Tiny freshwater hydra, stinging jellyfish, and flowerlike coral all belong to the phylum Cnidaria. Animals in this phylum are called cnidarians. As you can see in Figure 33-3, the body of a cnidarian may be either bell-shaped or vase-shaped. The bell-shaped medusa (me-DOO-suh) is specialized for swimming. In contrast, the vase-shaped form, called a polyp (PAHL-ip), is specialized for a sessile existence.
Figure 33-3 also shows that all cnidarians have bodies constructed of two cell layers—an outer epidermis and an inner gastrodermis. Between these layers is a jellylike material known as mesoglea (mez-uh-GLEE-uh). In the center of the body is a hollow gut called the gastrovascular cavity, which has a single opening, or mouth. Surrounding the mouth are numerous flexible extensions called tentacles.
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