Evolution of the
Animal Phyla
21.1 General Features of Animals
All animals are multicellular heterotrophs
They all require oxygen for respiration
Animals are diverse in form
There are ~ 10 million living species
~ 99% are invertebrates (lacking a backbone)
~ 1% are vertebrates (possessing a backbone)
There are about 36 phyla
Most occur in the sea
Three phyla dominate life on land
Arthropoda; Mollusca; Chordata
Animals lack cell walls
They are usually quite flexible
Animals are mobile
They move more rapidly and in more complex ways than
members of other kingdoms
Most animals reproduce sexually
An animal develops from a zygote by a characteristic
process of embryonic development
Morula Æ Blastula Æ Gastrula
Details vary widely between phyla
Provide clues to evolutionary relatedness
21.2 The Animal Family Tree
Taxonomists have traditionally created phylogenies
(family trees) based on two main criteria
Anatomical features
Embryonic development
Fig. 21.3
Parazoa (“beside animals”)
Lack tissues and symmetry
Eumetazoa (“true animals”)
Possess tissues and symmetry
Eumetazoa have two branches
Radiata
Have radial symmetry
Bilateria
Have bilateral symmetry
Fig. 21.3
Fig. 21.2
Evolutionary
trends
among the
animals
“Evo-Devo” and Roots of the Animal Family Tree
Nearly all the major animal body plans can be seen
in Cambrian rocks dating from 543-525 mya
Biologists have long debated what caused this
Cambrian explosion of animal diversity
Proposed reasons include
Emergence of predatory lifestyles
Geological factors, such as the buildup of
minerals in the oceans
“Evo-Devo” and Roots of the Animal Family Tree
Another idea…from the Cambrian explosion comes
the new field of “evo-devo”
A synthesis of evolutionary and devolepmental
biology
Much of the variation in animal body plan is
associated with changes in the Hox gene complex
Family of genes controlling animal development
21.3 Sponges: Animals
Without Tissues
The Kingdom Animalia consists of two subkingdoms
Parazoa
Animals that lack symmetry and possess
neither tissues nor organs
1 phylum: Porifera
Eumetazoa
Animals that have symmetry and in most cases
tissues and organs
About 35 phyla
Sponges are the simplest animals
Bodies consist of little more than masses of
specialized cells embedded in a gel-like matrix
Fig. 21.7
Fig. 21.8
The choanocytes of sponges very closely resemble a kind of protist called
choanoflagellates
These may be the ancestors of all animals
21.4 Cnidarians: Tissues Lead to
Greater Specialization
The structure of eumetazoans is much more
complex than that of sponges
Radially symmetric eumetazoans form two distinct
embryonic layers
An outer ectoderm Æ epidermis
An inner endoderm Æ gastrodermis
A jelly-like layer called the mesoglea forms
between the epidermis and gastrodermis
Cnidaria
Hydra
Corals
Fig. 21.9
Jellyfish
Sea anemone
Cnidarians
Carnivores that capture their prey with tentacles
Bear unique stinging cells called cnidocytes
Contain a small but powerful harpoon called a
nematocyst
A major evolutionary innovation among the radiates
is extracellular digestion of food
In radiates, digestion begins in the gastrovascular
cavity
Fig. 21.10
Cnidarians have two basic body forms
Medusae
Free-floating,
gelatinous and often
umbrella-shaped
Polyps
Cylindrical, pipeshaped and usually
attached to a rock
Fig. 21.11
Cnidarians may exist exclusively as either/or
Others alternate between the two phases
21.5 Solid Worms: Bilateral Symmetry
Radiates are radially
symmetrical
Have a regular arrangement
of parts around a central axis
All other eumetazoans are
bilaterally symmetrical
Have right and left halves
that are mirror images
Dorsal (top) vs.
Ventral (bottom)
Anterior (front) vs.
Posterior (back)
Fig. 21.13
Bilaterally symmetrical animals have evolved a
definite head end, a process called cephalization
Solid worms are the simplest of all bilaterally
symmetrical animals
Nervous system
Ectoderm
Mesoderm
Endoderm
Lack internal
cavities, except
for digestive tract
Acoelomate
+
Fig. 21.14
Flatworms
Members of Platyhelminthes
The largest phylum of solid worms
Simplest animals in which
organs occur
Some species are free-living
Most species are parasitic
Tapeworms
Fig. 21.15a
Flukes
Many require two or more hosts
to complete their life cycle
Planaria
Fig. 21.16
Life cycle of
the human
liver fluke,
Clonorchis
sinensis
Fig. 21.17