Download Animal Diversity Background

Kingdom Animalia (select phyla)
Background information on Select Phyla of the Kingdom Animalia
I.
Phylum Porifera (sponges)
Background: The sponges are the least complex of all multicellular animals. They are asymmetrical, with
poorly defined tissues and no organs. In fact, if the cells of a sponge are separated, the cells become amoeboid
and reaggregate and redifferentiate into a new sponge without regard to their previous roles.
The body of a sponge is organized around a system of water canals. Water is drawn through small pores
into a central cavity, the spongocoel, and then flows out through a larger opening, the osculum. Cells of the
sponge body are differentiated by function. Flattened epithelial cells cover the outer surface. On the inner
surface, special flagellated cells called choanocytes or “collar cells”, strain extremely small particles from the
water and thus serve in filter-feeding. In the middle jellylike layer, wandering amoebocytes secrete a skeleton
composed of calcium carbonate (CaCO3), silicon dioxide (SiO2), or a protein called spongin. Calcareous and
siliceous sponges are hard due to the presence of tiny rodlike skeletal elements called spicules. The natural
sponges you might buy for bathing or to wash your car are soft and are made of a skeletal network of spongin
fibers.
Most sponges are marine, but a few live in fresh water. As adults, all are sessile (attached to a
substrate). They can reproduce asexually by budding or fragmentation and sexually by production of eggs and
sperm. Most sponges are hermaphroditic (or monoecious); each individual has both male and female gonads.
The zygote develops into a free-swimming, flagellated larva—free-swimming hollow ball of flagellated cells
that resembles the embryonic blastula of other organisms. When the larva settles and attaches to a substrate, the
external cells lose their flagella and move to the interior in a process of cellular reorganization much like that of
gastrulation in other animals.
II.
Phylum Cnidaria (“stinging” Animals)
Background: Organisms in the phylum Cnidaria, and all those that are higher on the phylogenetic tree, have
distinct cell layers and are radially symmetrical. Symmetry implies a higher degree of complexity and
organization than the asymmetrical organization characteristic of the sponges. These organisms are diploblastic
(2 embryonic tissue layers), and the body contains a gastrovascular cavity that is responsible for digestion,
circulation and excretion of particles into and out of the body.
All remaining phyla of animals consist of organisms that are triploblastic and possess bilateral symmetry at
some time in their life history. The representatives of these diverse phyla considered in this exercise exhibit
many important evolutionary advances. One of these is the mesoderm, a third distinct embryonic tissue layer
between the ectoderm and endoderm (hence the term triploblastic). Our study of these phyla will reveal
additional advances in organization and function.
III.
Phylum Platyhelminthes
Background: “Flatworms”—flattened, unsegmented worms—include planarians (class Turbellaria), flukes
(class Trematoda), and tapeworms (class Cestodea). They are acoelomate (lack a coelom), with body organs
embedded in their mesodermal tissues. Platyhelminthes exhibit the first extensive organ-system level of
development.
Free-living flatworms, the turbellarians, are small, and most are marine, living on or in the bottom
sediments. Locomotion is by cilia and, in some larger flatworms undulating muscular movements may help.
The nervous system includes a small anterior ganglionic “brain” and longitudinal nerve cords. “Eyespots”
consist of concentrations of pigment (melanin) that shade photoreceptive neurons (ocelli). The turbellarian
digestive tract is a blind sac with no anus—the mouth is used for both ingestion and egestion. Primitive
osmoregulatory structures, protonephridia (flame cells), are also present. Turbellarians are hermaphroditic and
larvae are free-swimming.
Lab: Kingdom Animalia
Adult flukes, the trematodes, are all parasites, either internal or external. Flukes are flattened and have a
ventral sucker or other adhesive organ for attaching to their host. In some trematodes, a second sucker is
associated with the anterior mouth. Most flukes are hermaphroditic. The life cycle may involve one to four
hosts—intermediate hosts (hosts that harbor the immature stages) may be invertebrates, but the definitive host
(the host that harbors the sexually mature stage) is always a vertebrate.
Tapeworms, the cestodes, are internal parasites of vertebrates and are highly adapted for a hostile
environment, where they nonetheless enjoy a rich food supply provided by their host. Like fluke, tapeworms
are hermaphroditic, and their life cycle may involve an intermediate host in which a “bladder worm” stage
encysts, awaiting ingestion by the definitive host.
IV.
Phylum Mollusca
Background: Molluscs represent the second largest phylum, consisting of more than 93,000 living species of
marine, freshwater, and terrestrial animals. They are bilaterally symmetrical and apparently unsegmented.
Note that we have now reached a branching point in the family tree of animal phyla. The molluscs, and all
organisms above them in the phylogenetic tree, have true coeloms (coelomates). The mollusc coelom is formed
by a splitting of the embryonic mesoderm and is said to be schizocoelous (schizo means split). This type of
coelom is also found among arthropods and annelids. Note that these three groups are on the same branch of
the phylogenetic tree. Organisms on the other side of the branch point are also coelomate, but their coelom is
formed by an outpocketing, or evagination, of the primitive gut, or archenteron. Hence the resulting coelomic
cavity is said to be enterocoelous. This type of coelom is found among the echinoderms and chordates.
The general body plan of a mollusc includes three regions: the head-foot (used in locomotion and food
capture), the visceral mass (containing the major organ systems), and the mantle (soft tissue that secretes the
calcium-containing shell present in many molluscs).
Molluscs have an open circulatory system with a chambered heart (one ventricle and two atria) and their
blood contains an oxygen-carrying respiratory pigment, hemocyanin. Excretory organs, the metanephridia,
drain the relatively small coelom surrounding the heart and a portion of the intestine. Gills are present in the
mantle cavity of most molluscs. Molluscs may be filter-feeders, sediment-feeders, herbivores, or carnivores.
Molluscs with shells include species that have shells formed from several plates (chitons), hinged shells
(bivalves—including clams, oysters, and scallops), conical twisted shells (gastropods—including snails), and
reduced or internalized shells (cephalopods—including squids and octopi).
V.
Phylum Annelida
Background: Phylum Annelida contains the segmented worms, almost all of which are free-living.
The coelom of annelids is compartmentalized into segments by septa. Coelomic fluid within the body
cavity acts like a hydrostatic skeleton against which muscles work to change body shape. Like nematodes,
annelids have a one-way digestive tract with a mouth, anus, and several specialized regions.
A dorsal mass of nerve cells forming a ganglion or “brain” and a ventral nerve cord provide a primitive
nervous system. The circulatory system is closed, blood being confined to blood vessels.
Marine polychaetes (sand worms), oligochaetes (freshwater annelids and earthworms), and leeches are
among the most common annelids. There are over 8,700 known species of annelid worms.
Many oligochaetes such as the earthworm have taken up a terrestrial life but have not completely
adapted to land; they secrete a thick mucus to keep their skin moist and must remain within their damp burrows
in the ground unless the surface is soaking wet. They eat soil and decaying vegetation and have a welldeveloped complete digestive system to process their vegetarian diet. Earthworms occur in the soil in large
numbers and are extremely important in maintaining the soil’s loose, aerated condition, because they process
their body weight in soil each day.
The earthworm’s head is much reduced to adapt to its burrowing way of life. As an oligochaete (“few
bristles”), it has only four pairs of setae (chaetae) on each segment for traction instead of the parapodia found in
polychaetes. Gas exchange takes place over the entire moist body surface and is aided by an efficient closed
circulatory system in which the blood, containing hemoglobin, is moved by 10 little pumps (“hearts”).
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Lab: Kingdom Animalia
The individual earthworm is hermaphroditic, producing both eggs and sperm, but copulates with another
worm rather than fertilizing itself.
The polychaetes (class Polychaeta) make up the largest group of annelids. Most are marine and are an
important food source for fish and crustaceans. Polychaetes have parapodia, fleshy appendages on the body
segments. Setae (chaetae) are found on the parapodia. The polychaetes have well-developed sense organs on
their heads, including eyes, antennae, and chemoreceptors. Some polychaetes build tubes to live in. Many of
these sedentary forms use tentacles covered with cilia to trap food such as tiny animals and decaying organic
matter and transport it to the mouth. Others pump water through their burrows and filter food from the water.
Most leeches (class Hirudinea) live in fresh water. They are parasitic or predaceous, feeding on tissue
fluids, blood, or small invertebrates. Leeches lack the setae (chaetae) characteristic of other members of the
phylum
VI.
Phylum Nematoda
Background: Nematodes (roundworms) are small, generally free-living animals with an anterior mouth and
sense organs, but no well-defined head. Cilia are reduced and the body is covered with a secreted cuticle. The
digestive tract is usually complete and has a specialized “pharynx”. Most of these organisms have a
protonephridia. Many of the nematodes have a constant number of nuclei in various organs, and mitosis ceases
following embryonic development. The sexes are separate (dioecious) in most of these organisms. The phylum
Nematoda consists of about 25,000 known species.
The nematodes are unsegmented worms that may be either free-living or parasitic. Many free-living
forms are inhabitants of the soil. Parasitic forms invade plant bodies and destroy tissues. Common animal
parasites include hookworms, intestinal roundworms (Ascaris), Trichinella, and pinworms.
In contrast to the acoelomate flatworms, the nematodes have a type of body cavity (coelom). A true
coelom is completely lined with mesoderm and will not be found until we progress to organisms in more
advanced phyla. In nematodes, the coelom is a pseudocoelom because it is only partially lined by mesoderm.
Nematodes also have a true gut with two openings—a mouth and an anus—allowing regions of the digestive
tract to assume specialized functions
VII. Phylum Arthropoda
Background: Arthropods are by far the most numerous and diverse of all animals, with more than 1 million
known species. Marine, freshwater, or terrestrial forms are found in every conceivable habitat due to their high
degree of evolutionarily adaptability and their great mobility, including for some the ability to fly. Two out of
every three known species are arthropods!
The arthropod body is covered by an exoskeleton made of layers of protein and chitin, a polysaccharide.
The arthropod body is typically divided into three segments: the head, thorax and abdomen. Each of these may
be subdivided into several segments to which are attached jointed appendages that carry out a variety of
functions. As arthropods grow they shed their chitinous exoskeleton by the process of molting (ecdysis).
During this growth, many arthropods may also undergo a marked change in form (metamorphosis). If this is the
case, the larva, feeding stage, often bears no resemblance to the adult.
The arthropod circulatory system is open: a distinct muscular heart pumps blood through open spaces in
the tissues—the hemocoel. The coelom, correspondingly, has been reduced and is represented in most
arthropods only by the cavity of the gonads. The digestive tract of arthropods is well developed and modified
into several distinct parts. The nervous system and associated sense organs are particularly well-developed and
control a variety of complex behaviors, including flight in winged insects.
The group of organisms we collectively call arthropods appears to have diverged into 4 subphyla. We
will be looking at 7 classes within these subphyla. The subphylum Crustacea includes Class Malacostraca
which are the crabs, lobsters, and shrimps, and Class Maxillopoda which include barnacles and copopods. The
crustacea have biramous (two-branched) appendages, two pairs of antennae, mandibles (jaws), and a pair of
compound eyes. The subphylum Chelicerata includes Class Arachnida (spiders, mites, ticks, scorpions) and
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Lab: Kingdom Animalia
Class Merostomata (horseshoe crabs). Chelicerates have 4 pair of walking legs and lack antennae and
mandibles (jaws). Instead, the first pair of appendages, the chelicerae, are in the forms of pincers or fangs.
Members of the subphylum Hexapoda includes Class Insecta (insects), which have 3 pair of walking legs in
adult form, one pair of antennae, compound eyes, and usually 2 pair of wings. They have 3 distinct body
segments and go through either complete or incomplete metamorphosis during their lifetime. Subphylum
Myriopoda includes Class Chilopoda (centipedes), and Class Diplopoda (millipedes). These organisms have
1 pair of antennae and unbranched (uniramous) appendages. The centipedes have one set of legs per body
segment, whereas the millipedes have two sets of legs per segment.
The origin of arthropods remains a mystery. Systematists originally believed that members of the
phylum Onychophora, a group of worm-like animals with claw-bearing legs and a thin chitinous cuticle,
represented a type of common ancestor, a “missing link” between the annelids and arthropods. Ribosomal RNA
evidence, however, suggests that onychophorans are true arthropods. Members of a second phylum,
Tardigrada, “the water bears”, are similar to the onychophorans. Most likely these two groups are similar in
structure to early arthropods but do not represent common ancestral lines.
VIII. Phylum Echinodermata
Background: The phylum Echinodermata includes four major groups of marine bottom dwellers or burrowers:
the sea stars, brittle stars, sea urchins, sand dollars, sea lilies, and sea cucumbers are all examples. Echinoderms
are noted for their spiny protective skin, their “five-part” body plan, and the presence of numerous small
appendages, the tube feet, which function as part of a water vascular system derived from the coelom. The tube
feet are used for locomotion, feeding, and respiration. The coelom carries out circulatory, respiratory, and
excretory functions.
Developmentally and evolutionarily, echinoderms are more closely related to chordates than any of the
groups studied thus far in lab. Like chordates, they have deuterostome development, and DNA comparisons
have reinforced the Deuterostomia as a true clade (monophyletic grouping).
Unlike arthropods, which have exoskeletons, echinoderms have internal skeletons. The skeleton is
composed of flattened calcareous plates called ossicles. Spines are outward extensions of the plates and are
characteristic of the echinoderms, often called the “spiny-skinned” animals. In addition to spines, some
echinoderms also have pedicellaria extending from their surfaces. These are small pincers that aid in capturing
food and keeping the body surface clean.
Though often showing radial symmetry in their adult form, all echinoderms have bilateral symmetry at
some point in development, and many show superficial rather than true radial symmetry as adults. The terms
dorsal and ventral are not usually used to describe radially symmetrical organisms. Instead, the terms oral (on
the side of the mouth) and aboral (on the side opposite the mouth) are preferred. The mouth of most radially
symmetrical echinoderms is on the lower surface. Class Asteroidea (star fish) and Class Echinoidea (sea
urchins and sand dollars) and Class Holothuroidea (sea cucumbers) are represented in lab today.
IX.
Phylum Chordata
Background: All chordates share 4 common characteristics: 1) a flexible but incompressible supporting
skeletal rod called the notochord, from which the name Chordata is derived; 2) a dorsal hollow nerve cord lying
above the notochord; 3) pharyngeal pouches, also called “gill pouches”, located in the pharynx (anterior region
of the gut); and 4) a post anal tail.
The phylum Chordata includes two subphyla, Urochordata and Cephalochordata, that do not have
backbones. These invertebrate chordates are also referred to as protochordates. A third subphylum, Vertebrata,
contains organisms that have a backbone, a bony spinal or vertebral column that replaces most of the notochord
and encases the nerve cord and a skull that encloses the anterior brain. There are seven living classes of
vertebrates that we will examine in this lab.
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Lab: Kingdom Animalia
A) Subphylum Urochordata (sea squirts and tunicates)
Background: Tunicates or urochordates are a relatively diverse group of marine filter-feeders. They are
solitary or colonial and may be either attached to the substrate or free-floating. The most familiar forms are
ascidians or sea squirts. Adult sea squirts can be found attached to pilings or rocks in the intertidal zone of our
coasts. Larvae of solitary forms, however are free-swimming, bilaterally symmetrical “tadpoles” with the four
characteristic chordate features (notochord, pharyngeal gill slits, dorsal hollow nerve cord, and a post anal tail).
The larvae locate an appropriate substrate, attach with a sucker at their head end, and undergo a metamorphosis
during which most of the chordate characteristics are lost. Great interest has focused on this larva because it is
a typical chordate, whereas the adult sea squirts bear little resemblance to any other representative of the
phylum.
B) Subphylum Cephalochordata (lancelets)
Background: Another small subphylum of chordates, the cephalochordates, contains small, bilaterally
symmetrical marine organisms with all characteristic features of chordates. However, unlike all other
representatives of the phylum, in cephalochordates the notochord extends all the way to the front of the head,
beyond the anterior end of the dorsal nerve cord. In other chordates, the notochord ends behind the expanded
part of the dorsal nerve cord or cerebral vesicle (behind the forebrain in vertebrates). Thus the name
cephalochordata (cephalo-, head) is appropriate for this group.
C) Subphylum Vertebrata
Background: The third subphylum of the Phylum Chordata contains organisms that have a backbone, a bony
spinal or vertebral column that replaces most of the notochord and encases the dorsal nerve cord, and a skull
surrounding the brain, an anterior expansion of the nerve cord. These organisms are called vertebrates. There
are seven vertebrate classes with living representatives.
1. Aquatic Vertebrates (generally speaking):
According to the fossil record, the Superclass Agnatha or, “jawless fish”, are the oldest known fish. At
one time, they had bony skeletons, but the skeletons of modern representatives in Class Cephalaspidomorphi
(Class Petromyzontida), the lampreys, are composed of cartilage. Lampreys feed upon their prey by attaching
to the skin and sucking blood from soft tissues—that is, the adult forms are parasites. The jawless fish have
external gill slits.
Modern representatives of the Class Chondrichthyes include the rays and sharks. These cartilaginous
fish, like the Agnathans, have external gill slits. They also have no swim bladder.
The Superclass Osteichthyes is made up of the “modern, bony” fish and is divided into 2 classes, Class
Actinopterygii (ray-finned fish) and Class Sarcopterygii (lobe-finned fish, e.g., coelacanth & lung fish). Most
have bony endoskeletons. This group includes many popular marine and freshwater fishes. The bony fish have
gills covered by a flap, or operculum. The skin of the bony fish is composed of scales.
2.
“Transitional” Vertebrates (Class Amphibia)
Organisms of the Class Amphibia are incompletely adapted to terrestrial environments. Even in those
species that possess lungs, some gas exchange must occur through their skin. This requires that the skin be kept
moist, a condition that prevents the amphibians from living in a strictly terrestrial environment. Amphibians
also depend on water for reproduction. In many amphibians, such as the frog, eggs are laid in freshwater ponds
or streams and are fertilized externally. The tadpole larva is a swimming stage that undergoes metamorphosis
into an adult frog. Frogs may live on land, but most must return to the water to lay their eggs.
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Lab: Kingdom Animalia
3. Terrestrial Vertebrates (generally speaking): Reptilia, Aves, and Mammalia
Unlike amphibians, most reptiles lead strictly terrestrial lives. There are two major adaptations that
allow this lifestyle. First, the skin of most reptiles is a tough, scaly skin that retards water loss and does not
need to be kept moist. Reptiles respire primarily through the respiratory membranes of the lungs as cutaneous
(skin) respiration is restricted by their tough skin. Second, reptilian eggs have hard or leathery shells reducing
water loss from the embryo to the environment. Although they are usually laid in moist environments, they do
not need to be laid in aquatic environments like amphibian eggs. The egg of the reptile contains all the food and
water needed for complete embryonic development.
Birds (Class Aves) trace their ancestry to groups within the Class Reptilia. The birds exhibit special
adaptations to allow flight for most organisms. In lecture (and in your textbook), we will discuss the potential
origins of flight, and the adaptations that led to this phenomenon. In short, the forelimb musculature of birds
has adapted to provide the lift necessary to attain and maintain flight. Birds also evolved feathery covering over
most of their skin, although the scaly feet generally lend a clue to their evolutionary origin (reptilian). Avian
eggs generally have even thicker shells than reptilian eggs allowing for them to be exposed to dry environments
without desiccation. In lab, you will examine homologous skeletal structures of avian skeletons and
mammalian skeletons.
Class Mammalia also trace their ancestry to the Class Reptilia. Mammals have several adaptations that
distinguish them as a unique class of vertebrates. Females have mammary glands that produce milk to nourish
their young. Most young are incubated internally by the female, and born live. Mammals also have hair (fur)
as their external covering. Most mammals are homeothermic and endothermic maintaining a constant body
temperature largely through metabolic chemical reactions.
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