Body Structure

Animal bodies range from those that lack true tissues and an organized body shape to those that have very organized tissues and a consistent body shape.

Patterns of Symmetry

A body plan describes an animal's shape, symmetry, and internal organization. Symmetry is a body arrangement in which parts that lie on opposite sides of an axis are identical. An animal's body plan results from the animal's pattern of development. Sponges have the simplest body plan of all animals. Sponges, as shown in Figure 32-5a, are asymmetrical—they do not display symmetry. Animals that have a top and bottom side, but no front, back, right, or left end, display radial symmetry—a body plan in which the parts are organized in a circle around an axis. Cnidarians, such as the sea anemone in Figure 32-5b, are radially symmetrical.

Most animals have a dorsal (back) and ventral (abdomen) side, an anterior (toward the head) and posterior (toward the tail) end, and a right and left side, as shown by the squirrel in Figure 32-5c. Such animals have two similar halves on either side of a central plane and are said to display bilateral symmetry. Bilaterally symmetrical animals tend to exhibit cephalization (SEF-uh-li-ZAY-shuhn)— the concentration of sensory and brain structures in the anterior end of the animal. As a cephalized animal moves through its environment, the anterior end precedes the rest of the body, sensing the environment.

Germ Layers

Germ layers are tissue layers in the embryos of all animals except sponges, which have no true tissues. The embryos of cnidarians and ctenophores have two germ layers. All other animals have three germ layers. Every organ and tissue arises from a germ layer.

figure 32-5

(a) The sponge lacks a consistent pattern of structure. (b) The sea anemone, an aquatic animal, displays radial symmetry. (c) The squirrel displays bilateral symmetry and cephalization.

Word Roots and Origins cephalization from the Greek word kephale, meaning "head"

Body Cavities

Most animals have some type of body cavity, a fluid-filled space that forms between the digestive tract and the outer wall of the body during development. Some animals, such as flatworms, have three germ layers but have a solid body. These animals lack a body cavity.

In the roundworm shown in Figure 32-6, the body cavity aids in movement by providing a firm, fluid-filled structure against which muscles can contract. The body cavity also allows some degree of movement of the exterior of the body with respect to the internal organs, resulting in more freedom of movement for the animal. Finally, the fluid in the body cavity acts as a reservoir and medium of transport for nutrients and wastes, which diffuse into and out of the animal's body cells.

Body Structure and Relatedness

Biologists use similarities in body plans and patterns of development to help them classify animals and hypothesize about the evolutionary history of animals. Biologists use information from extant (living) species and extinct species to develop phylogenetic diagrams, such as the one shown in Figure 32-3. Animal phyla shown on the same branch of the phylogenetic diagram, such as flat-worms and rotifers, are thought to be related to each other more closely than they are to other animals and are characterized by similarities in morphology and rRNA sequences. Conversely, animals shown in different parts of the diagram are thought to be more distantly related.

• Multicellularity and a limited degree of cell specialization characterize the sponges. Sponges have no organized body shape and no true tissues.

• True tissues in two layers are found in the cnidarians and the ctenophores.

• True tissues in three layers and bilateral symmetry characterize all of the other animal phyla.

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