Chapter 32: An Introduction to Animals
o Animals are a particularly species-rich and morphologically diverse lineage of multicellular
o Major groups of animals are recognized by their basic body plan, which differs in the number of
tissues observed in embryos, symmetry, the presence or absence of a body cavity, and the way in
which early events in embryonic development proceed.
o Recent phylogenetic analyses of animals have shown that there were three fundamental splits as
animals diversified, resulting in two protostome groups (Lophotrochozoa and Ecdysozoa) and the
o Within major groups of animals, evolutionary diversification was based on innovative ways of
feeding and moving. Most animals get nutrients by eating other organisms, and most animals move
under their own power at some point in their life cycle.
o Methods of sexual reproduction vary widely among animal groups, and many species can reproduce
asexually. It is common for individuals to undergo metamorphosis during their life cycle.
Why Do Biologists Study Animals?
Animals are heterotrophs—they obtain the chemical energy and carbon compounds they need from
Animals are the consumers that occupy the upper levels of food chains in both aquatic and
Animals are a species rich and morphologically diverse lineage of multicellular organisms.
Humans depend on wild and domesticated animals for food and, in preindustrial societies, for
transportation and power.
Humans study our closest relatives to understand ourselves.
How Do Biologists Study Animals?
Animals are very diverse, particularly in morphology.
About 34 major animal phyla are recognized.
Analyzing Comparative Morphology
Most morphological diversity in animals is based on differences in mouths and limbs.
The basic architecture of the animal body has been highly conserved throughout evolution though.
Four features define an animal's body plan:
1. the number of tissue types in embryos
2. the type of body symmetry and degree of cephaloization
3. the presence or absence of a fluid–filled cavity
4. the way in which the earliest events of embryo development proceed. The Evolution of Tissues
All animals have tissues—tightly integrated structural and functional units of cells.
Diploblasts are animals whose embryos have two types of tissues or germ layers:
the ectoderm and endoderm.
Triploblasts are animals whose embryos have three types of tissues: the ectoderm, endoderm,
and mesoderm. These germ layers develop into distinct adult tissues.
Symmetry and Cephalization
A basic feature of a multicellular body is the presence or absence of a plane of symmetry (Figure
Animals with radial symmetry have at least two planes of symmetry. Organisms with bilateral
symmetry have a single plane of symmetry and face their environment in one direction.
Bilateral symmetry allowed cephalization: the evolution of a head, or anterior region, where
structures for feeding, sensing the environment, and processing information are concentrated.
Why Was the Evolution of a Body Cavity Important?
Animals may or may not have an enclosed, fluid–filled body cavity called a coelom (Figure 32.6).
The coelom forms from within the mesoderm and is lined with cells from the mesoderm.
The coelom creates a container for circulation of oxygen and nutrients, and also acts as an
efficient hydrostatic skeleton that allows an animal to move even without fins or limbs (Figure
What Are the Protostome and Deuterostome Patterns of Development?
With the exception of the echinoderms, all coelomates are bilaterally symmetric and have three
embryonic tissue layers.
This group, called the Bilatera, can be divided into protostomes (arthropods, mollusks, and
segmented worms) and deuterostomes (chordates and echinoderms).
Three events in early development (cleavage, gastrulation, and coelom formation) differ in
protostomes and deuterostomes (Figure 32.8).
Cleavage is a rapid series of mitotic divisions that occurs in the absence of growth.
Gastrulation is a series of cell movements that results in the embryonic tissue layers endoderm,
mesoderm, and ectoderm.
As gastrulation proceeds, the coelom forms.
Protostome and deuterostome patterns of development result from differences in these three
The Tube–within–a–Tube Design
The basic animal body plan is a tube–within–a–tube design in which the outer tube forms the body
wall and the inner tube forms the gut.
Web Animation: The Architecture of Animals Evaluating Molecular Phylogenies
The phylogenetic tree shown in Figure 32.10 indicates that the choanoflagellates are the closest
living relatives of animals and that the Porifera (sponges) are the most ancient animal phylum.
Placement of diploblasts near the base of the tree implies that endoderm and ectoderm were the first
tissue types to evolve and that radial symmetry evolved before bilateral symmetry.
After the split between the protostomes and the deuterosteomes, the protostomes split to form two
major subgroups, the Ecdysozoa and the Lophotrochozoa.
Segmentation evolved independently in annelids and arthrophods, as well as in vertebrates and
possibly in molluscs.
What Themes Occur in the Diversification of Animals?
Within each animal phylum, the basic features of the body plan do not vary from species to species.
Animals obtain food in four general ways and have three general sources of food.
How Animals Feed: Four General Tactics
The feeding tact