28
The Animal Kingdom: An Introduction to Animal
Diversity
Lecture Outline
I.
Members of kingdom Animalia inhabit most environments
A. Animals probably evolved in shallow Precambrian seas during the Cambrian
explosion, and many still inhabit the seas
1. Sea water is isotonic with the body fluids of many animals
2. Marine life may be difficult due to currents
B. Fresh water is a less constant environment than sea water with respect to
oxygen, temperature, turbidity, and volume of the body of water
1. Food is generally less available and water tends to diffuse into the cells of
the organism
C. Terrestrial life is more difficult due to the problems of dehydration, and only
some invertebrates such as snails and arthropods, and vertebrates have been
successful on land
II.
Reconstructing phylogeny is a major challenge; often 35 phyla are identified
A. Animals are multicellular eukaryotes
B. Animals are heterotrophs
C. Animals are characterized by specialized cells and tissues
D. Animals respond to the environment
E. Most animals are motile
F. Most animals are diploid and reproduce sexually
III.
Animals inhabit varied habitats
A. Animals may be oceanic and planktonic
B. Animals may also inhabit hypotonic environments in fresh waters
C. Animals may be terrestrial, with the challenges of living in a non-aqueous
habitats
IV.
Animals can be classified according to body symmetry
A. Symmetry refers to the arrangement of structures relative to an axis of the
body
B. Organisms with radial symmetry have more than one plane that can divide
the body into equal halves
1. Many cnidarians exhibit radial symmetry
C. Bilaterally symmetrical organisms have one plane that divides the body into
equal halves
1. Bilaterally symmetric organisms have a right and left side
2.
Bilateral symmetry is an adaptation for cephalization (development of a
head end)
D. Bilaterally symmetrical animals have an anterior or cephalic (head) and
posterior or caudal (tail) end, a dorsal (back) and ventral belly side, and
medial (midline) and lateral (to the side) structures
1. The terms inferior and cephalic (superior) are relative terms referring to
structures below and above other structures
2. Sagittal planes divide the animal into right and left halves, frontal planes
divide it into dorsal and ventral parts, and transverse sections cross the
body and divide it into anterior and posterior parts
E. Animals can be grouped according to type of body cavity
1. During embryological development, three germ layers develop in
triploblastic organisms (cnidarians and ctenophores are diploblastic)
a) The ectoderm gives rise to the outer covering and nervous system
b) The mesoderm is the middle layer and gives rise to most internal
organs
c) The endoderm is the inner layer and forms the lining of the
gastrointestinal tract
2. In the "solid worms," which are the flatworms and a few relatives, there is
no body cavity (except for the digestive cavity), hence the name
acoelomates
3. Pseudocoelomates, which include nematodes and rotifers, have a false
body cavity between the mesoderm and the endoderm
4. Coelomates, which include all other animals, from annelids to vertebrates,
have a true coelom that is completely lined with mesoderm
a) The coelom develops between the ectoderm and the gut cavity
F. Coelomate animals form two main groups based on differences in
development
1. Animals can be classified as protostomes or deuterostomes
2. Animals with a true coelom may be divided into two groups that reflect
separate lines of evolution
3. In early embryonic development, the blastula invaginates form an opening,
the blastopore
a) In protostomes, the blastopore develops into the mouth
(1) Protostomes include mollusks, annelids and arthropods
(2) Protostomes also exhibit spiral cleavage, as cell divisions are
diagonal to the polar axis
(3) Protostomes exhibit determinate cleavage, so twinning is not
possible
(4) Coelom formation is schizocoelous, as the mesoderm splits to
form the coelom
b) In deuterostomes, the blastopore develops into the anus and a second
opening develops into the mouth
(1) Deuterostomes include echinoderms and chordates
(2) Deuterostomes exhibit radial cleavage, as cell divisions are
parallel or at a right angle to the polar axis
(3) Deuterostomes exhibit indeterminate cleavage, so twinning is
possible
(4) Coelom formation is enterocoelous, as the mesoderm forms by
outpocketings of the gut
G. Biologists are rethinking animal relationships based on molecular data,
including DNA, ribosomal RNA and Hoxgenes
1. Many biologists think most of the 35 extant phyla evolved during the
Cambrian explosion, but the knowledge of placozoans may change the
currently held theories
2. Others feel animal diversity began much earlier
3. Many scientists now divide the protostomes into two branches:
Lophotrochozoa and Ecdysoza
V.
Sponges have unique flagellated cells and are the only living members of the
Parazoa
A. Phylum Porifera consists of the sponges, and about 10,000 species have been
identified
B. Sponges are aquatic, mostly marine, and typically asymmetrical
1. Live sponges may be brightly colored, often colored by symbiotic bacteria
or algae
C. Sponges have choanocytes (collar cells), indicating evolutionary links to the
choanoflagellates
D. Sponges feed by bringing water through ostia into the spongocoel and finally
out the osculum
E. Sponges are multicellular, but do not have tissues
1. Epidermal cells cover the sponge and line the canals
2. Porocytes regulate the openings of the ostia
3. The choanocytes create the water current that moves food, dissolved gases,
and other materials in and out of the sponge
a) Choanocytes have a flagellum and a collar of microvilli
b) Food is trapped in the collar of the choanocyte
c) Gas exchange and excretion of wastes happen by diffusion in and out
of cells
4. Ameboid cells wander in the acellular mesophyl, which is supported by
spongin and/or spicules
a) These cells aid in digestion and food transport; others secrete the
spicules
F. Sponges may reproduce asexually by budding or fragmentation
G. Sponges may reproduce sexually by production of egg and sperm
1. Most sponges are hermaphroditic, but typically produce eggs and sperm at
different times to avoid self-fertilization
2.
Sperm are expelled from the sponge and taken in with the water current
into a nearby sponge
3. Fertilization and early development take place in the mesophyll
4. Embryos move out of the parent through the osculum, and swim about in
the plankton for a while, then settle down on the substrate
H. Sponges have a remarkable regenerative ability
VI.
Cnidarians possess radial symmetry and unique stinging cells
A. Hydrozoans, jellyfish, and corals belong to phylum Cnidaria
B. Approximately 10,000 species of cnidarians are classified in 4 classes (3 are
covered here)
1. The name Cnidaria refers to the cnidocytes, which are stinging cells
possessed by all members of this group
2. Cnidarians have two body shapes, the polyp and the medusa
a) Many cnidarians have both forms in their life history
b) The polyp has the mouth pointed up; the medusa has the mouth
pointed down
3. Some cnidarians are solitary, others are colonial in the traditional sense of
living in an aggregation, others (like the Portuguese man-of-war) are
colonies of dissimilar forms (medusae and polyps) forming one
"individual"
4. Cnidarians are radially symmetrical with a centrally located mouth
5. The mouth leads to the gastrovascular cavity
a) The mouth serves both for ingestion of food and expulsion of wastes
6. Cnidarians are diploblastic—having an outer epidermis, inner
gastrodermis, and acellular mesoglea in between
C. Class Hydrozoa includes solitary and colonial forms
1. Hydra is often studied in classrooms, but is not typical of the class, as it is
solitary and freshwater
a) Hydra has a hydrostatic skeleton formed by two sets of epidermal
contractile cells
2. Cnidocytes located in the epidermis, particularly on the tentacles, possess
organelles called nematocysts
a) When triggered, nematocysts release a coiled thread, which may be
sticky, barbed, or contain toxins
b) Triggering may be due to touching the cnidocil, or various chemicals
sensed in the water
3. After ingestion of the prey, digestion begins in the gastrovascular cavity,
and is completed intracellularly
4. Gas exchange and excretion occurs by diffusion
5. A simple nerve net is present
6. Reproduction occurs by budding, or sexually by production of eggs and
sperm
7. Colonial cnidarians form by budding of a single individual
a) Some specialized individuals may arise for feeding, reproduction, or
defense
8. The Portuguese man-of-war, Physalia, is not a jellyfish; rather it is a colony
of both hydrozoan medusae and polyps
9.
Some marine cnidarians alternate sexual and asexual stages—in contrast to
alternation of generation in plants, the cnidarians are all diploid (except for
the gametes)
10. The distinctive larva of the marine cnidarian is the planula
D. The medusa stage is dominant among the jellyfish
1. The jellyfish belong to class Scyphozoa
a) The polyp is only a small larval stage
E. The anthozoans have no medusa stage
1. Both the hard and soft corals, and the sea anemones, belong to the class
Anthozoa
2. The anthozoans only have a free-swimming planula larva
3. The gastrovascular cavity is divided into a number of chambers, which
increases the surface area available for digestion
4. Many anthozoans are dependent on symbiotic algae (zooxanthellae) for
nutrition
5. Coral reefs are among the most productive ecosystems on earth, and are
home to a wide variety of vertebrate and invertebrate species
VII. Comb jellies move by cilia and have adhesive glue cells that trap prey
A. Phylum Ctenophora includes the comb jellies
B. Members of this small (both in size and number of species) phylum are also
called sea walnuts or sea gooseberries as they are commonly spherical
C. The name comb jelly refers to the 8 rows of cilia that aid in locomotion
D. They are bilaterally symmetrical and have tentacles with sticky cells, which
previously were believed to be identical to cnidocytes (hence their prior
lumping with the cnidarians in the older terminology of phylum
Coelenterata)
Research and Discussion Topics

The presence or absence of a coelom, and the distinction between a true and
pseudocoelom, have been emphasized. What really is the importance of a body
cavity? Is it just a distinction for the sake of making taxonomic groupings? Do
humans have a coelom?

Coral reef ecosystems have been very much threatened in recent years, due to a
phenomenon known as "coral bleaching," collecting of coral for aquariums, the use
of poisons in coral reefs to collect fish, anchors from cruise ships and pollution in
general. Find out more about these causes of coral reef destruction.

One class of cnidarians not mentioned in the text is class Cubozoa, the sea wasps.
Find out about this small group, which may be quite problematic in some areas,
including Australia.