Number of Body Layers
Figure 3.24: Most animals develop three layers of cells at an early stage in the growth of the embryo.
Symmetry and Body Plans
Figure 3.25: (A) the cnidarian that is known as a hydra shows radial symmetry. (B) The turtle shows bilateral symmetry.
Body Cavities
Figure 3.26: (A) Acoelomate animals have flattened bodies. (B) Coelomate animals have a body cavity in which complex internal organs can
develop.
Two Body Forms
Figure 3.29: There are two basic forms of cnidarians: polyps and medusae. (A) Polyps, such as adult sea anemones, are sessile. (B) Medusae, like
the jellyfish shown in the photo, are mobile.
Worms
Figure 3.31: Segmented worms, like this earthworm, are coelomates that are segmented on the inside as well as on the outside.
Mammals
Table 3.6: Characteristics of Mammal Groups