• All animals are members of the Kingdom Animalia, also called
• All members of Animalia are:
• Most ingest food and digest it in an internal cavity.
• Animal cells lack the rigid cell walls that characterize plant cells.
• Most animals are capable of complex and relatively rapid movement
compared to other organisms.
• Most reproduce sexually, by means of differentiated eggs and sperm.
• Most animals are diploid.
• The development of most animals is characterized by distinctive
- a zygote, is the product of fertilization;
- a blastula, which is a hollow ball of cells formed by the
- a gastrula, which is composed of three germ layers:
• The classification of animals is based on:
• body plan
• germ layers
• level of organization
• coelom (cavity lined by epithelium)
Animals- Body Plans
• A sac body plan: is one in which the
mouth is used as both an entrance for
nutrient molecules and an exit for waste
• A tube-within-tube body plan: Animals
with a mouth and an anus have this body
plan. This plan allows specialization of
parts along the digestive canal to occur.
• Radial symmetry means that any
longitudinal cut that passes through a
midpoint gives two equal halves.
• Bilaterally symmetrical animals can
be cut into right and left halves and
generally have well-developed head
Animals- Number of Germ Layers
• During the embryological development, there may be only
two germ layer, ectoderm and endoderm; or three germ
layers, ectoderm, endoderm, and mesoderm.
• The presence of mesoderm is associated with the development of
internal organs associated with higher animals such as skeletons
and reproductive organs.
Animals- Level of Organization
• Some animals do not have organs and instead have a tissue level of
• Most animals have the organ system level of organization.
• Is the body cavity in which
the internal organs are found.
• Acoelomates: are animals that
do not have a coelom.
• Pseudocoelomates: have a
body cavity incompletely lined
• True coelomates: have a body
cavity completely lined by
• When an animal is segmented, it has repeating parts like the
vertebrae in the human backbone. Segmentation has led to
specialization of parts in advanced animals.
• Cephalization, having a well-developed head, is also a characteristic
of the most advanced animals.
Phylum Porifera (Sponges)
• The name stems from the meaning of ("pore bearers").
• are multicellular
• Still, they have a very simple grade of organization
• the body consists of a loose aggregation of cells embedded in a gelatinous
• They lack any real organs or true tissues,
• Nonetheless, they do contain different types of cells.
• Cells include:
• epithelial-like cells called pinacocytes that cover the
exterior and non-flagellated interior surfaces
• flagellated cells called choanocytes that line inner
chambers and canals
• amoeboid cells called archeocytes that move about in the
mesohyl (the gelatinous matrix within a sponge).
• They can differentiate into other types of cells that form parts
of the skeleton
composed of collagen and/or spongin fibers often
embedded with calcareous or siliceous crystalline spicules).
• Sponges show radial symmetry or none.
• A spongocoel is the large, central cavity of sponges.
• Water enters the spongocoel through hundreds of tiny pores (Ostia)
• Water exits the spongocoel through the larger opening (osculum).
• Most species are marine
• a few are found in freshwater.
• few animals appear to eat sponges, probably
because a mouth full of spongin and spicules is
not too appetizing!
• However, a few reef fish and the hawksbill turtle
feed exclusively on sponges.
• Sponges provide homes for many organisms,
particularly those anchored to coral reefs.
• Cnidarians are radially symmetrical animals that have a circle of
tentacles surrounding the mouth.
• Cnidarians are characterized by the presence of stinging cells called
cnidocytes that are used for the capture of prey and defense.
• Cnidarians have two true tissue layers:
• an outer epidermis
• an inner gastrodermis.
• A connective tissue layer called the mesoglea separates the two tissue layers.
• The basic body plan includes a mouth that opens into a blind
gastrovascular cavity, which is lined with the gastrodermis.
Cnidarians can be:
- Sessile polyps in which the
mouth points upwards
(hydra and sea anemones)
- swimming medusas in
which the mouth points
downwards (e.g., jellyfish)
Phylum Platyhelminthes (Flatworms)
• Members of this phylum are bilaterally symmetrical and dorsoventrally
• They have three embryonic body layers: ectoderm, mesoderm, and
• The gut is a gastrovascular cavity and has the functions of both digestion
• the mouth serves as both entrance and exit from the gut.
• These animals are the simplest form of animal life to have organ systems, a
central nervous system, and a well-defined rostral or head end, with an
opposite caudal or tail end.
• Flatworms are acoelomate, i.e., the outside of the gut wall is directly
continuous with the mesoderm; there is no coelomic cavity and so the gut
is not free inside the body.
• The mesoderm is muscular and is used for movement.
• Ectodermal cells may be ciliated and provide another means of
• Three classes of the phylum Platyhelminthes exist:
- class Turbellaria
- class Trematoda
- class Cestoda.
• Members of the class Turbellaria are free living (not parasitic).
• Planaria lives in fresh water ponds
• is a carnivore (animal eating).
• The diet consists of such foods as insect larvae, small crustaceans, and
other small living and dead animals.
• Planarians reproduce asexually and sexually; individuals have both ovaries
and testes. 24
• They glide over surfaces by the action of cilia; muscular contractions
wrinkle or bend the body.
• There is clearly a head end with a pair of black, light sensitive spots.
• The color of planaria would make them blend in and be invisible
among the leaves on the bottom of a pond.
• The gastrovascular cavity is highly branched, because it serves also
for circulation of nutrients to every cell.
• A planarian cross section has
three or four slices through
• Most obvious is the one from
the middle with the great
circular, muscular tube of the
• around it exists a space.
• The gut is continuous with the cells of the surrounding mesoderm.
• the gut is lined with cells of endoderm
• the outside of the animal is covered with ciliated cells of ectoderm.
• The mesoderm is muscular.
• Trematodes are commonly called flukes, and they are parasitic.
• The life cycles of parasites are complex, with intermediate hosts and
Liver flukes- Opisthorchis
• Opisthorchis is the liver fluke.
• The anterior end has a mouth at the end, a sucker disc for holding on; and
a ventral sucker, which is somewhat farther posterior.
• The intestine divides into two simple tubes near the head end.
• The dark organ (colored) in the center of the body is the uterus and it
is filled with eggs; the ovary is just posterior to it and appears as a
• Flukes have a seminal receptacle at the posterior end. The branched
organ in the posterior part of the body is the testes.
• Trematode life cycle: The liver fluke is an important human parasite
that is widely distributed in the Far East. Human infection occurs due
to eating raw fish.
• The small brown eggs of the fluke exit from a person's body in the
feces. The eggs hatch when they are eaten by certain kinds of fresh
• The hatchling is called a miracidium; after several metamorphoses,
the parasite leaves the snail and infects fresh water fish; it encysts in
their flesh. 31
• When a person eats these fish uncooked or insufficiently cooked, the
live parasites enter the digestive system and migrate from the small
intestine via the bile duct to the liver.
• There they mature into adult liver flukes and in about 3 weeks begin
to produce eggs. Eggs return via the bile duct to the digestive system
and exit from the body in feces.
• The Cestoda are parasitic tapeworms. The life cycles are complex.
• All tapeworms are extremely flat
• the body is divided into segments, and there is no digestive system.
• They absorb nutrients across their body walls.
• is the term for the head end of a tapeworm
• it has a disc of hooks at the tip, which anchor it into the lining of the
host's intestine, and four large suckers for holding on.
• New segments or proglottids are generated behind the scolex.
• As you move down the worm away from the head, these segments
get larger. Each is a complete reproductive machine
with testes, ovaries and uterus.
• The figure below shows mature proglottids. You can see a branched
uterus containing hundreds of eggs in each segment.
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