Section 1

The Nature of Animals

• Animals are either invertebrates—lack a backbone—or vertebrates—have a backbone.

• Animals are multicellular organisms that lack cell walls and are heterotrophic. Most animals reproduce sexually and can move.

• Movement and response to the environment are governed by an animal's nervous and muscle tissues.

• The first animals may have evolved from colonial protists.

• At some stage of their lives, chordates have a notochord, dorsal nerve cord, postanal tail, and pharyngeal pouches.

• Most animals have tissues and radial or bilateral symmetry. Bilateral symmetry is associated with cephalization. Most animals have a body cavity that aids movement and the transport of nutrients and wastes.

• Animals are classified by degree of cell specialization, number of tissue layers, type of symmetry, and most recently, by sequencing of rRNA.

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