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Anatomy of Animals


Cnidarians have radial symmetry, and there is typically a ring of tentacles surrounding the mouth region. Some cnidarians, referred to as hydroids or polyps, have a tubular shape, with the mouth region directed upward. Others, which have a bell
Cnidaria Hydra Anatomy shape with the mouth region directed downward, are called jellyfishes, or medusae. The polyp is adapted to a sessile life, while the medusa is adapted to a floating or free-swimming existence. At one time, both body forms may have been a part of the life cycle of all cnidarians, because today we see an alternation of generations life cycle of these two forms in certain cnidarians (see Figure 05d). In such life cycle, the polyp stage produces medusae, and the medusae, which produce eggs and

Figure 05a Cnidaria [view large image]

Figure 05b Hydra Anatomy [view large image]

sperm, disperse the species. Cnidarians are quite diversified including the Portuguese man-of-war, sea anemones, hydra and many other species.

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