Animals are classified according to morphological and developmental characteristics, such as a body plan. With the exception of sponges, the animal body plan is symmetrical. This means that their distribution of body parts is balanced along an axis. Additional characteristics that contribute to animal classification include the number of tissue layers formed during development, the presence or absence of an internal body cavity, and other features of embryological development.
Figure 15.3 The phylogenetic tree of animals is based on morphological, fossil, and genetic evidence.
- Eumetazoa have specialized tissues and Parazoa do not.
- Both acoelomates and pseudocoelomates have a body cavity.
- Chordates are more closely related to echinoderms than to rotifers according to the figure.
- Some animals have radial symmetry, and some animals have bilateral symmetry.
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