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Chapter 8:Protista
Members of the kingdom Protista evolved from the Archaea approximately 1.5 billion
years ago. All protists are not likely to have evolved from a single common ancestor, so
they are considered to be a polyphyletic group. The group includes plant-like
(autotrophic) forms called algae, and animal-like (heterotrophic) forms.
Modern forms are highly evolved, but surface area to volume ratio constraints prevent
much differentiation in cell size or shape. Protozoans are bounded by a single plasma
membrane and maintain homeostasis within their unicells, but they use organelles to
carry out all the functions of more complex organisms:
Some protozoans have a pellicle composed of microtubules under the plasma
membrane. The outer portion of the cytoplasm is clear and firm and is referred to as
the ectoplasm, whereas the inner portion is granular and thinner in consistency and is
referred to as the endoplasm.
Contractile vacuoles function in excretion of water.
Some protozoans take in food via a cytopharynx, and digestion of food occurs in food
vacuoles. Following digestion, wastes may be ejected via the cytopyge.
Reproduction of protozoans may be asexual, via binary fission, budding, or multiple
fission, or sexual, via the formation of gametes. Many protozoans are symbiotic, taking
the life styles of parasites, commensals, or mutualists. Protozoologists study the >38,000
species of protozoans; they are loosely arranged into 7 “phyla.”
Members of Phylum Sarcomastigophora are the most speciose protozoans.
Sarcomastigophorans (18,000 species) move by flagella, pseudopodia or both, but the
groups are divided by the means of locomotion such that organisms classified in
subphylum Mastigophora have flagella. The Mastigophora is divided into two groups
based on feeding type:
Members of the class Phytomastigophora include the dinoflagellates, Euglena,
Volvox, and are usually autotrophs, but some like Euglena can be heterotrophic under
certain local conditions. Some dinoflagellates cause “blooms” or “red tides.”
Protozoans in class Zoomastigophora are heterotrophs and include many important
parasites: 3 species of Trypanosoma, vectored by tsetse flies, cause sleeping
sickness; Giardia lamblia causes giardiasis.
Members of the Subphylum Sarcodina are amoebae and move by extending various kinds
of pseudopodia distinguished by shape (lobopodia, filopodia, reticulopodia, or axopodia).
Amoeba is classified in superclass Rhizopoda, class Lobosea; amoebae may be naked or
shelled (have a test). Tests are calcareous, proteinaceous, siliceous, chitinous, or
composed of environmental materials. No sexual reproduction is known in this group.
Most amoebae are free-living; but some, like Entamoeba histolytica, are parasitic.
Members of subphylum Actinopoda are typically planktonic organisms enclosed in a test.
The group includes radiolarians with a glassy test, heliozoans (aquatic amoebae), and
foraminiferans, a primarily marine group with a calcareous test and a long fossil record.
Phylum Labrinthomorpha includes a small species that was responsible for the death of
the eel grass on the Atlantic coast in the last decade.
Members of phylum Apicomplexa are all parasites and share an apical complex used to
penetrate host cells. They have complex life cycles with both sexual and asexual phases.
Schizogony is the multiple fission of asexual stages, gametogony is the first stage of the
sexual cycle, and sporogony is the formation of sporocytes that infect a new host. The
most important species of this phylum are members of class Sporozoa:
With a mosquito as a vector, Plasmodium causes malaria and has been a significant
cause of death in humans for centuries. Other sporozoans cause coccidiosis,
toxoplasmosis, cryptosporidosis, and many diseases of livestock. Toxoplasma is
carried by cats and is spread by contact with feces and is a significant cause of
stillbirths and miscarriages. Toxoplasma and Cryptosporidium are particularly
dangerous for immunosupressed persons.
The phyla Microspora, Acetospora and Myxozoa are parasites with relatively few
species. The Myxozoa affect the nervous systems of trout and salmon. The Acetospora
are extra-cellular parasites of molluscs, and Microspora are important parasites of
silkworms and honeybees.
Phylum Ciliophora includes protozoans that move via cilia, have a cytostome (“mouth”),
and dimorphic nuclei. Many short cilia move in a coordinated wave, but they may also
join together to form cirri which aid in a directed movement. Trichocysts are used as
protection. Most ciliates are free-living; they feed on small particles or are predators. A
few ciliates are commensal, parasites, or mutualists (like those in the rumen of
ungulates). Ciliates have both a macronucleus, not involved in reproduction, and a
micronucleus that functions in sexual reproduction. Sexual reproduction occurs by
conjugation.
The protozoan lineage has been suggested to be 1.5 billion years old, but the fossil record
is limited to those with hard parts, typically foraminferans and radiolarians. The
choanoflagellate group lacks mitochondria, suggesting a very early origin.
Acoelomates/Flatworms
Lecture Outline
The three acoelomate phyla (Nemertea, Platyhelminthes, and Gastrotrichia) are bilateral
and triploblastic. The evolutionary relationships of the members of these groups are
controversial. The acoelomates may represent an evolutionary side branch—they have
primitive characters somewhat between the radial diploblasts and the triploblastic
coelomate plans. Evolution of these forms by paedomorphosis has been proposed, but
they might also represent degenerate forms of coelomate taxa. They have bilaterally
symmetry correlated with active forward locomotion.
Members of the phylum Platyhelminthes are known as the flatworms. Parenchyma is a
mesodermally derived tissue that fills the spaces between the gut, organs, and the outer
body wall. Platyhelminths have an organ system level of organization. The gut is
typically incomplete, and may be absent, as in tapeworms. Platyhelminths are mostly
monoecious with complex reproductive systems. There are four classes of flatworms.
Members of the Class Turbellaria are typically free-living, and are carnivores or
scavengers. Turbellarians are up to 60 cm long, move via muscular movement and cilia,
and are typically bottom dwellers. The main body features are detailed below:
The epidermis typically contains rhabdites (making mucus), and adhesive and
releaser glands for attachment and release from the substratum.
The digestive tract generally has one opening to the external environment. Some
have no pharynx nor digestive cavity, but most flatworms that have highly branched
cavities where increased surface area brings food in close proximity to all cells. A
circulatory system is not required in this case. Food digestion is partially
extracellular.
Respiration and loss of many wastes across the body wall occurs by diffusion.
Excretory organs (protonephridia) contain flame cells that move fluid into a system of
tubules and the excretory fluid ultimately exits via the nephridiopore. These organs
function in osmoregulation, so the invasion of freshwater has occurred in many
flatworm species.
The nervous system of turbellarians is a nerve net, or a ladder-like system of nerves.
Most turbellarians have ocelli (photoreceptors), and chemoreceptors to aid in locating
food.
Asexual reproduction by fission is common, and sexual reproduction typically occurs
by reciprocal exchange of sperm in these monoecious animals. The eggs hatch, then
development is usually direct, but a few have a Muller’s larva.
The members of class Monogenea are usually ectoparasites of fish; they attach to the host
via a posterior attachment organ called an opisthaptor. They have an external tegument.
These flukes have a single generation in their life cycle.
Most members of the very speciose class Trematoda are digenetic flukes.
Digenetic flukes parasitize a variety of hosts and have multiple stages (some asexually
produced) in the life cycle; there are typically two hosts for the parasite. The
intermediate host is typically a snail, and most use a vertebrate as the definitive host
(where a metacercaria develops into the adult parasite that is sexually mature). As
examples, the human liver fluke, Clonorchis sinensis, the sheep liver fluke, Fasciola
hepatica, and Schistosoma spp., the dioecious blood flukes, all have an enormous impact
on the human population.
Members of subclass Aspidogastrea are mostly endoparasites of molluscs but may have a
second vertebrate host. They have no oral sucker and the opisthaptor is highly subdivided.
Flukes are ovate to elongate and flattened. They have a syncytial tegument that allows
for transport of nutrients, wastes and gases across the body wall. The complex structure
of the syncytial tegument includes microvilli, a glycocalyx, and the majority of the cell
body internal to the basement membrane, connected to the outer zone by cytoplasmic
bridges.
The members of class Cestoidea are often called the ultimate parasites. Tapeworms are
endoparasites, typically living in the gastrointestinal tract of vertebrates. Tapeworms are
uniquely adapted to the endoparasitic lifestyle; they lack a mouth and gastrointestinal
tract and absorb nutrients directly across the body surface via the tegument.
Tapeworms in the small subclass Cestoidaria are endoparasites of fish in the
gastrointestinal tract; they possess some tapeworm features and other features similar to
those of digenetic trematodes.
The true tapeworms belong to the subclass Eucestoda. The anterior end (scolex) has
suckers and often hooks for attachment to the host gut. The narrowed portion of the neck
is posterior to the scolex. The neck is followed by the strobilae; a series of proglottids
that contain the both male and female reproductive organs (tapeworms are monoecious).
As example of the varied and complex life cycles of tapeworms, examine the life cycles
of Taeniarhynchus saginatus, the beef tapeworm, Taenia solium, the pig tapeworm, and
the broad fish tapeworm, Diphyllobothrium latum.
The members of phylum Nemertea are characterized by a ciliated epidermis and mucus
glands. The have a complete digestive tract, compartmentalized for food processing.
Nemerteans feed with a proboscis that may be everted from the rhynchocoel (an interior
cavity) for prey capture. Nemerteans have a closed circulatory system without a heart,
but with two lateral blood vessels that branch extensively. Blood does not circulate, but
flows back and forth. Protonephridia are present. Nemerteans are dioecious animals;
fertilization leads to the pilidium, a free-swimming larva.
Gastrotrichs are small (up to 4 mm) freshwater and marine organisms that live between
sediment particles. Gastrotrichs have ventral cilia for locomotion and a dorsal surface
covered with scales or bristles. They have a syncytial epidermis with adhesive glands, a
complete gut, and unique protonephridia. Most marine species are hermaphroditic and
reproduce sexually; most freshwater species are parthenogenetic.
Recent molecular evidence indicates that the traditional view of acoelomate phyla is in
question. The acoelomates do not seem to be closely related to the Platyhelminthes.
Many of the acoelomates are carnivores, seeming to be under selection pressure to
increase body size.
Pseudocoelomates/Aschelminthes
Lecture Outline
The aschelminths include 7 primarily worm-like phyla. The aschelminths may be
evolutionary related, as pseudocoelomates sharing a cuticle and a few other
characteristics; however, convergent evolution could account for these features, making
the group polyphyletic. Most are small, are bilaterally symmetrical, unsegmented,
triploblastic, and round in cross section. Most aschelminths have a complete digestive
tract with a sequence of digestive processes occurring throughout its length. They also
exhibit eutely (cell constancy), both within organisms in a species, and in organs of an
individual. Some workers divide the aschelminths into two groups, one characterized by
an extracellular cuticle, the other characterized by an intracellular cuticle. The 7 phyla
are listed below:
1. Rotifers. Rotifers are 0.1 to 3 mm in length, and are typically freshwater, but a few
are marine. They are usually solitary, free-swimming organisms. Rotifers may have a
lorica (or may be “naked”), and suspension feed via their ciliated corona. The mastax
grinds the food articles. There are some sexually reproducing species and these
commonly use hypodermic impregnation. However, rotifers are remarkable for their
parthenogenesis, and for using haplo-diploidy as part of their amazing reproductive
potential. Amictic eggs are those that are produced by mitosis; mictic eggs are
produced by meiosis, and if fertilized, over winter and hatch out as females; if not
fertilized, hatch as males.
2. Kinorhynchs. Kinorhynchs are small (< 1 mm) burrowing worms that feed on marine
benthic material; they are part of the marine meiofauna. They have a complete gut.
The body is divided into units called zonites. They have no cilia on the body surface,
instead there is a cuticle and an epidermis. The larvae molt the cuticle, but adults do
not. They are dioecious animals.
3. Nematodes. Nematodes are the most speciose of the aschelminths and are
exceedingly numerous in most habitats; they may be free-living as carnivores,
herbivores, omnivores, and saprobesm or they may be parasites. Nematodes are
elongate, round in cross-section, and tapered at both ends. They may reach several
meters in length. They have an acellular cuticle that is continuous with many of the
internal organs. The cuticle is underlain by an epidermis that surrounds the
pseudocoel (functions as hydrostatic skeleton). The cuticle protects the body, and in
parasitic species, resists digestion by the host; it may be smooth or have bristles or
spines. Juvenile nematodes typically go through 4 molts of the cuticle. Nematodes
have a complete gut, and some have glandular systems. Aquatic nematodes have
renette glands which function in excretion; parasitic nematodes have a tubular system
developed from the renettes. Sensory organs include amphids and phasmids; some
have ocelli. Nematodes are dioecious, and males are smaller than females.
Nematodes may be ovoviviparous or viviparous; gonads are located in the
pseudocoel.
Nematodes may be distinguished from other worms by their unsegmented bodies
having a complete digestive tract, and by possessing only longitudinal muscles that
results in their characteristic thrashing movement. They share some characteristics
with the arthropods, including the absence of cilia, except in sensory structures,
molting the cuticle, and possessing amoeboid sperm.
Many nematodes are parasites of humans: Ascaris lumbricoides may infect nearly 1
trillion people worldwide; the less harmful pinworm, Enterobius vermicularis, is the
most common roundworm parasite in the U.S., and lives in the large intestine;
Necator americanus is the New World hookworm, and persons may become infected
by walking barefoot through infected soil; Trichinella spiralis may be acquired by
eating undercooked pork; filarial worms are common in the tropics, and various
species may cause elephantiasis (by blocking lymph channels), as well as heartworm
in dogs.
4. Nematomorphs. Members of the Phylum Nematomorpha are also referred to as
horsehair worms or Gordian worms. The juvenile stage is an endoparasite of
arthropods, but the adults are free-living. Adults have very elongate thin bodies with a
cellular epidermis and a thick cuticle. Nematomorphs are dioecious. Nematomorphs
are not common, but are particularly distinctive when encountered.
5. Acanthocephalids. Members of phylum Acanthocephala are typically small
endoparasites (but ranging in size from 40 mm to 80 cm) in the gut of vertebrates
(mainly fish), and are digenetic, so they require two hosts. Juveniles live in
crustaceans or insects, but the adults live in mammals, fishes or birds.
The acanthocephalans have a proboscis covered with spines; hence the common name
spiny-headed worms. A syncytial tegument, covered by a glycocalyx, prevents the
worm from being digested by its host, yet allows absorption of food molecules.
There is no gut. Acanthocephalans are dioecious; fertilization is internal and eggs
develop in the pseudocoelom of the female. The huge number of fertilized eggs
produced pass out of the host in the feces.
6. Loriciferans. Members of the phylum Loricifera were only recently described (1983).
Loriciferans live in the interstitial spaces between marine sediment grains. The lorica
is the outer covering of the body, into which the introvert (head) and thorax may be
retracted. The lorical cuticle is molted. The animals are dioecious, and possess a
complete gut.
7. Priapulids. Members of the phylum Priapulida are a small group of marine worms
that range in size from 2 mm to 8 cm. They live buried in sediments, and have
introvert used in feeding and burrowing that can be withdrawn into the trunk. These
animals have a simple nerve cord and are dioecious.
The phylogenetic relationships among the aschelminths are not well understood, but it is
clear that grouping “worms” together will not produce a reasonable phylogeny. Rotifers
appear to be closely allied to the acoelomate taxa. Loriciferans and kinorhynchs appear
to be closely related to each other. The acanthocephalids and priapulids appear to share a
retractable spiny anterior end. The nematomorphs may be related to priapulids or to
nematodes. Nematode ancestral groups have not been identified.
Molluscs-Coelomates
Lecture Outline
Two groups of coelomates animals have been distinguished: the protostomes and the
deuterostomes. Protostomes include molluscs, annelids, and some lesser phyla; both the
annelids and the molluscs have a trochophore larval stage in development, suggesting an
evolutionary relationship. Because molluscs are the first group of coelomates to be
covered here, a brief discussion of coelom formation is needed. Most protostomes form
the coelom by splitting the mesoderm (schizocoel hypothesis), whereas most
deuterostomes form the coelom by outpocketings of the gut (the enterocoel hypothesis).
There has been some question as to which method of coelom formation was present in
the ancestral coelomate, but given that both forms now exist, some people believe that the
coelom evolved independently in each lineage.
Molluscs have been very successful in a myriad of habitats. Members of this very
diverse group range in size from a few mm to the 18 meter giant squid. Bivalves and
gastropods are the most successful of the 8 molluscan classes. Molluscs have bilateral
symmetry, a trochophore larva, and basic protostome characteristics, but they also have 3
unique features:
1. A body composed of a head-foot and visceral mass.
2. A mantle that encloses the visceral mass, secretes the shell (if present), and forms
the mantle cavity.
3. A radula, a rasping structure used in feeding that is supported by a cartilaginous
odontophore. The radula has been lost in the bivalves.
Although mollusc are coelomate, the coelom is greatly reduced. Molluscs have an open
circulatory system, in all classes other than the cephalopods.
The class Gastropoda is the most speciose molluscan class, but it is characterized by
unique development process called torsion. All gastropods undergo torsion, an 180°
counterclockwise twisting of the body that causes the gills and anus to be located behind
the head. This position of the anus allows wastes to fall onto the head and gills, fouling
them. The function of torsion is unknown, but has been suggested to relate to shell
coiling. Ancient shells were coiled in a single plane with new coils wrapping around old
coils. Modern shells coil asymmetrically with new coils at the side of old coils; this has
resulted in asymmetry in the body systems. The foot is flattened. Because of torsion, the
snail’s head is drawn into the shell first, and as the foot retracts, an operculum may cover
the aperture, so perhaps protecting the head is a benefit of torsion.
The basic features of gastropods are outlined below. Most gastropods are herbivores,
feeding with a radula, but some are predatory. Once food is ingested, the protostyle in
the stomach aids in breaking down food particles. Gastropods have an open circulatory
system and the hydraulic skeleton (where blood or other fluids under pressure are forced
into tissues to extent them) functions in circulation, as well as movement and support.
Gastropods have well developed senses, including eyes on tentacles, statocysts, and
osphradia. Nephridia are the excretory structures in gastropods. Snails may be
monoecious or dioecious, and may be fertilized externally or internally.
In marine gastropods, the trochophore larva develops into a veliger larva with torsion
occurring in the veliger stage. The veliger stage settles and undergoes metamorphosis.
Subclass Prosobranchia is the most speciose sub-group of the gastropods; its members are
mostly marine. Subclass Opisthobranchia includes sea hares and slugs; these animals are
mostly marine, and many have lost or greatly reduced their shell.
Members of the Subclass Pulmonata are mostly freshwater and terrestrial snails. The
terrestrial forms use a lung, made from the vascularized mantle cavity, in place of gills.
The opening to the lung is the pneumostome.
Class Bivalvia includes clams, mussels, oysters and scallops, and is the second largest
molluscan class. Bivalves are found in almost all aquatic habitats, buried, or attached to
rock or man-made substrata by byssal threads. Bivalves are laterally compressed and
covered by two valves, or shell halves. A pair of adductor muscles keeps the shell
closed.
Bivalves are typically sedentary filter feeders. The incurrent siphon is the conduit for
providing the water current, and. Filter feeding is accomplished by the lamellae of the
gills; gills are used in both respiration and feeding. Once collected, food is directed to the
labial palps, which surround the mouth, and then into the stomach. A crystalline style in
the stomach aids in digestion, along with the gastric shield. Feces pass through the anus,
and then out via the excurrent siphon.
Circulation and respiration involve blood vessels in the heart, tissues, sinuses, and gills.
The circulatory system is open. The nervous system is primitive, including several
ganglia, sense organs, around the margin of the mantle, and may include complex eyes as
seen scallops.
Most molluscs are dioecious, undergo external fertilization, with both trochophore and
veliger stages of development. Freshwater bivalves tend to brood their young in their
gills; the young may develop into a larval stage known as a glochidium. The glochidium
may become parasitic in fish gills or other parts of a fish; it uses the fish as a dispersal
agent.
Class Cephalopoda, including octopuses, squid, cuttlefish, and the nautilus, contains the
most morphologically complex invertebrates, particularly with respect to the nervous
system. Ancestral cephalopods were shelled; most extant cephalopods have reduced or
lost the shell (the nautiloids are the exception). The cephalopods move by contraction of
longitudinal and circular muscles in the mantle which produces a rapid water jet from the
funnel (a modified foot). Their rapid locomotion aids in their predatory habits. This
increased activity is supported by the unusual closed circulatory system allowing more
efficient blood flow for excretory and respiratory functions. Carnivorous cephalopods
feed using their jaws and radula; digesting food is moved by peristalsis through the
gastrointestinal tract; wastes pass out the anus and exhalent water flushes it out of the
mantle cavity. Discharge of ink from the mantle cavity may also deter predators.
The cephalopod nervous system is the most advanced of any invertebrate. They have
large brains tied to chromatophores, pigment sacs in the skin that allow cephalopods to
change colors rapidly. The cephalopod eye is complex and image-forming; it is
convergent evolution with the vertebrate eye.
Cephalopods are dioecious, and typically males have a specialized tentacle (the
hectocotylus) for transferring spermatophores to the female. Many cephalopods tend
their eggs; hatchlings are miniature adults.
The class Polyplacophora contains the chitons. All chitons have 8 plates on their dorsum,
and a ventral food to adhere to the substrate. They are mainly herbivorous grazers,
feeding on algae is accomplished by a radula. Gills in the mantle cavity provide for
respiration. The nervous system is ladder-like with a nerve ring around the esophagus.
Chitons are dioecious.
The class Scaphopoda contains the tooth or tusk shells. Tusk shells are burrowers,
feeding on protists, while lying partially buried in the marine sediments.
The tubular or cone-shaped shell is open at both ends. Various sensory structures are
found in many places on the body. These dioecious animals have both a trochophore and
veliger larva in the life cycle.
The class Monoplacophora contains primitive molluscs with an extensive fossil record;
living forms have been known only since 1952. Neopilina is the only extant genus.
They have one limpet-like shell, in spite of the serially repeated retractor muscles and
gills that are also present. They are dioecious.
The class Caudofoveata contains poorly known worm-like molluscs.
The class Aplacophora contains 2 subclasses of primitive shell-less molluscs. The
subclass Neomeniomorpha houses the solenogasters (some with a radula). The subclass
Chaetodermomorpha (previously the Caudofoveata) contains animals with scale-like
spicules on the body surface. Most burrow or creep on the substrate. They have a
nervous system similar to that of flatworms.
Phylogenetic studies of the Mollusca indicate that the group is more than 500 million
years old and did not have a segmented ancestor. Many characters, such as segmentation,
may be secondarily derived. The evolutionary relationships among the classes are not
well understood.
Annelida-Roundworms
Lecture Outline
Annelids are coelomate worms exhibiting typical protostome characteristics, including a
trochophore larva and coelom formation by schizocoely. One of the most obvious
characteristics of the annelids is the internal and external segmentation of the body. This
segmental arrangement produces a metameric body of serially repeated units, each with
its own coelomic, excretory, circulatory and nervous systems. This body plan allows
efficient burrowing, crawling and swimming because the coelom serves as a hydrostatic
skeleton under the alternating forces of the circular and longitudinal muscles. Tagmata
are specialized groups of segments that form distinct body regions. Tagmatization is
better exhibited in the arthropods than the annelids.
The evolutionary relationship of the annelids to the acoelomates and to other coelomates
are not fully understood; fossil evidence is sparse and the discovery of organisms that
share characteristics of both acoelomates and coelomates has not helped much.
One hypothesis states that annelids and arthropods had a common ancestor that was
marine, wormlike, bilateral, and metameric. Another hypothesis links the annelids and
the molluscs on the basis of the shared trochophore larva.
The class Polychaeta contains many species of marine worms and is the most speciose
annelid class. Polychaetes range in size from 5 mm to 10 cm. Polychaetes may be motile
or be sessile tube-dwellers. Possession of parapodia distinguishes polychaetes from
members of the other class; these body extensions allow the worm to crawl, burrow, or
swim. The head has two regions: a prostomium and a peristomium. The prostomium is
an anterior lobe that is sensory in function.
Polychaetes may be predators, herbivores, scavengers, or deposit or filter feeders.
Some polychaetes may absorb 20–40% of their energy requirements across the body
surface (non-living cuticle) from the organic muds that they inhabit.
Respiration in polychaetes, as in most annelids, is via diffusion across the body wall.
Circulation is closed in polychaetes. Excretion of ammonia occurs across the body wall,
but there are specialized excretory organs, nephridia, in each segment. Protonephridia
are primitive excretory structures; metanephridia are found in most polychaetes.
Chloragogen tissue, surrounding the dorsal blood vessel, has a similar function as the
vertebrate liver (it works in amino acid metabolism). Most polychaetes cannot
osmoregulate in freshwater.
The nervous system is typical of annelids; it consists of sets of paired ganglia and a
double ventral nerve cord. A variable number of paired eyes are located on the
prostomium; other nerve receptors are found elsewhere in the body.
Most polychaetes are dioecious. Epitoky, as exhibited by the palolo worm, involves the
separation of the main part of the worm from the reproductive sections, which swarm and
release gametes. Most polychaetes use external fertilization followed by development of
a trochophore larva. Polychaetes may also regenerate lost parts and reproduce asexually
by budding or transverse fission.
The class Clitellata includes earthworms and leeches. The shared presence of a clitellum
used in cocoon formation, monecious reproduction, direct development, and few or no
setae unite two previous classes into this class. The two previous classes remain as
subclasses of the Clitellata.
The subclass Oligochaeta contains terrestrial or freshwater worms without parapodia and
having fewer setae than polychaetes. Oligochaetes swim, creep, or burrow by alternate
contractions of longitudinal and circular muscles in conjunction with the hydrostatic
skeleton. They use short setae as an anchoring mechanism. They are primarily
scavengers and also ingest much earth while burrowing. The digestive system consists of
a pharynx, esophagus, a stomach, crop, and/or gizzard, and intestine. The intestine
contains a U-shaped fold, the typhlosole, that increases surface area for nutrient
absorption. Respiratory, circulatory and nervous systems are similar to those seen in
polychaetes. Metanephridia function in excretion, chloragogen tissue functions much
like the vertebrate liver.
Oligochaetes are monoecious, and reciprocally exchange sperm from seminal vesicles
(sperm matures and is stored here) in one animal to seminal receptacles in the other
animal. Subsequent to mutual copulation, the clitellum forms a cocoon for the zygote (no
larva is formed). Freshwater oligochaetes may also reproduce asexually by transverse
division.
Lumbricus terrestris is a common species for study, as it is large and common in the
United States, although introduced from Europe. Native worms are present in the U.S.,
but they are smaller in size than this introduced species.
The subclass Hirudinea contains the leeches. Leeches typically have 34 segments, but
many more external rings called annuli. Their septa are reduced. Leeches are
dorsoventrally flattened, and have oblique muscles, in addition to longitudinal and
transverse muscles. Leeches lack parapodia, head appendages, and most lack setae.
Gas exchange occurs across the body wall and coelomic fluid, in sinuses, takes on the
function of blood.
Most leeches are freshwater, but some are terrestrial or marine. Leeches may be
temporary ectoparasites, or predators. Parasitic leeches have jaws to bite into the prey,
and secrete an anticoagulant to keep blood and body fluids flowing into the muscular
digestive system. Leeches are often class specific with respect to their host; e.g., the
attack fish, but not reptiles, etc. Leeches have specialized sense organs that allow it to
find its prey or host—some are temperature-sensitive, but others follow chemical trails.
All leeches reproduce sexually and are monoecious. The clitellum is present only during
the breeding season. Some males have a penis; other males hypodermically impregnate
the females by inserting the spermatophore directly through the integument of the female.
No leeches reproduce asexually and they do not have regenerative capacities.
Traditional analysis states that annelids evolved in marine habitats, and polychaetes are
the most primitive of the phylum. Paired epidermal setae are the diagnostic character for
the phylum. Oligochaetes evolved in freshwater, some ultimately moved into the
terrestrial environment, feeding on terrestrial plant debris—the freshwater oligochaetes
gave rise to the leeches.
There is some question as to the status of this phylum as a monophyletic group. The
echiurans, pogonophorans and vestimentiferans may be added to the polychaetes, but the
addition of these taxa calls the features of the ancestral polychaete into question. Did the
ancestor have metameric segments and parapodia like a polychaete or was it a burrower
lacking parapodia? The evolutionary relationship between annelids and arthropods is
also currently under scrutiny.
Arthropods
Lecture Outline
Arthropods are clearly the most numerous organisms on earth. To date, about 1
million species of arthropods have been identified. Of the arthropods, a species of
copepod may be the most abundant animal on earth. Copepods are a primary species
at the base of the marine food chain. Five specializations seen in arthropods are:
1. Metamerism and tagmatization. Metamerism is evident externally, less
evident internally in arthropods. Tagmatization is the specialization of body
segments for a particular function.
2. A chitinous exoskeleton with jointed appendages. The exoskeleton is
composed of an epicuticle (composed of a lipoprotein), and the deeper procuticle
(=endocuticle), composed of chitin and some proteins. The exoskeleton of some
arthropods is further hardened by the process of sclerotization, and impregnation
of calcium carbonate. Some areas of the exoskeleton are thinner, allowing for
movements at the joints.
3. A reduced coelom, formed from the blastocoel. The coelom is part of the open
circulatory system and functions as a hemocoel.
4. Molting. Growth of the animal is regulated by molting of the exoskeleton
(ecdysis is a 4-step process).
5. Metamorphosis. The change from an immature stage to an adult results in less
competition between the life stages.
Insights from molecular biology suggest new protostome relationships. Annelids belong
with the lophotrochozoans, whereas arthropods and other animals that molt belong in a
group called the Ecdysozoa. This classification scheme means that metamerism in the
annelids and arthropods is convergent—a highly controversial conclusion.
The phylum Arthropoda is currently divided into 5 subphyla: the extinct Trilobitomorpha
and 4 subclasses with living members: Hexapoda, Chelicerata (see below in the section
on pycnogonids for a new name and new groups), Crustacea, and Myriapoda.
The subphylum Trilobitomorpha contains the extinct aquatic trilobites. These organisms
dominated the Paleozoic seas from 600 to 345 million years ago. The name trilobite is
derived from the three vertical lobes of the dorsal exoskeleton. The body had 3 tagmata
(head, thorax and pygidium) and biramous appendages.
The subphylum Chelicerata includes spiders, scorpions, ticks, horseshoe crabs and sea
spiders. The chelicerates have two tagmata. The anterior tagma, called the prosoma or
cephalothorax, typically bears eyes and chelicerae, but no antennae. Pedipalps are
directly posterior to the chelicerae, and are involved in feeding, movement, or
reproduction. The walking legs follow the pedipalps. The posterior tagma is the
opisthosoma, containing most internal organs. There are 2 classes of chelicerates:
Members of the class Merostomata include the extinct eurypterids (subclass
Eurypterida—the water scorpions that flourished from 600 to 280 million years ago),
and the extant horseshoe crabs (subclass Xiphosura). The most common horseshoe
crab (Limulus) is found in the Atlantic Ocean and Gulf of Mexico, and is used in
biology classes as a classic example of stabilizing evolution. Horseshoe crabs are
scavengers. On the prosoma, horseshoe crabs have chelicerae and pedipalps; the first
three pairs of walking legs have chelae (pinchers) and are used for walking and food
handling. The fourth pair of legs is for locomotion and digging to mate and lay eggs.
The opishtosoma has a long telson and book gills for respiration (open circulatory
system). During the mating season, males and females (they are dioecious) gather in
shallow sandy areas at the shoreline to mate (via external fertilization) and lay eggs.
Members of class Arachnida include the spiders, mites, ticks, scorpions and some
others. It is believed that the arachnids arose from the eurypterids, and the arachnids
were among the first terrestrial animals in the Devonian period. The exoskeleton may
have pre-adapted them for terrestrial life by enabling them to resist dessication.
Arachnids are typically carnivores, and use chelicerae for food handling—they
consume liquified prey via a pumping stomach. Coxal glands and/or malpighian
tubules are used to excrete nitrogenous wastes (semi-solid uric acid). Arachnids may
respire via book lungs or tracheae (which are not homologous to the tracheae of
insects). The circulatory system of arachnids includes a contractile dorsal aorta that
pumps blood through the hemocoel, where it bathes the tissues. The nervous system
of all arthropods is ventral, similar to the annelids. Sensilla are sensory structures that
project through the exoskeleton and respond to vibrations and chemical stimuli. They
may have one or more pairs of eyes.
Arachnids are dioecious, and often courtship is performed before mating. Sperm
transfer may be direct (via the pedipalps) or indirect, by presentation of sperm packets
by the male to the female. Development is direct, and many exhibit some parental
care of eggs and/or young.
The scorpions (order Scorpionida) have small chelicerae, but their large pedipalps
are chelate and aid in grabbing prey items. The prosoma is fused into a shield-like
carapace. The opisthosoma is divided into two sections; the anterior preabdomen,
and the long, thin postabdomen (often called the tail), terminating in the stinger.
Mating occurs subsequent to a period of courtship: the male inserts a
spermatophore into the female. Most scorpions are oviparous; some are
ovoviviparous, yet others are viviparous. After hatching/birth, the young are
brooded on the back of the female.
The spiders (order Araneae) has the largest number of currently identified species
of arachnids (34,000 species). On the prosoma, spiders have chelicerae with
poison glands and fangs. (The brown recluse and the black widow spiders have
venom that is toxic to humans.) Spiders are predators, typically feeding on other
arthropods. Pedipalps may be leg-like, and in males, are involved in sperm
transfer. A pedicel attaches the opisthosoma (abdomen) to the prosoma. The
abdomen contains openings to the reproductive tract, book lungs and trachea, as
well as the spinnerets that produce silk. Silk functions in prey capture, formation
of the egg case, and dispersal of young spiders by ballooning. Reproduction
involves complex signaling between partners, and then the male’s pedipalp,
holding sperm that he previously deposited there, is inserted into the female.
Other arachnids, such as the harvestmen (daddy longlegs) are members of order
Opiliones. Unlike spiders, there is no thin pedicel, and the body appears to be
oval.
Some harvestmen are omnivores; others are carnivores. Harvestmen tend to
inhabit warm moist places.
The mites and ticks may be ectoparasites, but some are free living in both
terrestrial and aquatic habitats. Mites and ticks are the arachnids that cause the
most harm to human health, ticks via Lyme disease, tularemia, Rocky Mountain
spotted fever and other diseases. Most parasitic mites and ticks are temporary
ectoparasites; the follicle mite is an exception. Ticks reach 3 cm in size; they feed
and copulate on the host, then drop to the ground to lay eggs.
Other small (in numbers of species and size as well) orders of arachnids include
whip scorpions, whip spiders, pseudoscorpions, etc.
The sea spiders are all marine (as per their name) and feed on cnidarians and
ectoprocts. They are characterized by 8 long knobby legs. They are dioecious and
the male broods the young. In some classifications this group is in the class
Pycnogonida, but some workers propose renaming the subphylum Chelicerata to
the subphylum Cheliceriformes. The new subphylum would have 2 classes: the
Pycnogonida and the Chelicerata. Everything currently under the subphylum
Chelicerata would fall within the class Chelicerata in the Subphylum
Cheliceriformes.
The subphylum Crustacea contains animals very well known to most people as a food
item, as well as others that are abundant and important, but commonly overlooked.
Crustaceans have two pairs of antennae, and have biramous appendages where the two
parts of the basal segment of the appendage are the endopodite and exopodite.
There are 5 crustacean classes; members of largest crustacean class, Malacostraca,
include shrimp, crabs, lobsters and crayfish.
Within the crustaceans, Decapoda is the largest order; including many organisms we
consider delicacies and/or lab animals (e.g., crayfish are used to illustrate internal and
external morphology). The body has two tagmata: a cephalothorax and an abdomen.
The cephalothorax is composed of the head and thoracic segments. The appendages
on the head include 2 pair of antennae, mandibles, 2 pair of maxillae, and 3
maxillipeds. The abdomen is posterior, and functions in locomotion by forming a
tail, which bears swimmerets. Gills are located laterally, and are attached to the bases
of some of the walking legs and the last two pair of the maxillipeds. Circulation is
accomplished via sets of arteries leading from a muscular heart, which empty blood
into the hemocoel. The nerves of a crayfish are ventrally located, ladderlike, and
various ganglia aid in swimming functions. Sensory structures include compound
eyes (with 25 to 14000 ommatidia), simple eyes, statocysts, chemoreceptors,
proprioceptors, and tactile setae. Crayfish are dioecious, like all other crucstaceans,
except the barnacles. Gonads are located in the thorax. In some, development is
direct, in others a nauplius larva forms, and may be followed by the zoea.
The text has a lengthy aside on the decapod endocrine system because this system has
been well studied and is very important in molting. The endocrine system controls
molting, sex determination, and color changes. Endocrine glands release hormones
into the circulatory system and ultimately affect target tissues. In crustaceans,
hormones mostly affect nervous functions, sexual development and behaviors.
X organs are neurosecretory, and along with the Y organ, control ecdysis.
Excretory glands are the antennal or green glands, similar to the coxal glands of
arachnids
There a few other crustaceans worth mentioning:
Pillbugs are classified in order Isopoda, and are dorsoventrally flattened
Isopods may be aquatic or terrestrial and are typically scavengers.
Amphipods are similar to isopods, but are laterally compressed.
The class Branchiopoda typically contains freshwater crustaceans, many living in
temporary bodies of water. They are characterized by flattened leaf-like appendages used
in respiration and locomotion. Examples of branchiopods include fairy and brine shrimp
(order Anostraca), and the water flea, Daphnia, in the order Cladocera.
The Class Maxillopoda contains copepods, barnacles, and pentastomids. Copepods
(subclass Copepoda) are extremely abundant in marine and freshwater habitats. A few
copepods are predatory, some are parasites or commensals; most are herbivores. The
barnacles (subclass Thecostraca) are all marine. The adult is sessile, but the motile larva
attaches to the substrate as in a late larval stage. The acorn barnacles are directly
attached to the substrate; gooseneck barnacles have a stalk. Some are ectoparasites on
whales and ship bottoms, and a few are highly modified endoparasites. The subclass
Pentastomida contains animals parasitic to the respiratory passages of birds, mammals
and reptiles. This group was previously in its own phylum.
Echinoderms
Lecture Outline
Echinoderms are sea stars, sand dollars, sea urchins and other odd beasts. There are 7000
living species, all marine. They have a rich fossil history; the number of extinct classes is
greater than the number of extant classes. Echinoderms are believed to share a common
ancestry with the other deuterostomes (hemichordates and chordates), despite their
relatively unique characteristics. Deuterostomes, in contrast to protostomes, have an anus
that forms from the area of the blastopore, and have radial, indeterminate cleavage.
It is probable that echinoderms evolved from bilaterally symmetrical ancestors, but adult
echinoderms have secondarily derived pentaradial symmetry. The anatomy of modern
echinoderms is strongly influenced by their pentaradial morphology. This radial
symmetry is not adaptive for rapid movement; most slowly creep, or are sessile.
Echinoderms have an endoskeleton composed of calcium carbonate. The skeleton of
echinoderms is composed of ossicles, mesodermal in origin, and covered with a thin
epidermal layer.
The water vascular system is unique to the echinoderms. The water vascular system
originates from the coelom and is an interconnected series of water filled canals.
This system opens to the outside through a madreporite on the aboral surface of the
animal. Descending into the body from the madreporite is the stone canal, which
communicates with the central ring canal (around the mouth). Five radial canals radiate
from the ring canal. Lateral canals branch from the radial canals. Lateral canals
terminate in tube feet. Tube feet have an upper portion called an ampulla that serves as a
water reservoir. When water is pushed out of the reservoir, the tube feet stiffen for use in
locomotion. Tube feet may have suction cups for adhesion to the substrate or their prey.
The hemal system is associated with the water vascular system and functions in
circulation of some substances, perhaps nutrients. The digestive, circulatory, and nervous
systems are unusual and uniquely adapted to their symmetry and vascular system.
The class Asteroidea contains the sea stars. The surface with the madreporite plate is the
aboral surface; the surface with the mouth is the oral surface—it is normally oriented
downward. Sea stars have five arms with ambulacral grooves, containing tube feet,
running down the length of the arm. The digestive system is simple, but it has 2 parts: an
eversible cardiac stomach and branches of the pyloric stomach that extend into the arms.
Sea stars are predators, and may feed on small marine organisms, or larger bivalves by
pulling the shell apart using the tube feet. When feeding on large bivalves, when the
gape of the bivalve is open slightly, the cardiac stomach of the sea star everts into the
bivalve, and partial digestion occurs, which weakens the adductor muscles of the bivalve,
allowing the starfish then to completely ingest it.
Sea stars have dermal branchiae for gas exchange, and many have pincer-like pedicellaria
that clean the surface of the body and are protective. Transport of dissolved materials
occurs via diffusion. The nervous system of sea stars consists of a typical nerve net and
radial nerves. Ensory structures allow the sea star to respond to light, chemicals, and
mechanical stimuli. Sea stars have photoreceptors on the ends of the arms, on the oral
surface. Most sea stars are dioecious with external fertilization. Their reproductive
processes and embryological development of the free-swimming larvae have been
extensively studied. Sea stars may regenerate lost arms.
The Class Concentricycloidea was once considered a valid group, but its members are
now considered modified sea stars and were absorbed into the Class Asteroidea. These
animals were only recently been discovered in the deep ocean, with only one described
species. They are also known as sea daisies and are very small. The sea daisies have two
concentric water vascular rings, one with tube feet, but they lack a digestive system.
Little is known about their reproduction or ecology.
The class Ophiuroidea contains the brittle stars and basket stars--the arms of ophiuroids
are distinctly set off from the central disc. The arms may branch. Ophiuroids lack dermal
branchiae and pedicellaria; the tube feet lack suction discs and ampullae and are not used
for locomotion. Ophiuroids have closed ambulacral grooves; rather they use the skeletal
ossicles for locomotion. The madreporite is on the oral surface.
This is the most speciose group of echinoderms, but often overlooked, as the brittle stars
hide in sandy areas under rocks. Brittle stars are predators and scavengers; basket stars
are filter feeders. The digestive system is simple and is restricted to the central disc.
Ophiuroids are dioecious, and males are so small that females may carry them around.
The larvae, the ophiopleuteus, is planktonic. Ophiuroids can also replace lost arms, and
indeed may shed them easily (autonomy) as an escape reaction.
The class Echinoidea contains the sea urchins, sand dollars and heart urchins. The body
is globular and there are no arms in this group. Sea urchins have long tube feet and
pedicellaria; the pedicellaria in some species have venom. Sea urchins are adapted for
living on hard surfaces; sand dollars and heart urchins live in or on soft substrates (some
are burrowers). Sea urchins have a specialized chewing structure (Aristotle’s lantern) for
grazing on algae, but sand dollars and heart urchins use their tube feet to trap organic
particles. Echinoids are dioecious; they shed gametes into the water, so fertilization is
external. The larval form is a free-swimming pluteus, but it eventually settles in
metamorphoses into a sessile adult.
The class Holothuroidea contains the sea cucumbers. Sea cucumbers have no arms, and
are secondarily bilaterally symmetrical. They are elongated along the aboral/oral axis.
Sea cucumbers are soft bodied with only microscopic ossicles; the madreporite is
internal, and they lack spines or pedicellaria. They have tube feet typically on one side
(the side they lay on); other tube feet surround the mouth and function as tentacles. They
are deposit feeders or suspension feeders. The coelom of the sea cucumber is large and
functions in circulation of gases, wastes, and nutrients. The unique respiratory trees
function in respiration and circulation of materials. Sea cucumbers may defend
themselves by extending Cuverian tubules through the anus, or by expulsion of the entire
set of internal organs. The dioecious sea cucumbers typically reproduce with external
fertilization, but some brood their eggs. Sea cucumbers may also reproduce asexually by
transverse fission; they later regenerate lost parts.
The class Crinoidea contains about 630 species of sea lilies and feather stars. They are the
most primitive and unusual echinoderms—crinoids were abundant in the Paleozoic and
although there are extant forms, most biologists have not even seen a living specimen.
They have branched arms and use the tube feet for suspension feeding—trapped food
items are carried to the mouth via cilia in the ambulacral grooves. This may be the
ancestral form of feeding for the echinoderms. Sea lilies are attached by stalks; feather
stars can swim and crawl. Crinoids lack a nerve ring, but do have radial nerves. Some
crinoids are dioecious, but others are monoecious, exhibiting protandry. Some crinoids
brood embryos on their arms. Crinoids can regenerate lost parts.
Echinoderms evolved most likely from bilateral ancestors prior to 600 million years ago.
As crinoids are the most primitive echinoderms, and use their water vascular system for
suspension feeding, this may likely have been the first function of this system. The
earliest echinoderms may have been bilaterally symmetrical and had the mouth facing
upward; a downward facing mouth may have been associated with the sessile, radially
symmetrical body form. The mobility of modern echinoderms may have evolved along
with the development of ampullae and suction discs of the tube feet.
Hemichordates
Lecture Outline
The Chordates and their relatives, the echinoderms and hemichordates, share several
important characteristics indicative of common ancestry. They are all deuterostomes.
Chordates possess a dorsal, tubular nerve cord, a notochord, pharyngeal gill slits and a
post-anal tail. Hemichordates share a variable number of ciliated pharyngeal gill slits on
the trunk and a post-anal tail with the chordates; they are surmised, therefore, to be more
closely related to the chordates than are the echinoderms. However, the two lineages are
thought to have diverged from a common stock a very long ago.
The phylum Hemichordata contains 2 unusual groups of marine animals that live in or on
sediments. Aside from the above-mentioned features, they have an acorn-shaped
proboscis, and a collar, followed by a long trunk where the coelom is divided into 3
cavities. They also have an open circulatory system and dorsal, sometimes hollow, nerve
cord. This phylum contains the enteropneusts (acorn worms) and the pterobranchs.
Class Enteropneusta contains acorn worms living in U-shaped burrows. Their size is
10 to 40 cm typically, but they may reach 2 meters. They have a ciliated epidermis
with gland cells. Acorn worms feed by ingesting benthic particles collected on the
proboscis via cilia and mucus. Captured food becomes part of a food string.
Respiration is via diffusion and the pharyngeal gill slits. A glomerulus is involved in
accumulation of excretory wastes from the blood. They have a nervous system that is
ectodermal in origin and has both dorsal and ventral tracts. Enteropneusts are
dioecious, and fertilization is external. Enteropneust larvae are ciliated, planktonic,
and are known as tornaria. They settle and become adult worms.
Class Pterobranchia are relatively uncommon, small (0.1 to 5 mm), colonial
hemichordates living in deep oceanic waters of the southern hemisphere and in
European coastal areas. The body is divided into 3 regions, including a shield-like
proboscis that secretes a tube and a collar that has up to 9 arms and numerous ciliated
tentacles. Pterobranchs filter feed, but only one genus has the pharyngeal gill slits.
They typically reproduce asexually to form colonies. Sexual reproduction occurs in
these mainly dioecious animals, but fertilization is external and the larva does not
feed.
Phylum Chordata includes the speciose vertebrates, as well as the lesser-known
urochordates and lancelets. The chordates have been successful in both aquatic and
terrestrial environments. Chordates are bilaterally symmetrical deuterostomes, with 4
unique chordate characteristics that must be present at some time in the life cycle:
1. notochord. The notochord is a supportive rod composed of a connective tissue
sheath around cells. The notochord is often replaced by cartilage or bone (vertebrae)
in the adult animal. The notochord or vertebrae may support the post-anal tail.
2. pharyngeal gill slits or pouches. Pharyngeal gill slits are used in primitive
chordates for filter feeding.
3. dorsal tubular nerve cord.
4. post-anal tail.
The subphylum Urochordata contains tunicates, often called ascidians or sea squirts.
Tunicates derive their name from their outer covering, the tunic, which is usually
composed of proteins, salts, and cellulose. The largest class of tunicates holds the
solitary or colonial ascidians that are sessile as adults, although they have a freeswimming larva. Members of the other classes are typically planktonic as adults.
Ascidians filter feeding, directing water into and out of the body via 2 siphons. Food is
trapped on mucus secreted by the endostyle. They have a complete gut with the anus
exiting into the outgoing siphon. The pharynx is used in gas exchange via the flow of
water through the animal. Blood flow in the tunicates is bi-directional.
Tunicates are monoecious, fertilization may be external or internal, but the animals
generally outcross. A non-feeding tadpole larva with all four chordate characteristics
develops. The larva settles and, after metamorphosis, the sessile adults retain only the
pharyngeal gill slits. The planktonic tunicates, appendicularians and thaliaceans, have
different life cycles and may be very common in the open ocean.
The subphylum Cephalochordata contains the lancelets. The two genera of lancelets
exhibit all four basic chordate characteristics in the adult animals. Unique characteristics
of these small (up to 5 cm), benthic animals include contractile cells in the notochord
(which aid in swimming) and a ciliated oral hood, with cirri, used in filter feeding.
Cephalochordates live partially buried in clean sand. They sit with the oral end up and
filter-feed, trapping food on cilia. Food is collected as a food string using mucus from
the endostyle; it is passed to the pharynx via the ciliated cirri on the oral hood.
Pharyngeal slits are present in the pharynx and are supported by cartilaginous gill bars.
The animals have no true heart, and a reduced coelom.
They are dioecious, and external fertilization produces a free-swimming larva, that settles
and metamorphoses. Paedomorphosis has been proposed to explain the movement of the
4 chordate characters from the larva of a urochordate to the adult of the cephalochordate.
The subphylum Vertebrata houses the most successful group of chordates. In the
vertebrates, vertebrae of bone or cartilage completely or partially replace the notochord.
Synapomorphies that distinguish urochordates and vertebrates from hemichordates
include the tadpole larvae and an endostyle; other homologies are questionable.
Cephalization is particularly pronounced in the vertebrates. Notable group members
include the jawless fish that became common in the Ordovician period and the first
terrestrial vertebrates that appeared in the Devonian period.
The text discusses two evolutionary hypotheses. The first hypothesis is that the
triploblastic deuterostomes originated from a cnidarian diploblast by enterocoely. The
second hypothesis states that nervous system homologies between arthopods and
chordates suggest that the main body axis of protostomes was inverted in the evolution of
the deuterostomes, reversing dorsal and ventral surfaces.
Insects!
Lecture Outline
The subphylum Hexapoda contains two classes: Class Entognatha and Class Insecta. The
insects have become the most speciose group of animals due to a variety of adaptations
that have allowed them to successfully invade the terrestrial habitat during the late
Silurian and early Devonian. Approximately 750,000 species have been identified to
date, making up 75% of all named species. Their success owes much to:
an exoskeleton that supports body weight
a waxy epicuticle that allows insects to avoid desiccation
flight, and a light-weight body, permits rapid dispersal
a rapid reproductive rate allows insects to become quickly established in new
habitats.
Previously the hexapods were grouped with the myriapods in the subphylum Uniramia on
the basis of their shared uniramous appendages, and similar excretory and respiratory
structures, but biologists now think these two groups are not closely related. We now see
the subphylum Myriapoda as a distinct group whose members have 2 tagmata; a head and
a trunk.
The Subphylum Myriapoda contains 2 classes, the Diplopoda and the Chilopoda.
The class Diplopoda contains the millipedes. Millipedes were among the first
terrestrial animals, appearing during the Devonian period. Millipedes have 2 pairs of
legs per segment, indicating fusion of ancestral segments; they now have between 11
and 100 trunk segments. Millipedes are rounded in cross section, as opposed to the
more flattened centipedes. Millipedes are scavengers or herbivores, and are typically
found in moist habitats. Anti-predator defenses include repellent chemicals such as
hydrogen cyanide.
The class Chilopoda contains the centipedes. Centipedes are mainly nocturnal and
require a moist habitat. Centipedes have a single pair of legs on each segment.
Centipedes are predators, with a venomous poison claw, but they are relatively
harmless to humans.
Classes Pauropoda and Symphyla contain elongate, segmented organisms that may
resemble centipedes. Pauropods have a soft body with a thin exoskeleton; they have
11 segments and live in leaf litter. Symphylans have 12 leg-bearing segments, no
eyes, and they resemble centipedes.
The subphylum Hexapoda contains the most successful land animals. Hexapods have 5
pairs of head appendages and 3 pairs of legs on the thorax. In the class Entognatha, the
mouthparts are inside the head capsule, whereas the mouthparts project from the head
capsule in the 30 orders of the Class Insecta.
The body of an insect is divided into 3 tagmata; head, thorax, and abdomen with one pair
of antennae on the head. The thorax is divided into the prothorax, mesothorax, and
metathorax. Legs are attached to each thoracic segment; wings, if present, are attached to
the thorax. Spiracles are located on both the thorax and abdomen. The abdomen has
reproductive structures for copulation and oviposition.
Insect locomotion includes walking, running, jumping, or swimming, in addition to flight.
Insect flight required wings, but the original function of wings may have been to protect
the spiracles. Early insects may have been gliders rather than wing flappers. Flight
required the ability to thermoregulate because the body must be kept warm to allow flight
muscles to contract. Insect flight may be accomplished by direct or indirect flight
mechanisms:
Direct, or synchronous flight is accomplished by contraction of muscles for both the
up and down strokes. Examples are butterflies, dragonflies and grasshoppers.
Indirect, or asynchronous flight is accomplished by deformation of the exoskeleton to
provide some of the thrust. Each wing beat does not require a separate nervous
impulse because the resilient exoskeleton stores energy. Examples are flies and
wasps. Indirect flight muscles are often called fibrillary flight muscles, and can beat
amazingly rapidly.
Insects feed on a diverse array of food items by biting, piercing, sucking or chewing;
their mouthparts are similarly diversified. The digestive tract consists of a foregut, a
midgut for digestion and absorption, and a long straight hindgut that may include a crop
and gizzard. Malpighian tubules and the rectum accomplish excretion and resorb water.
Excretion of uric acid is advantageous for terrestrial life, as it conserves water.
Gas exchange occurs through the tracheae that form a finely branching network that pipes
air directly to cells. Ventilation is usually aided by muscle contraction to exchange air in
the tracheae. Aquatic insects may rely on tracheae, gills, or diffusion. Circulation is
accomplished by the blood, which carries dissolved materials, but is not important in
transfer of gases. Thermoregulation is critical for flying insects; they produce a variable
body temperature (heterothermy) via basking or shivering thermogenesis.
Chemical regulation controls ecdysis and other behaviors. Insect sensory systems include
receptors for touch, vibration, stretching, and chemicals. Tympanic organs are found in
orthopterans and some lepidopterans and function in sound reception. Compound eyes
are well developed in most adult insects, and are composed of ommatidia. The eye of an
insect functions primarily in detecting movement, and can also see light waves that
humans cannot; some can even detect polarized light for navigation. Receptors for odor,
mechanoreceptors, and stretch receptors are all relatively well developed—Johnson’s
organs and tympanal organs sense pressure waves for hearing. Pheromones are released
by insects and function in intraspecific signaling. The segmental nervous system is well
developed in insects; some insects can learn and some even exhibit memory (e.g., some
bee species).
The most primitive insects, such as silverfish, have indirect sperm transfer via a
spermatophore. The develop via ametabolous metamorphosis in which the young are
miniatures of the adult, and simply go through instars by growing in size.
Some of the relatively primitive insects have hemimetabolous metamorphosis where the
eggs hatch to form a nymph that goes through a species specific number of molts to
gradually become an adult. Adults have wings and sex organs. In primitive aquatic
insects, the larvae are called naiads and often have gills.
Most insects have direct fertilization, and females lay eggs using an ovipositor. Most
insects exhibit holometabolic or complete metamorphosis where the stages of the life
cycle are the egg, larva, pupa and adult. The pupa may be encased in a cocoon, a
chrysalis, or a puparium. Emergence from the cocoon is also called eclosion.
Insects have many innate (non-learned) complex behaviors. The social insects (order
Hymenoptera; the bees, ants and wasps and order Isoptera; the termites) show the most
complex behaviors. Several different castes compose the colony. Reproductive females
are the queens; sterile females are workers; males are drones (exception: males which are
infertile in the termite colony are also workers). Pheromones released by the queen
control the castes.
A very small proportion of insects have a negative effect on humans; indeed many are
extremely helpful in our endeavors. Insects may be involved in biological control; they
pollinate our agricultural and ornamental plants; they provide honey, silk, and wax, and
are useful for biocontrol. Harmful insects include those that eat our agricultural products
or carry diseases such as malaria, yellow fever, bubonic plague, and typhus, which affect
humans; others are pests of domestic animals. Some harmful examples are lice, bedbugs,
and fleas.
Recent phylogenetic considerations suggest that the arthropods are a monophyletic group.
Some debates in classification center on the homologies of limbs, compound eyes,
tracheal systems, malpighian tubules, and mouthparts. The earliest fossils, from more
than 600 million years ago, may be protocrustaceans. Some believe that hexapods and
crustaceans shared an ancestor—the hexapods underwent several rapid diversifications
associated with flight and with their relationship with flowering plants.
Fishes
Lecture Outline
The group Craniata, where a skull encloses the brain and sensory, includes the fishes.
The jawless fishes, the hagfishes and the lampreys, were previously groups as the
Agnatha, but this term is no longer in use. The hagfishes are now placed in the
Subphylum Hyperotreti, but still belong to the Class Myxini. The vertebrates may be
about 750 million years old, but fish ancestry is not well known—fishes may have
evolved from jawless ancestors of fish. To this end, more recent molecular and cladistic
analysis suggests that the jawless hagfish may be the most primitive craniates.
Fossils dated to 530 million years ago, from China, illustrate vertebrate fish-like animals
with a brain and segmented muscle blocks. Conodonts are odd fossil forms with toothlike structures composed of dentine—they may also be craniates dating from 510 million
years ago. In any event, bone was well developed by 500 million years ago in the
ostracoderms, but they had neither jaws nor paired fins, and were benthic unspecialized
filter feeders. One hypothesis for the evolution of shark denticles suggests a mineral
storage function. The ancestors of fish probably evolved in marine habitats, but the
radiation of fish occurred during the Devonian in fresh waters. Approximately 40% of
the species of fish occur in freshwaters, although this habitat makes a relatively small
percentage of the earth’s surface.
The subphylum Hyperotreti contains the hagfishes. They have a brain enclosed in a
fibrous sheath, keep the notochord as adults (no vertebrae are present). They feed on
small invertebrates and scavenge dead and dying fishes.
The subphylum Vertebrata includes the jawless ostracoderms (extinct), but most living
members of the group have jaws (the lampreys are an exception). Ostracoderms lacked
paired fins and had bony external armor; they were 15 cm bottom-dwellers, primarily
filter-feeding, although the bony mouth plates could have been used to crack prey.
The class Cephalaspidomorphi contains the lampreys. Sea lampreys are found in the
North American Great Lakes and have been problematic as they kill or harm many
economically important fish. Lampreys are found in both marine and freshwater habitats,
and the adults are typically predators or ectoparasites of other fish (as blood feeders they
rasp a hole in the side of other fishes). Lampreys typically migrate to a freshwater stream
and bury their eggs in gravel. The larval lamprey is similar to the cephalochordate and is
a filter feeder.
The Gnathostomata contains the jawed fishes. Jaws evolved from the anterior pair of
skeletal supports for the gill slits (pharyngeal arches). The development of jaws and
paired appendages were responsible for the development of a predatory lifestyle in the
ancestors of modern fishes. The evolution of paired fins is not understood, but they
control roll and pitch in swimming. Class Chondrichthyes and class Osteichthyes are two
of the extant gnathostomes.
The class Chondrichtyes has two subclasses—subclass Elasmobranchii includes 820
species of sharks, skates and rays. Elasmobranchs are mainly marine carnivores or
scavengers. Sharks and their relatives evolved in the Devonian period. Modern forms,
like unusual filter-feeding basking shark, reach 10 meters in length. Shark’s placoid
scales were modified into teeth that can be used and replaced. The subclass Holocephali
contains the ratfish; they possess an operculum (cover) over the gills and lack scales.
Ratfishes use modified teeth to form large plates used to crush mollusc shells.
The class Osteichthyes contains fishes with some bone in the skeleton; they have scales
and a bony operculum to cover the gills. They usually lungs or a swim bladder.
Approximately 20,000 species of fish have been identified—indeed they are a successful
group with fossils dating from the late Silurian (over 400 million years ago).
The subclass Sarcopterygii includes the lungfish, the coelacanths, and the
rhipidistians. Lungfish have fleshy, muscular fins, and have gills, but respire via
lungs to a varying extent. They breathe air during the seasonal drought. The deep
water coelacanths are well known from the fossil record, but living specimens were
found off South Africa and Madagascar in the 1900s. The osteolepiforms are an
extinct subgroup, but one hypothesis places them as the ancestors of the amphibians;
another hypothesis places lungfishes in this position.
The subclass Actinopterygii is the most speciose and diverse group of fishes. Known
as the ray-finned fishes, they typically have swim bladders. Within the group,
the chondrosteans include the sturgeons (which migrate between freshwaters and
marine habitats, and feed on benthos) and paddlefishes (large, freshwater filterfeeders with a cartilaginous skeleton that was secondarily decalcified—their ancestors
were bony). The teleosts are the most common subgroup, with many species present.
The teleosts include some primitive fishes like the gar and the bowfins.
The bodily systems of fish are adapted to an aquatic life with efficient respiratory,
locomotory, sensory and reproductive systems. There are 24,000 species of fishes.
Swimming is relatively energetically inexpensive, and fish morphology is related to their
mode and speed of swimming. The earliest fish were filter feeders and scavengers.
Some fish, particularly juvenile fish, are filter feeders, and trap food by their gill rakers.
Adult modern fish are typically predators and constantly swim in search for prey. Some
are suction-feeders.
The circulatory system of fishes is closed, with a ventral heart composed of only one
atrium and one ventricle. Blood passes the heart once every circuit through the body—
there is a no distinct pulmonary and systemic circulation in fishes (but, there are distinct
pulmonary and systemic circulatory routes in the lungfish—these are indicative of the
circulatory pattern in the rest of the vertebrate taxa). In fish, blood flows to the sinus
venosus, atrium, ventricle, conus arteriosus, ventral aorta, gills through the body, then
back to the sinus venosus.
Fish extract oxygen from the water via a pumping mechanism involving their mouth,
pharynx, and operculum. Fish that ventilate by keeping their mouth open practice ram
ventilation. Oxygen diffuses across the epithelium of the gill lamellae of the gill
filaments. The countercurrent exchange mechanism provides a very efficient exchange of
gases.
Some fish have pneumatic sacs that may function in respiration; in others, they function
in buoyancy regulation (as a swim bladder). The lung was initially a supplemental
oxygen source for life in stagnant water—it is presumed that the use of the internal air
sacs as lungs predates their use as a swim bladder. Other mechanisms to aid in vertical
regulation include fins to create lift, and changing body density by the addition of oils.
Specializations of the nervous system include:
the olfactory system,
receptors for equilibrium,
the lateral line system. The lateral line system is a set of pits that detect changes in
water pressure.
electroreception—it is widely used among fish to find prey, and some fish produce
electricity to stun prey.
external nares leading to a blind sac for olfaction—this is especially important in
migratory fish.
fish eyes—they are typical vertebrate eyes, but focusing is accomplished by moving
the lens.
Fish need to balance their internal concentrations of electrolytes and water—they do this
via osmoregulation. Elasmobranchs reduce the need for osmoregulatation by having a
high concentration of urea in the body tissues; this makes them isosmotic with seawater.
Freshwater fish live in an environment that lacks ions, so they take up ions via their food
and via cells in the gills; they take in water by diffusion and never drink water—they
excrete hypoosmotic urine in a large volume to reduce bloating. Marine fish lose water
to their environment, so they conserve water by excreting a small volume of isosmotic
urine. To get more water, they drink seawater, and then excrete all extra ions via cells in
the gills. Migration between marine and fresh waters makes great demands on the
osmoregulatory system. Most fish excrete ammonia via diffusion across the surfaces of
the gills.
Most fish produce a large number of eggs, which are externally fertilized. A few have
evolved internal fertilization. Some fishes show elaborate mating behaviors, or school to
increase the chances of fertilization of eggs. Although parental care is seen in some
fishes, most do not practice parental care. Most fish are oviparous, some are
ovoviviparous, and some are similar to viviparity.
Molecular data suggest that the lungfish lineage is closest to the amphibians, but an
alternate hypothesis suggests that osteolepiforms, like Eusthenopteron, may be the most
likely the ancestors of terrestrial vertebrates.
Amphibians
Lecture Outline
Amphibians were the first tetrapods (4-legged vertebrates), evolving in the late Devonian
period—the diagnostic feature is an immobile skull roof. Two lineages of amphibians
evolved from the sarcopterygians; one (Ichthyostega) became extinct in the late
Carboniferous period and the other gave rise to the living amphibians as well as some
extinct forms. The defining character of this lineage is the unique attachment of the skull
roof to the brain case and the presence of mucoid secretion from the skin. The extant
amphibians (non-amniotes) are the sister taxon to the amniote lineage that gave rise to the
reptiles, birds and mammals. Amphibians today occur on all continents except
Antarctica. The lineage now has only 3000 species divided among 3 orders:
Order Caudata contains the salamanders. Caudates have four legs (typically), and a
tail present in the adult stage; eggs and larvae are typically aquatic. The
plethodontids are the most fully terrestrial salamanders—they lay eggs on land and
the developing young are miniature adults. The newts comprise family Salamandridae
and are aquatic throughout life. Salamanders typically are internally fertilized via a
sperm packet picked up by the female. Some salamanders are paedomorphic
(Nectarus is an example).
Order Gymnophiona contains the caecilians. These amphibians are tropical, legless,
wormlike, and superficially resemble an oligochaete, albeit a large one! Fertilization
is internal in caecilians, and in many incubation occurs internally, with the young
born as minature adults.
Order Anura contains frogs and toads. This is the most speciose group of
amphibians. Adult anurans lack tails, and hind limbs are adapted for jumping (via
strong muscles and webbed feet). Fertilization is almost always external with aquatic
eggs and larva. Larval anurans (tadpoles) are tailed, and typically aquatic. Tadpoles
are herbivorous, and typically feed on algae and aquatic plants. After metamorphosis,
the tailless adult may be terrestrial.
Amphibians are characterized by adaptations for both aquatic and terrestrial life. In the
water, the body weight is supported, an osmoregulatory system is required, and gas
exchange occurs via gills. On land, the body weight must be supported, gas exchange
requires a moist surface, and water must be conserved. Adult amphibians are typically
carnivores, using their tongue to capture prey—this is the first appearance of a “true”
tongue. Larval amphibians are usually filter feeders.
The skin of amphibians is highly glandular, lacks scales, is typically moist and may
function in respiration. The gland secretions keep the skin moist, may contain
anti-predatory toxins, and help the male and female cling during reproduction.
Cutaneous respiration is important in most amphibians, although the majority also
possess lungs.
Air has 20 times the oxygen of water, so the highly vascularized skin can take advantage
of this oxygen source.
The fish skeleton protects the internal organs, allows muscle attachment and keeps the
body from collapsing—the amphibian skeleton has similar functions, but it is smaller, has
fewer bones and weighs less. Amphibians have neck vertebrae, unlike fish. A pelvic
girdle (ilium, ischium and pubis) is characteristic of amphibians, and other tetrapods, but
does not occur in fish. Appendicular bones, and the musculature is associated with these
bones for movement, is homologous within the tetrapods and the ancient sarcopterygians.
The skeletal system of salamanders is light, compared to the strengthened skeleton of
anurans.
The circulatory system of amphibians has a systemic and a pulmonary (lungs) portion.
The systemic circulation does pick up oxygen because of the cutaneous respiration in this
group. Lungs are used in air, but when under water, amphibians respire through the skin.
Thus the heart atrium of anurans is divided completely, partially separating the
pulmonary and systemic circulation. Lung ventilation is accomplished by a buccal pump,
but the lungs supply only 1 to 7% of the oxygen needed—lungs are used more at higher
temperatures. Larvae and some adult amphibians respire with external gills. Lymph
flow is also controlled; amphibians have unique lymphatic hearts that pump fluid in the
lymphatic system.
As ectotherms, amphibians rely on behavior for any regulation of temperature—they may
bask or burrow and are generally tolerant of a broad temperature range. The nervous
system of amphibians is typical of vertebrates (they have a 3-part brain); aquatic forms
have lateral line organs like fish; chemoreception is accomplished by receptors in the
nasal passages, as well as in the skin. Amphibians have well developed visual
capabilities, as most search for food by sight. They have rods and cones for color vision.
A transparent nictitating membrane acts to protect the eye—distance ccommodation is
accomplished by moving the lens forward. Hearing is an adaptation to life on land, and
most amphibians have a tympanic membrane and a middle ear with a single columella
(stapes) that transmits high frequency vibrations to the inner ear. Low frequency
vibrations use a different pathway.
Freshwater amphibians excrete ammonia like fish—life in freshwater presents the same
osmotic problems faced by fish. Terrestrial amphibians have adaptations to conserve
water including physiological and behavioral mechanisms. Water loss is reduced by
behavioral changes in posture, hardening the skin, or increasing skin permeability to take
up water. They may store water in the bladder and the lymph system—up to 35% of
body weight. The bladder of amphibians stores the hypotonic urine in the form of urea.
Terrestrial forms excrete urea.
Reproductive patterns of amphibians are extremely diverse, but fertilization is usually
external and eggs are laid in water (but with some interesting exceptions—salamanders
make a spermatophore that the female picks up for internal fertilization; caecilian males
develop and intromittent organ and use internal fertilization). The timing of reproduction
is tied to hormonal and environmental factors. Many reproductive activities involve
vocalizations (the male has a vocal sac). The tadpole larva has a different form of
feeding, respiration, and locomotion from the adult. The larval metamorphosis to the
adult is under hormonal control. There are numerous examples of paedomorphosis in the
amphibians due to breaks in the hormonal pathways regulating metamorphosis. There
are interesting parallels to tetrapod evolution in amphibian life cycles—the larval tail,
caudal fin, and gills are lost, and the adults use lungs and limbs.
Amphibian declines and disappearances worldwide have caused alarm—some proposed
causes include habitat loss, increased ultraviolet light and acid deposition.
The classification of amphibians is problematic. Some biologists place all in one
subclass, Lissamphibia, abut another interpretation is that this group evolved from at least
two non-amniotic lineages, making the amphibians a paraphyletic group.
Reptiles
Lecture Outline
The reptiles adapted to life on land. They were the first animals to lay amniotic eggs—
this was the beginning of the amniote lineage. Amniotic eggs have 4 extraembryonic
membranes for protection and cushioning the embryo, as well as for exchange of gases
and storage of wastes. The egg also provides a food source for the developing young.
The amniotic eggs differentiate reptiles, birds, and mammals from other vertebrates.
Cladistic analysis of the amniotes based on molecular data suggests that the class Reptilia
is not a monophyletic group without the class Aves added to it. The class Mammalia also
shares a common ancestry with reptiles. The entire amniote lineage together forms a
monophyletic group.
Amniotes underwent adaptive radiation in the late Carboniferous period, coincident with
the insects, which were the major prey of the early amniotes. Three reptilian subclasses
formed, they are distinguished by changes in the jaw musculature as indicated by the
openings in the skull:
Subclass Anapsida has no openings in the temporal region of the skull; turtles
represent extant anapsids.
Subclass Diapsida has two openings in the temporal portion of the skull; extant
diapsids are the crocodiles, snakes, lizards, and tuataras. Both dinosaurs and birds are
also diapsids.
Subclass Synapsida has a single opening in the temporal area; it was the reptilian
synapsids (therapsids) gave rise to the mammals.
As terrestrial adaptations, reptiles have a epidermally-derived keratin scales in the skin
that resist water loss, one occipital condyle, metanephric kidneys, they respire via lungs,
and they use internal fertilization with amniotic eggs. Reptiles are found on all
continents except Antarctica, although they are not common except in tropical and
subtropical regions. There are several reptile orders:
The order Testudines (Chelonia) includes the turtles, terrapins and tortoises.
Turtles have a dorsal carapace, to which the vertebrae and ribs are fused, and a
ventral plastron. Turtles are long-lived organisms, grow slowly and lay their eggs on
land. Many marine species are endangered due to a variety of human activities such
as hunting, shrimp harvesting, and predation by introduced carnivores on hatchlings,
as well as intrinsically slow growth rates and long maturation times.
The order Rhynchocephalia has only 2 extant species of tuatara (Sphenodon
punctatus and S. guntheri). Tuataras resemble lizards and have remained virtually
unchanged for 200 million years. They are only found in New Zealand, and are now
protected, as they have been adversely affected by human influences and predation by
introduced domesticated animals.
The order Squamata includes the lizards, snakes, and worm lizards. The lizards
belong to suborder Sauria and are characterized (typically) by 4 legs, an elongate
body and their jaws unite anteriorly. Lizards vary from big to small, burrowers or
tree dwellers, some are oviparous, but some are ovoviviparous or viviparous.
Geckos are nocturnal and characterized by toe pads that facilitate climbing.
Both the Gila monster and the Mexican beaded lizard are venomous lizards.
Chameleons are arboreal and can change color. The suborder Serpentes contains the
snakes—they lack limbs (but may have some vestigial pelvic girdle appendages) and
eyelids. Some snakes are dangerous to people: worldwide, on average 35,000
humans die from snakebites per year. Snake jaws are loosely jointed to facilitate
swallowing large prey. Most snakes are oviparous. Snake evolution may taken place
in aquatic environment or in dense vegetation—both places where their body form
would be adaptive. The suborder Amphisbaena contains the worm lizards; they are
burrowers that move both forwards and backwards with ease.
The order Crocodilia) contains alligators, crocodiles, gavials, and caimans.
Crocodilians are characterized by a diapsid skull, and a secondary palate. In
morphological terms, they are a good example of stasis. Crocodiles are oviparous
and may display parental care. Crocodilians also have many characteristics shared
with dinosaurs, including a gizzard-like stomach and the habit of swallowing stones
for grinding.
Reptiles have a number of adaptations for terrestrial life—many are also predators. The
scaly skin of reptiles is shed periodically, and has no respiratory functions. The skeletal
system is highly ossified to support locomotion on land. Reptiles are typically
carnivorous; in particular the skull of snakes is highly adaptive for swallowing large prey.
Many snakes have venom glands modified from salivary glands, which may inject
hemotoxins or neurotoxins into their prey. A larynx is present, but vocal cords are absent.
Metanephric kidneys produce urine composed of uric acid and it is excreted in a pastelike form to conserve water. Reptiles are ectothermic, but most behaviorally
thermoregulate by basking and regulating the time of activity.
The circulatory system has similarities with amphibians. As in amphibians, reptiles have
two separate atria (the veins from the body and lungs empty into them), and in crocodiles,
the ventricles are separate as well—the separate circulation to the lungs can be stopped
for intermittent breathing, as when the turtle is in its shell or diving under water. Reptiles
respire via lungs, but lack a diaphragm; therefore they inspire and expire via the
contraction of thoracic muscles. These muscles cannot be used it turtles; instead they
force the viscera up and down.
The typical nervous and sensory system of reptiles is characterized by better olfactory
capabilities than amphibians; for example, the forked tongue is olfactory. A Jacobson’s
organ is present. Reptiles rely on vision as the dominant sense. Accommodation is as in
amphibians, but they have more cones in the retina suggesting color vision. There is a
median (parietal) eye, likely used for detection of light and orientation to the sun.
Pit vipers have pit organs between the eye and nostril opening to detect endothermic prey
Hearing varies; there is no middle ear, auditory tube, or tympanic membrane.
Courtship behaviors in reptiles are somewhat more complex than seen in amphibians; sex
pheromones can be used. The intromittent organ of the male typically facilitates internal
fertilization, but sperm can be stored in the female for up to 4 years in some. After
fertilization, the shelled, amniotic egg can be deposited on land. The eggs are typically
abandoned, but some, like the American alligator, do parental care. Parthenogenesis is
seen in several groups of lizards and one species of snakes. In some reptiles, sex is
determined by the temperature during incubation.
Birds
Lecture Outline
Birds are traditionally classified in the class Aves because of their unique adaptations for
flight, but birds evolved from the archosaur lineage of reptiles. Birds share one occipital
condyle, one ear ossicle, the lower jaw structure, nucleated red blood cells, nesting
behavior and parental care with other reptiles. The unique features of birds are wings,
feathers, endothermy, a modified vertebral column and light bones, as well as a horny bill
without teeth. Archaeopteryx is one fossil link between reptiles and birds,
(approximately 150 million years old), but recent finds of three species of feathered
dinosaurs from China from 135 million years ago complicate the story.
Evidence from Archaeopteryx appears to suggest that they crawled up trees, using clawed
digits, then glided and flew short distances. Other ideas describe early birds running or
hopping along the ground, and trapping their prey with the wings. It has been suggested
that bird wings may have provided stability during jumps, but some fossils show early
birds with a short tail and a sternum with a large surface area suitable for the attachment
of flight muscles. Both Archaeopteryx and modern birds possess clavicles and feathers.
Sinornis is another fossil bird, approximately 135 million years old; it appears to have
characteristics associated with true flight, including folded wings. Eolulavis, 115 million
years old, has characteristics (including an alula) indicative of slow, hovering flight.
Other fossil birds were terrestrial and flightless, some were aquatic, and it is not certain
which, if any, of these is the ancestor to today’s birds. Today, over 9,000 species of
extant birds are classified in about 27 orders.
Feathers (plumage) allow flight, but they also function in species recognition, mate
attraction, endothermy, and waterproofing. The color of feathers may be due to
pigments, reflected light, or iridescence. Feathers are keratinized epidermal structures
that evolved from epidermal scales of reptiles. Contour feathers cover the body, wings,
and tail. Birds clean their feathers by preening; feathers are molted and replaced.
The skeleton of birds is lightweight; most large bones have air spaces, other bones are
reduced in size. Uncinate processes, also seen in reptiles, strengthen the rib cage.
The rear appendages of birds are adapted for running, hopping, or perching. The
flexibility of the neck and the bill make up for the use of the forelimbs as wings and the
concomitant loss of hands. The synsacrum (a structure formed by fused vertebrae and
pelvic bones) and the pygostyle support and steady the pelvic region while walking,
hopping, and flying.
Bird flight alternates between gliding and flapping flight; it is a complex process
requiring a lot of energy; many mitochondria produce ATP and extensive vascularization
brings oxygenated blood to the flight muscles. The keel on the sternum is enlarged for
attachment of the strong pectoral flight muscles. The pectoral muscles are attached to the
sternum and the furcula (united clavicles); they run to the humerus. The airfoil design
creates lift. Slotting of feathers and the alula decrease turbulence; turbulence would
decrease lift. The distal part of the wing provides most of the force of flight—the down
stroke is the power stroke; the up stroke is the recovery stroke. Differences in wing
shape, length and aspect result in different flying patterns, speeds, etc. The tail functions
in balance, steering, braking, and increases lift.
Other adaptations seen in birds include bills and tongues of birds adapted to the diet, and
a storage compartment in the esophagus (crop). The stomach is modified into the
proventriculus, involved in chemical digestion, and the ventriculus (gizzard), involved in
mechanical digestion. Birds have a circulatory system like that of other reptiles, with a
separate pulmonary and systemic circulation. A four-chambered heart, rapid heart rate,
and a large heart characterize birds—in birds, the sinus venosus is reduced, and only
remains as the pacemaker in the right atrium. Birds are endothermic, and maintain
resting temperatures higher than most mammals, but some birds become torpid to save
energy. The syrinx produces sound and is located at the bifurcation of the trachea.
The nervous system of birds is adapted for a highly active life. The cerebrum (forebrain)
is much enlarged, compared to other reptiles—it is involved in visual learning, courtship,
nesting and feeding. Vision in birds is also advanced with large, often double-focusing,
eyes. These eyes have two foveae (focal points) per eye; one for monocular vision, and
the other for binocular vision and depth perception. Some birds have a 360° monocular
field of vision. Many birds have well-developed hearing, but olfaction is of minor
importance.
Birds, and other reptiles, excrete uric acid through the cloaca, which allows them to
conserve water. Some birds, particularly marine birds, have supraorbital salt glands for
excretion of salt.
All birds are oviparous. Most birds are monogamous and exhibit parental care
(correlated with the lack of ability to control resources), although monogamy typically
lasts for a single season. Some birds are polygynous, a small number are polyandrous.
Many exhibit elaborate courtship behaviors. Nesting of birds varies from species to
species; the number of eggs laid (clutch size) also varies, as does the incubation period.
Precocial birds are able to care for themselves soon after hatching, altricial birds are
much more helpless. Migration (defined as periodic round-trips between breeding and
non-breeding areas) allows birds to exploit environments that are favorable at different
seasons. Birds navigate by auditory and visual cues (using landmarks, the sun, moon and
stars), as well as the earth’s magnetic field.
The respiratory system in birds is unique because the connection of air sacs and the lungs
creates a flow-through system with maximal efficiency. Air flows from the bronchi to
the posterior air sacs, and then with the exhalation, the air moves to the parabronchi.
With the next inhalation, that bolus of air moves to the anterior air sacs, and with the next
exhalation, out. This is not an in-and-out form of respiration; it is a efficient flowthrough system with almost no dead air space
Mammals!
Lecture Outline
The adaptive radiation of the mammals was coincident with the extinction of many
reptilian lineages at the beginning of the Tertiary period. Early synapsids included the
pelycosaurs) and the therapsids; these groups contained both predators and herbivores.
The last therapsids were a group of animals called cynodonts where mammal features
evolved over a 200 million year period. There were changes in the animal’s gait and
longer legs to hold the body off the ground. The first mammals were small (< 10 cm),
probably nocturnal, and had well-developed senses of smell and hearing. They were
possibly endothermic and probably had hair.
One group of mammals, the prototherians, are all extinct, but the extant species of the
class Mammalia may be characterized by hair, mammary glands, specialized teeth and
three middle ear bones. The subclass Theria is divided into three infraclasses:
Infraclass Ornithodelphia contains the monotremes.
Infraclass Metatheria contains the marsupials.
Infraclass Eutheria contains the placental mammals.
Some of the most distinctive features of mammals involve adaptations of the epidermis
and the skeletal system. In the epidermal system:
Hair is a unique structure with many functions, including protection, sensory
perception, recognition, camouflage, and temperature regulation—hair must be
molted, gradually or all at once. Claws, nails, or hooves are keratinized epidermal
structures. Sebaceous glands are associated with hair follicles and produce oil.
Sudoriferous glands produce sweat, which functions in evaporative cooling in a few
mammals and in making odiferous products in more mammals. Scent glands may
produce pheromones. Mammary glands are derived from apocrine sweat glands, and
function to provide nourishment for the young—in monotremes, these glands lack
nipples.
In the skeletal system:
Mammalian skulls are characterized by three middle ear bones, a secondary palate,
heterodont teeth, and two sets of teeth. The adult set of teeth has up to 4 different
kinds of teeth. Incisors are the most anterior teeth, canines are used for biting prey,
premolars and molars are used for chewing. The diet of a mammal (herbivore,
carnivore, or omnivore) can be distinguished from the dentition. Horns are
keratinized sheaths covering a bony spike; antlers are made entirely of bone.
Many organ systems of mammals are similar to those of other vertebrates. The digestive
system of mammals reflects their diets, so some herbivorous mammals have a cecum to
aid in digestion of cellulose. Ruminants have a four-chambered stomach to allow
fermentation of cellulose by microorganisms.
The circulatory system of a fetus differs in several ways from the adult, mainly due to the
lungs not being inflated and the fact that the placenta supplies gases to the fetus.
A muscular diaphragm facilitates inspiration in mammals. The hearts of birds and
mammals are four chambered, as a result of convergent evolution.
Mammals may produce heat by shivering thermogenesis and non-shivering
thermogenesis. Heat conservation may be accomplished by a thick pellage, fat, and
countercurrent heat-exchange systems. Heat loss takes place by evaporative cooling
(sweating or panting), or by radiation into the air from surface blood vessels. True
hibernation includes many metabolic changes, including a drop in body temperature, and
is distinguished from the more alert state of winter sleep.
Adaptations of the nervous system that are particularly prominent in mammals include
well-developed senses of touch, smell, hearing and vision—the cerebral cortex is
enlarged.
The metanephric kidney of mammals produces urea that is excreted with some water loss.
A lengthened loop of the nephron allows mammals to produce hypertonic urine,
particularly important in desert dwellers.
Behaviors of mammals (vocalizations and tactile communications) are associated with
interspecific and intraspecific interactions. Pheromones facilitate intraspecific
communication in the areas of sexuality, territoriality, and recognition of young.
Territoriality involves defense of an area or resource.
The monotremes are oviparous, but nearly all other mammals are viviparous. Estrus is a
time in which a female mammal is sexually receptive. Delayed fertilization (e.g., sperm
storage in bats) and embryonic diapause occur in a few mammals. All mammals, other
than monotremes, have a gestation period in which the embryo receives nutrition via the
placenta. Mammals typically exhibit parental care.