Polyp and Medusa Structures

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During the life cycle of cnidarians they may form two types of body structures, the polyp and the medusa. Some cnidarians alternate between polyp and medusa at different stages of their life cycles, some cnidarians live solely as polyps, and others only as medusae.

A polyp has a cylindrical shape. The mouth, surrounded by tentacles, is located at one end of the polyp, and the opposite end of the polyp typically attaches to a rock or other object on the sea bottom. Polyps may be solitary, as in some types of hydras, or they may attach to other polyps to form a colony, as corals do. In some cnidarians, such as those from the genus Obelia, the polyps of a single colony may perform specialized functions. Some polyps in a colony capture food, others reproduce, and others protect the colony.

The medusa body resembles a saucer or umbrella. The mouth is located in the center of the undersurface of the saucer-like body, surrounded by tentacles. Medusae are solitary and most swim freely in the water. The gelatinous mesoglea layer in medusae is quite thick, which is why cnidarians with the medusae body form are commonly referred to as jellyfish.

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