SYMMETRY
Sponges diverged from other animal lineages very early and are very different from other animals. They function like colonial, unicellular protozoa and lack body symmetry.

Most animals except sponges show radial or bilateral symmetry. Jellyfish, sea anemones, and adult echinoderms (sea stars), in the phylum Cnidaria, exhibit radial symmetry. Their bodies resemble a cylinder and multiple planes divide the animal into mirror images.

Most other animals (see Table 28-1 in your textbook) exhibit bilateral symmetry in which one unique plane of symmetry produces right and left halves. Animals with bilateral symmetry evolved a head with sensory structures.

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