Body Plan Of Cnidarians

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.

Tentacle

Mouth

Epidermis Mesoglea

Gastrovascular cavity

Gastrodermis Tentacle

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