic girdle is peculiarly modified. The girdle has two long, curved bars articulating in
front with the transverse processes of the sacral vertebra and uniting posteriorly in an
irregular vertical disc of mingled bone and cartilage. The disc bears on each side a deep
acetabulum into which fits the head of the femur. The ilia together form a long jointed lever
especially adapted for jumping. The frog lacks a stabilizing tail, and its center of gravity is
located just behind the sacrum.
Pelvic girdle of frog (Figure 16)
1. Pelvic girdle lies in the posterior
region of the trunk.
2. It gives support to the hind-limbs.
3. It is V-shaped and composed of two
similar halves each of which is
known as an os-innominatum.
4. Each os-innominatum is composed
of three bones called the ilium, pubis
and ischium.
5. Ilium is greatly elongated and forms the major part of each os-innominatum. It runs
forwards to meet the transverse processes of the ninth vertebra.
6. It bears a prominent vertical ridge called the iliac crest on its dorsal surface.
7. Pubis is much reduced. It is a triangular piece of calcified cartilage.
8. Ischium is a larger and slightly oval bone.
9. The disc formed by the union of the three bones contains a cup-shaped cavity called the
acetabulum.
10. The head of the femur fits into the acetabulum.
In the hind limb, the tibia and fibula are fused to form a single tibia-fibula. The two bones in
the proximal row of the tarsus, namely the tibiale or astragalus and the fibulare or calcaneum,
are greatly elongated and provide the leg with an extra segment. There are three tarsals in the
distal row. There are five well-developed digits. On the tibial side of the first there is an
additional spur-like structure or calcar. This extra digit is known as the prehallux.
39
Hind-limbs bones of frog (Figure 17)
Femur
1. Femur is the bone of thigh region of
hind-limb.
2. It is long and slender having a slightly
curved shaft.
3. The proximal swollen end is called
the head.
4. Head fits into the acetabulum of
pelvic girdle.
5. The distal end forms a condyle which
articulates with the tibia-fibula.
6. The head and condyle are covered by
calcified cartilage.
Tibia-fibula
1. Tibia-fibula is a compound bone of
the shank region of hind limb.
2. It is formed by the fusion of tibia and
fibula bones forming a single bone
called the tibia-fibula.
3. The proximal and distal ends are covered by cartilage.
4. Near the proximal end tibia bears a cnemial or tibial crest.
5. The proximal end articulates with the astragalus-calcaneum.
Astragalus-calcaneum
1. Astragalus-calcaneum is a compound bone of ankle of hind-limb.
2. The ankle consists of two rows of four bones. The first or proximal row consists of
two long bones fused together at their proximal and distal ends with a wide gap in the
middle.
3. The inner bone is thinner and slightly curved called the astragalus or tibiale.
4. The outer bone is thicker and straight called the calcaneum or fibulare.
5. The proximal and distal ends are covered by epiphyses of calcified cartilage.
Metatarsals and digits
1.
2.
3.
4.
The foot of frog is supported by five metatarsals bearing five true toes.
The metatarsals are long and slender bones.
The first, second, third and fourth metatarsals bear 3 phalanges each.
A small preaxial sixth toe composed of 2 or 3 bones is present on the inner side of the
first toe.
5. The sixth toe is called the prehallux.
Modifications of the appendicular skeleton in different amphibians are as follows: The
shoulder girdle of urodela has unossified coracoids of great size. The precoracoid is also
large, and there is no clavicle. The sternum is a rhomboid cartilage plate and there is no
omosternum. The pectoral girdle of anurans shows several modifications. In many frogs the
40
two halves are firmly united in the mid-line and are closely related to the sternum (the
firmisternal condition). In toads and some frogs the two halves overlap in the middle (the
arciferal condition). The overlapping of the coracoids is sometimes correlated with the
absence of an omosternum as in urodela. In the pelvic girdle of the urodela the combined
ischiatic and pelvic regions are united into elongated cartilaginous plates on each side, which
give rise to the rod-like ilia. In many urodela and some anurans there occurs a cartilage rod
called the epipubis, which is attached to the anterior border of the pubic region. The limbs of
urodela differ from that of anurans. There are usually four digits in the forelimb and five in
the hind limb. In Anura, the limbs are modified by the fusion of the radius and ulna and of the
tibia and fibula, and by the great elongation of the two proximal tarsals. A prehallux is
usually present.
g. Nervous system and sensory organs- Brain: Frog’s brain includes three parts: fore,
mid and hind brain (Figure 18). The fore brain consists of a pair of olfactory lobes, a
pair of cerebral hemispheres and a diencephalon. The olfactory lobes located at the
anterior most part of the brain are large-sized and fused at the median line. The
olfactory fibers from the nasal sac pass into the cerebral hemispheres through the
olfactory lobes. A pair of cerebral hemispheres is located behind the olfactory lobes.
These large elongated structures are separated from each other by a median
longitudinal groove. Floor and lateral walls of the two hemispheres are thickened to
form the corpora striata. Two commissures, the anterior commissure and the
hippocampal commissure join the hemispheres. Each cerebral hemisphere encloses a
cavity called the lateral ventricle and the lateral ventricles communicate with the
olfactory ventricles in front and the third ventricle below via the foramen of Monroe.
The cerebral hemispheres receive tactile and optic impulses from the skin receptors
and the eyes. The diencephalon or the thalamencephalon is placed behind the cerebral
hemispheres and prior to the mid brain. It encloses the third ventricle and has a non
nervous roof that forms a thin vascular covering called the anterior choroid plexus.
Behind the choroid plexus is a small hollow outgrowth called the pineal stalk, which
during the larval stage bears a pineal body. The floor of the third ventricle is called
the hypothalamus. The optic chiasma is found ventral to the hypothalamus. A hollow
median bilobed process called the infundibulum is found just behind the optic
chiasma and bears a flat oval body called the hypophysis. The infundibulum together
with the hypophysis constitutes the pituitary body. Diencephalon receives a number of
afferent optic fibers from the
eyes and some from the skin
receptors.
The mid brain consists of a pair of
large optic lobes and a thick crura
cerebri. The optic lobes are a pair of
oval bodies on the dorsal side of the
brain and have a pair of cavities
called the optic ventricles, which
open into a narrow passage called
the iter or aqueduct of Sylvius. The
optic lobes receive fibers of optic,
olfactory and auditory nerves. The
crura cerebri are a pair of thick
bands of nerve fibers extending
antero-posteriorly below the optic
lobes.
These
connect
the
41
diencephalon with the medulla oblongata.
The hind brain consists of the cerebellum and the medulla oblongata. Cerebellum is a
narrow band-like structure on the dorsal side of the brain just behind the optic lobes and
encloses the cerebellar ventricle. Cerebellum is less developed in frogs and not
differentiated into lobes. Medulla oblongata is the posterior most part of the brain that is
thick at its anterior end but tapers posteriorly into the spinal cord. It encloses the fourth
ventricle that communicates with the iter in front and with the central canal of the spinal
cord behind.
The brain is enclosed in two meninges, the inner thin and highly vascular pia-arachnoid
membrane and the outer thick and tough dura mater. Between the two meninges is the
subdural space filled with cerebrospinal fluid. Between the dura mater and the bony wall
of the cranial cavity lies another space called the epidural space, also filled with
cerebrospinal fluid. There are ten pairs of cranial nerves in anamniotes; a detailed chart of
the nerves is given below:
Table 7: Cranial nerves of frog
S. No.
Roman Number
Nomenclature
Nature
Origin / Supply
1
I
Olfactory
Sensory
Olfactory epithelium
2
II
Optic
Sensory
Eye
3
III
Oculomotor
Motor
Four of the six eye muscles
4
IV
Trochlear
Motor
Superior
muscle
5
V
Trigeminal
Mixed
Head and jaw muscles
6
VI
Abducens
Motor
Posterior
muscles
7
VII
Facial
Mixed
Muscles of the face
8
VIII
Auditory/
Acoustic
Sensory
Internal ear
9
IX
Glossopharyngeal
Mixed
Tongue and pharynx
10
X
Vagus
Mixed
Viscera
oblique
rectus
eye
eye
Sensory Organs:
The Olfactory organs show modifications correlated with the terrestrial life. Each olfactory
chamber has an external nostril and an internal nostril, the latter opens into the mouth. These
two openings are separated by a nasal septum. The olfactory passage in amphibians is short.
In aquatic forms the passage is lined with folds. Olfactory sense cells are found in the
depressions between these folds, and ciliated epithelium covers the ridges. The olfactory
epithelium in terrestrial forms is located in the upper medial part of the nasal passages. It is
42
not folded to any extent. In some forms, a shelf-like fold from the lateral wall foreshadows
the appearance of the conchae or the turbinal folds, which become highly developed in more
advanced vertebrates. Glandular areas in the nasal passages keep the olfactory epithelium
moist. In addition, a new olfactory structure first appears in amphibians and is known as the
Jacobson’s / vomeronasal organ, which communicates with the olfactory chamber and the
buccal cavity. It arises as a ventro-medial or ventro-lateral evagination of the nasal passage
and is believed to be used in testing food substances held in the mouth. It is supplied with
branches of the terminal, olfactory and trigeminal cranial nerves.
Gustatoreceptors: Taste buds of amphibians are found on the roof of the mouth, tongue and
on the lining of the jaws. In frogs, they lie on the free surfaces of the fungiform papillae. The
more numerous filiform papillae do not have taste buds associated with them.
Lateral line sense organs occur in the larval stage and these are receptive to water vibrations.
Branches of facial, glossopharyngeal and vagus nerves supply these. These sensory structures
however disappear during metamorphosis.
The Visual sensory organs: Among amphibians, the anuran eyes are best developed and are
lodged in the orbital fossae on either side of the head. The upper eyelid is large and
immobile, while the lower eyelid forms a protective nictitating membrane, which covers the
eye whenever the animal is in water (Figure 19). The movement of the nictitating membrane
is regulated by the retractor bulbi and the levator bulbi muscles. The eyeball consists of three
concentric layers: the outermost fibrous tunic, middle vascular uvea and the innermost
nervous retina. The large posterior part of the fibrous layer is called the sclerotic while the
small anterior transparent part in front is the cornea. The curved surface of the cornea helps
the lens in focusing the light rays. Uvea is made of loose connective tissue having blood
capillaries and pigment cells. Its part lining the sclerotic is called the choroid. At the junction
of the sclerotic and cornea is the ciliary body but ciliary muscles are absent. The uvea in front
separates from the sclerotic to form the pupil, which is perforated in the middle by an
aperture called the pupil and has sphincter and dilator muscles to regulate the amount of light
entering in the eye by changing the size of the pupil. The iris divides the cavity of the eyeball
into a small anterior aqueous chamber and a large posterior vitreous chamber.
The innermost delicate part of the eye is called the retina. Its optic part consists of an outer
pigmented layer adjacent to the choroid and the inner nervous layer having receptor cells and
neurons. The inner sensory area has three regions: the outer light-sensitive cells, the middle
bipolar nerve cells and the inner ganglion cells. Rods and cones are found in the light
sensitive cells and contain the visual pigments, rhodopsin and iodopsin. An area called the
macula lutea or yellow spot is the small part of retina just opposite the center of pupil and is
the point of most acute vision. However, frog eye lacks the fovea centralis depression. The
lens is enclosed in a delicate lens capsule and lies just behind the iris.
The light rays entering the eyeball are focused on the retina by a combination of the
conjunctiva, cornea, aqueous humor, lens and vitreous humor. The inverted image formed on
the retina stimulates the receptor cells in the area centralis, which in turn generates nerve
impulses that are conveyed to the brain by the optic nerve. The eyes of a frog have limited
focusing power. The frog is short sighted on land but far sighted in water.
Auditory sense organs: The ears of frog located behind and below the eyes are organs of
hearing and equilibrium. Each ear consists of the middle and the inner ear. External ear is
absent. The middle ear is visible from outside and encloses an air filled cavity called the
tympanic membrane. It is limited internally by the auditory capsule and externally by the
43
tympanic membrane or tympanum. The cavity of the middle ear communicates with the
pharynx by a narrow passage called the Eustachian tube, which serves to conduct the external
sound waves into the ear and also keeps the air pressure inside the tympanic membrane equal
to that outside it. Externally, the cavity of the middle ear is limited by the tympanic
membrane, which is vibratile in nature. A club-shaped columella auris touches the center of
the tympanic membrane and extends across the tympanic cavity to a cartilaginous stapedial
plate, which is fused with an aperture in the auditory capsule called the fenestra ovalis. A
ring-like bone called the operculum is present in the fenestra ovalis and is covered by a
membrane attached to the scapula by a muscle.
44
The inner ear is enclosed inside a bony auditory capsule, the latter is filled with a watery fluid
called the perilymph in which floats the soft and compact inner ear / membranous labyrinth.
The latter consists of an irregular structure called the vestibule that is divided by a shallow
constriction into an upper large chamber called the utriculus and a lower small chamber
called the sacculus (Figure 19). Three small diverticula collectively known as the lagena are
given out by the sacculus. The lagena represents the coiled cochlear duct of the mammals.
The utriculus bears three semicircular canals, which are oriented at right angles to each other
thus assuming mutually perpendicular planes. At least one end of each semicircular canal is
always swollen into a small round structure called the ampulla. The hollow, membranous
labyrinth is filled with a fluid called endolymph and contains pieces of calcium carbonate in
the form of ear stones or otoliths. The wall of inner ear is lined by cubical epithelium that is
modified to form sensory spots at certain places. Each ampulla as well as sacculus, utriculus
and lagena possesses one sensory spot each. The sensory spots of the ampullae are called the
cristae while those of sacculus, utriculus and lagena are called the maculae. The macula of
utriculus is the large pars neglecta while the macula of the lagena is known as the basilar
papilla. Cristae and maculae consist of sensory cells associated with supporting cells. Each
sensory cell has a tapering hair-like process at its free end and a nerve fiber at its lower end.
Sound waves first strike the tympanum setting it into vibrations. These vibrations are
conveyed to the columella auris and the stapedial plate. The perilymph conducts the
vibrations to the endolymph, which in turn brings about disturbance of the sensory hair of the
maculae in the sacculus and lagena. The influenced auditory nerves convert these vibrations
into nerve impulses that are transmitted to the brain. As far as equilibrium is concerned, the
semicircular ducts along with utriculus help in maintaining the correct body posture.
Whenever the head is tilted, it alters the stress transmitted by the otoliths to the sensory hair.
This pressure change works as a stimulus for the auditory cells. The sensory cells in turn
generate a nerve impulse, which is carried to the brain by the auditory nerve. The disturbance
of equilibrium in any direction is detected by the cristae of the semi-circular canals that are
arranged in three different planes.
Modifications of the nervous system and the sensory organs among the various amphibian
groups are as follows: The urodele brain is more elongated and slender, with small optic
lobes and having a non-union of olfactory bulbs. The Jacobson’s organs are usually found;
the olfactory sacs open into the mouth by external nostrils located behind the vomers. The
eye has modifications for allowing long-sightedness in the terrestrial forms, while the aquatic
forms still retain the lachrymal ducts. Eyelids are vital in the terrestrial forms, but are absent
in a few primitive forms. Urodeles, Apoda and some Anura have no tympanic cavity and
tympanic membrane; one or both the middle ear ossicles may also be absent. In some
urodeles and anurans, the highly reduced stapes is associated with a cartilage inserted in the
fenestra ovalis and called the operculum. Possibly, vibrations from the forelimbs are
conveyed to the operculum and the passed on to the inner ear. Lateral line organs are retained
in the perennibranchiate urodeles and in the larvae of the terrestrial forms. Groups of
neuromast organs are found in the aquatic anurans that possibly correspond with those of
fishes.
h. Urinogenital system and Osmoregulation- The primitive archinephric type of
kidney found in the larval stage of the hagfish (cyclostomes) also occurs in the larval
caecilians in which there is a distinct metameric arrangement of kidney tubules, renal
corpuscles, and nephrostomes. In the adult apoda, the opisthonephros extends the
greater part of the length of the coelom and is lobulated. Although a small head
45
kidney with peritoneal connections is present in many larval amphibians, it does not
persist in the adult stage.
Urodeles have opisthonephric kidneys much like those of elasmobranchs. The kidney
consists of two regions: an anterior portion, which in males is concerned more with
genital than urinary functions, and a posterior expanded urinary region, which makes
up the main part of the opisthonephros. Numerous collecting ducts or tubules leave
the opisthonephros at intervals to join the persisting archinephric duct. The latter, in
both sexes open on either side of the cloaca at the apex of a small papilla. In Necturus
peritoneal connections with some of the kidney tubules persist throughout life.
The opisthonephric kidneys of anurans show a more posterior concentration of
tubules and are confined to the posterior part of the abdominal cavity. The kidneys are
flat, oval, dark-red organs, in the posterior region of the coelom. They are dorsally
located, retroperitoneal, and flattened in a dorso-ventral direction. There is no clearcut distinction between the anterior and posterior ends, as in urodeles. An adrenal
gland of a yellowish-orange color is located on the ventral side of the kidney. Blood
comes to the kidney from two entirely different sources. The adult frogs have ciliated
nephrostomes on the ventral surfaces of the kidneys. They are usually not connected
with kidney tubules but have become secondarily connected with the renal veins. The
kidneys of female anurans have no relation to the reproductive system, but in males
an intimate connection exists. Certain anterior kidney tubules have become modified
as efferent ductules connecting the testis with the kidney and archinephric duct, the
latter serves to transport spermatozoa as well as urinary wastes. Unlike the condition
in urodeles, the archinephric ducts are located within the kidney along its lateral
margin. They leave the opisthonephros near the posterior near the posterior end and
pass to the cloaca. The structure of a typical renal tubule of an amphibian kidney is
shown in Figure 20. A thin-walled urinary bladder connects with the amphibian
cloaca a short distance beyond the openings of the archinephric ducts. It is bilobed.
There is no direct connection of the ducts with the bladder, so that the urine first
passes into the cloaca.
46
In spite of several terrestrial adaptations incorporated into the amphibian body, the
kidneys of extant forms retain fish-like characters. However, when on land, the loss of
water is retarded by the reduction of glomerular filtration by a hormone from the pars
neuralis, which constricts glomerular arterioles. Water conservation is also brought
about by the passive absorption of water and salts through the integument. Certain
47
desert-dwelling frogs like Cyclorana, Notaden, and Heleioporus can survive severe
droughts by aestivating in dry surface mud and can absorb water very rapidly.
Furthermore, their kidneys have venous sinuses into which drain peritoneal funnels.
During aestivation, water stored in the peritoneal cavity is drawn through these
channels into the general circulation. It is also known that in Cyclorana the glomeruli
are reduced in size and vascularity.
Male reproductive tract: The shape of amphibian testes shows a correlation with the
body shape. In caecilians, these are elongated structures, resembling a string of beads.
Each swelling consists of masses of seminiferous ampullae, all of which are
connected by a longitudinal collecting duct. In urodeles, the testes are shorter and
irregular in outline. In anurans, these are compact, oval or rounded structures (Figure
21 b). A marked difference in size is visible during the breeding and non breeding
season. Fat bodies associated with the testes, show a fluctuating size correlated with
the seasons.
The relationship of the reproductive and excretory systems in male amphibians is
closer than in most fishes. Efferent ductules usually join a longitudinal canal inside
the testis or along its medial border. The efferent ductules move through the
mesorchium, enter the anterior part of the opisthonephros on its medial side, and may
connect directly to the archinephric duct or join certain kidney tubules, which in turn
connect to the archinephric duct. In urodeles, efferent ductules join a narrow
longitudinal Bidder’s canal, which moves within the mesorchium, but outside the
medial edge of the kidney. Bidder’s canal connects by a number of short ducts to
48
kidney tubules in the narrow anterior part of the opisthonephros. Certain kidney
tubules emerge from the lateral edge of the opisthonephros and join the archinephric
duct that moves posteriorly. The anterior part of the archinephric duct is concerned
mainly with the transport of spermatozoa, but the posterior part serves for the
elimination of urinary wastes also. The archinephric ducts enter the cloaca
independently.
Conditions in anurans are same as those found in urodeles, but with some variations.
Efferent ductules enter the anterior end of the opisthonephros along its medial edge.
In some forms they connect directly to the archinephric duct, but in others join the
Bidder’s canal, which lies within the opisthonephros close to its anterior border.
Spermatozoa are then transported from the Bidder’s canal through kidney tubules to
the archinephric duct, which also courses within the opisthonephros but along its
lateral border. (A Bidder’s canal of unknown function is also found in the female
kidney). The archinephric duct emerges from the kidney near its posterior end and
passes to the cloaca. In males of several species, a dilatation of the archinephric duct
near the cloaca forms a seminal vesicle in which spermatozoa may be stored
temporarily. Copulatory organs are absent in urodeles and anurans. In some
caecilians, the muscular cloaca is protrusible and serves as a type of intromittent
organ when the cloacae are in apposition.
Female reproductive tract: Amphibian ovaries are saccular structures, with the shape
varying with the body shape. They are long and narrow in caecilians; elongated in
urodeles; shortened and more compact in anurans (Figure 21 a). The cavity within
each ovary becomes lymphoid in character and the ova escape into the coelom
through the external walls of the ovaries. Fat bodies are closely associated with
amphibian ovaries, serving for the storage of nutriment. A peculiar structure in the
male toad, known as the Bidder’s organ, may under certain conditions develop into a
true ovary. Oviducts in amphibians have the same structural pattern throughout the
class. These are paired elongated tubes with ostia located well forward in the body
cavity. Posteriorly, each Mullerian duct is slightly enlarged to form a short uterus,
which mostly opens independently into the cloaca. In certain toads, the oviducts unite
before entering the cloaca by a common orifice. The uteri in most amphibians serve as
temporary storage places for ova. The oviducts have a glandular lining and prior to
the breeding season these become greatly enlarged and coiled and secrete a clear
gelatinous substance. As the eggs pass down the oviducts, several layers of this jellylike material are deposited about each ovum. This swells when the egg enters the
water.
External fertilization takes place in most anura. The male grasps the female in a
process called amplexus, and as the eggs emerge from the cloaca, spermatozoa are
shed over them. No copulatory organs are present. However, each surviving
amphibian group also contains ovoviviparous individuals in which eggs are retained
and development proceeds partly in the oviduct. For example, Nectophyrnoides
produces well-yolked eggs but larvae are retained in the oviduct till metamorphosis is
completed.
Internal fertilization occurs in most urodeles, but no copulatory organs are present.
Males deposit spermatophores, which are small packets of spermatozoa held together
by secretions of the cloacal glands. A complex mating ritual performed by the male
stimulates the female, the latter picks up the packet of sperms by muscular
movements of the cloacal lips. The spermatheca, which is a dorsal diverticulum of the
cloaca serves as the receptacle for the spermatozoa, which are available for fertilizing
49
the ova as they pass down the oviducts to the cloaca. Fertilization is external in
Cryptobranchus, Asiatic land salamanders, and also in the family Sirenidae. A few
salamanders are ovoviviparous. For example, larvae of Salamandra remain in the
parent tract to derive nutrition after depletion of egg yolk. They also possess long
plume like external gills during their oviducal existence, which are shed before birth.
Internal fertilization occurs in most caecilians. The eversible cloaca of the male is
considered to serve as the copulatory organ. Caecilians are oviparous or
ovoviviparous, with some forms retaining the developing embryos in the oviduct and
feeding on its lining.
Salamanders show many fascinating examples of physiological adaptations. Although
most salamanders undergo complete metamorphosis, but some of them also undergo
the process of incomplete metamorphosis referred to as paedogenesis. A classic
example of paedogenesis is shown by the Mexican Axolotl, which often breeds in the
larval state (neotenous forms). On the other hand, the experimental administration of
thyroxin will cause the larva to loose its gills, develop lungs and emerge from the
water in an adult-like form. Reducing the water level in which the larva lives, thus
making gill respiration difficult and facilitating respiration by lungs, can also induce
metamorphosis. Yet another case of paedogenesis is seen in the Alpine newt, in which
complete metamorphosis occurs in the habitats of French and Italian lowlands,
whereas the race that inhabits the colder Lombardy lakes is often neotenous.
However, among these perennially larval forms no known experimental manipulation
will induce metamorphosis.
V. PARENTAL CARE IN AMPHIBIA
Several remarkable instances of parental care are known among the amphibians (Figure 22).
A number of different species of frogs and toads construct nests or shelters of leaves or other
materials in which the eggs are deposited and the young are developed.
Parental care falls under two heads, which may be found combined in some forms: Firstly
protection by the parents, either by means of nests or nurseries, or by direct nursing;
Secondly, by shortening of the metamorphosis period.
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(i) Order Anura
a. Protection by nests and nurseries
A. In enclosures in the water- Brazilian frog, Hyla faber, protects its progeny by building a
basin-shaped nursery in the shallow water on the border of a pond (Figure 22 A). The female
scoops mud to a depth of 3-4 inches and with the material thus removed, a circular wall is
built that emerges above the water surface. The inside wall is smoothened by webbed,
flattened hands while bottom is leveled by belly and feet. The eggs and larvae are thus
protected from attack of many insects and fishes at least for some time, thereafter, heavy
rains destroy the wall and the larvae directly go to the water.
B. In holes near water- A still better mode of protecting the offspring during the early stages
of development has been adopted by the Japanese tree frog, Rhacophorus schlegelli. The
male and female together bury themselves in the damp earth on the edge of a ditch near a
flooded rice field, and make a hole or chamber, a few inches above the water level (Figure
22 B). The walls of this chamber are polished and during this process, the gallery by which
the frogs had entered into this chamber gets obliterated and the oviposition begins. The
female first produces a secretion from the cloaca, which is beaten into froth. The eggs are
deposited into the froth and are fertilized by the male. Thereafter, the parents make an exit
gallery towards the ditch. It runs obliquely downwards towards the water and is later on used
by the newly hatched larvae that come to the water to complete their development.
C. In nests on trees or on rocks overhanging the water- Some tree frogs like the South
American Phyllomedusa, the Indian Rhacophorus malabaricus and the African Chiromantis,
deposit their spawn on trees within nests of froth attached to one or many leaves stuck
together, and overhanging a pool. The larvae move about in the froth and after loosing their
external gills, fall in water to complete their metamorphosis
D. In transparent gelatinous bags in water- Phrynixalus biroi has large eggs that are enclosed
in a sausage shaped transparent common membrane, secreted by the female and this bag like
structure is left in the mountain streams. The entire development takes place within the eggs
and the young ones go out in perfect condition. No gills have been observed and the large tail
serves as the breathing organ of the larva.
E. On trees or on moss away from water- In several species of tropical American genus
Hylodes, the eggs are deposited in damp places under stones or on moss or plant leaves, and
are of large size. The metamorphosis is hurried up within the egg. There is plenty of yolk
within the egg and hence the entire development occurs there. The young frog leaps out as an
air breather with a vestige of a tail, which was fully developed and vascularized earlier and
had served as a respiratory organ. No gills have been observed in the larva.
b. Direct nursing by the parents
A. Tadpoles transported from one place to another by the male parents- Small South
American frogs Phyllobates and Dendrobates and Arthrolepis seychellensis carry well
developed tadpoles on their back (Figure 22 I). The young ones adhere by their sucker like
lips and flattened stomachs. They are thus carried from one place to the other effectively.
B. Eggs protected by the male parent who covers them with his body- The eggs of
Mantophryne robusta are strung together by an elastic gelatinous envelope. These are around
15-20 in number, and form a cluster over which the male sits, holding it with both hands. The
development takes place in this position. The larvae have no gills, but have a large tail, which
is vascular and respiratory.
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C. Eggs carried by the parentsI. Round the eggs by the male: In Alytes obstetricans, pairing and oviposition occur on the
land, and the eggs are deposited by the female in batches of 2-3, at short intervals. The male
then binds the eggs into a string like structure and wraps them around its legs for protection
(Figure 22 D).
II. On the back of the female:
(i) Exposed- In a Brazilian tree frog, Hyla goeldii, it is the female which takes charge of the
eggs, carrying them on her back (Figure 22 E).
(ii) In cell-like pouches- In Pipa Americana and Pipa dorsigera, the eggs are carried on the
back of the mother, the skin thickens and grows around the eggs until each is enclosed in a
dermal cell, which is finally covered by a lid, believed to be formed by a secretion from the
skin glands (Figure 22 G). The eggs are about 100 in number, they develop in these pouches
and the young leap out in a perfect condition. Pipa is aquatic and pairing occurs in water.
During egg laying, the cloaca projects as a bladder-like pouch directed forwards between the
back of the female and the breast of the male. It is by means of this ovipositor, that the eggs
get evenly distributed over the whole back of the female. However, the method by which
these eggs get fertilized is not very clear.
(iii) In a common pouch- In Nototrema, the entire brood is sheltered in a common pouch
(Figure 22 F), which develops only during the breeding season. How the eggs are introduced
into the pouch is still unknown, the opening of this pouch is small and located on the
posterior part of the back.
III. Exposed on the belly of the female: The female of Rhacophorus reticulatus carries its
eggs on the belly, which bears shallow impressions when the eggs are removed.
IV. In the mouth or gular pouch:
(i) By the male- A remarkable mode of nursing is shown by Rhinoderma darwini. It shelters
around 10-15 young ones in the gular pouch, which is a modified vocal sac, and development
is completed here (Figure 22 H).
(ii) By the female- The female of Hylambates breviceps carries the eggs in her mouth. These
eggs are large and few in number.
D. Viviparity- In East African toads, Pseudophryne vivipara and Nectophryne tornieri, larvae
are found in the uteri.
ii) Order Urodela
Urodeles show courtship of various types. During courtship, the male deposits the
spermatophores attached to the ground or on to the stones, and the female takes them up by
applying her cloaca on these spermatophores or else by pressing the spermatophores between
its legs. There are also certain forms as for example, Cryptobranchus, in which fertilization is
external. Thus, courtship may or may not involve copulation/amplexus:
1. No amplexus but a lengthy courtship occurs in water. Males have dorsal and caudal crests
and are more brightly colored than the females. Examples are Molge cristata and Molge
vulgaris.
2. Amplexus occurs; however no marked sexual differences of color are found, also neither
sex shows presence of dermal ornamental appendages. This type is further of two types:
52
a. Amplexus of a short duration occurs partly or entirely on land, as seen in Salamandra,
Plethodon and Autodax.
b. Amplexus of a more or less lengthy duration occurs in water, as found in Molge torosa and
Molge montana.
In some forms, the eggs are small and the larvae come out soon and no parental care is seen,
but in other examples, parental care is found to be as prominent as in anurans.
a. Protection by nests and nurseries
A. In holes on land or in trees- Autodax lays about 10-20 eggs in a dry hole in the ground or
in a hole on a tree, roughly up to 30 feet above ground. The mother or both the parents
remain in the nest during development to defend the brood and also to provide them with
moisture. The young ones remain in the nest for a considerable period with the parents.
B. In a transparent bag in water- Salamandrella keyserlingii deposits its eggs in a gelatinous
bag, which is attached at one end to an aquatic plant just below the water level. This bag is
more or less sausage shaped and contains approximately 50-60 eggs. The larvae remain
within the bag and hatch out at an advanced state of development.
b. Direct nursing by the parents
A. Female parent coils around the eggs- In Plethodon, the eggs are laid beneath stones, in
small clumps, and the mother coils its body around them. The larvae survive on large,
spherical mass of yolk and do not leave the gelatinous egg capsule until after the loss of the
gills. Thereafter, the larvae come out.
B. Male coils around the eggs- In Megalobatrachus maximus, it is the male parent that coils
around the eggs and protects them during the early stages of development.
C. Female parent carries the eggs on the back or around the legs- In Desmognathus fusca, the
eggs are laid in the form of rosary-like strings. The egg strings are then bound around the
body many times and the female parent nourishes them for some time.
D. Viviparity- Salamandra maculosa pairs on land, and several months later, the female goes
to the water and gives birth to 10-50 young ones of small size and similar to the newt larvae
with their fore limbs developed. In Salamandra atra, the young are retained in the uterus,
until the completion of metamorphosis.
(iii) Order Apoda
In Ichthyophis glutinosa, the female digs a hole close to the surface in damp ground, near the
water. It then deposits about a dozen large, yellow eggs, measuring 8-10 mm in diameter, and
coils its snake-like body around them (Figure 22 C). The mother thus, protects the eggs
against enemies and also against desiccation. Another genus of Apoda, known as Dermophis
thomensis is viviparous.
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15. http://www.zo.utexas.edu/research/salientia/salientia.html
16. http://tolweb.org/tree?group=terrestrial_vertebrates
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