Introduction to
Kingdom Animalia
Defining Animals
1. Animals are multicellular, heterotrophic eukaryotes.
2. Animals generally store their carbohydrate reserves as glycogen
3. Animal cells possess a number of unique cellular structures, e.g.,
gap junctions - intracellular junctions that allow the passage of
materials between cells
4. Animals possess special tissues that are responsible for impulse
conduction (nervous tissue) and movement (muscle tissue).
5. Most animals reproduce sexually, with the diploid stage
dominating the life cycle
Animal Phylogeny
• Origin of most animal phyla and major body plans took place in Precambrian era
and early Cambrian period of the Paleozoic era (circa 500-600 mya)
• Most systematists agree that the animal kingdom is monophyletic
• By the early Cambrian period (about 550 mya), virtually all known animal phyla
had evolved from the first animals of the late Precambrian
Trends in Animal Evolution
1. The first trend was a shift from a body plan called radial symmetry to
a body plan referred to as bilateral symmetry
• Radial symmetry - a circular body plan having a central axis from
which structures radiate outward
• Bilateral symmetry - a body plan in which the right and left sides of
the body are mirror images of each other; central longitudinal plane
divides the body into 2 equal but opposite halves
Trends in Animal Evolution cont.
2. A trend toward increasing cephalization - the development of a
distinct head, which has associated with it a brain and various
kinds of sensory structures
3. A change from a simple sac-like body with a single opening at
one end to a more complex, elongated body containing a tube
called the "gut" with openings at both ends
4. A change from a tube enclosed in solid tissue toward suspension
of the tube in a fluid filled space
5. An increase in body segmentation - the development of a series
of body units, each containing similar sets of muscles, blood
vessels, nerves. etc.
Major Events in
Animal Phylogeny
Major Events in Animal Phylogeny cont.
1. The Parazoa-Eumetazoa Split
Two basic kinds of animals
evolved early in animal evolution.
A. Parazoa ("beside the animal") animals that lack true tissues. e.g.,
Sponges
B. Eumetazoa - animals with well
defined tissue layers,
e.g., essentially all other animals.
Major Events in Animal Phylogeny cont.
2. The Radiata-Bilateria Split
• The eumetazoa are divided into
2 major branches depending on
the type of body symmetry
•Some organisms exhibit radial
symmetry, and are called the
Radiata
• Other eumetazoa exhibit
bilateral symmetry, and are
called the Bilateria
2. Radiata-Bilateria Split cont.
• The Radiata-Bilateria split is also defined by the differences in the
number of germ layers that are formed in the embryo during
gastrulation
• The Radiata are referred to as diploblastic - two germ layers form
• Ectoderm - covering the surface of the embryo; gives rise to the epidermis
and in some phyla the nervous system.
• Endoderm - inner most germ layer; lines the primitive gut; gives rise to the
lining of the digestive tract and associated organs, such as the liver and lungs
of vertebrates
• The Bilateria are triploblastic; in addition to the ectoderm and
endoderm they produce a third germ layer, the mesoderm
• Mesoderm - germ layer between the ectoderm and the endoderm; gives rise
to muscles and to most other organs
Major Events (Branch Points) in Animal
Phylogeny con’t
3. The Acoelomate-Coelomate
Split
A. Acoelomates - animals with solid
bodies; there is no body cavity
between the gut (endoderm) and the
outer body wall.
• The other 2 body plans are often
referred to as a tube within a tube
body plan; a fluid filled sac separate
the gut from the outer body wall.
• A second important difference
between acoelomates and animals
having a body cavity is that animals
with a body cavity have some sort of
blood vascular system
3. Acoelomate-Coelomate Split cont.
B. Pseudocoelomates animals in which the body
cavity is not completely lined
with mesodermal tissue
• The body cavity is called a
pseudocoelom
C. Coelomates - animals that
have a fluid filled body cavity
that is completely lined with
tissue that is derived from the
mesoderm.
• This kind of body cavity is
called the coelom
Animal Phylogeny cont.
4. The ProtostomeDeuterstome Split
Coelomates, can be
divided into 2 distinct
groups: protostomes and
deuterostomes.
They are distinguished
based upon fundamental
differences in early
development, including
cleavage, fate of the
blastopore, and coelom
formation
4. The ProtostomeDeuterstome Split con’t
A. Cleavage
Protostomes - Spiral,
determinate cleavage
Deuterostomes - Radial,
indeterminate cleavage
4. The Protostome-Deuterstome Split
cont.
B. Fate of the Blastopore
• During gastrulation, the
rudimentary gut or archenteron
forms; it has a single opening
called the blastopore
• A second opening forms later at
the opposite end of the
archenteron to produce a
digestive tube with a mouth and
anus
• Protostomes - the blastopore
becomes the mouth
•Deuterstomes - the blastopore
becomes the anus
4. The Protostome-Deuterstome
Split cont.
C. Coelom Formation
• Protostomes - coelom
formation is called
schizocoelous
development; coelom forms
by splitting of mesoderm
• Deuterostomes - coelom
formation is called
enterocoelous
development; coelom forms
as outpockets from the
endoderm