names for the two body forms, one nonmotile and one typically free swimming, found in the aquatic invertebrate phylum Cnidaria (the coelenterates). Some animals of this group are always polyps, some are always medusae, and some exhibit both a polyp and a medusa stage in their life cycle. The polyp is a sessile, or nonmotile, organism; well-known solitary polyps are the sea anemone and the freshwater hydra. The medusa, when free swimming, is popularly known as a jellyfish.

Anatomy

The two forms are similar in construction; both consist of a cylindrical body surrounding a digestive cavity, with a single opening, the mouth, at one end. The mouth is surrounded by tentacles, which are used to capture food and convey it to the mouth; these tentacles are armed with stinging cells which paralyze the prey. The body wall is composed of three layers of tissue. Thin layers called endoderm and ectoderm line the outside and inside, respectively; between these is a layer of jellylike material, called mesoglea, of varying thickness.

The polyp, also called the hydroid, tends to be elongated, with a thin body wall; it is attached to the ocean bottom or other surface by the end opposite the mouth, its tentacles pointing upward. The medusa tends to be rounded, with a thick body wall containing much mesoglea; it swims or is carried in the current with the mouth side down and the tentacles dangling.

Reproduction

In organisms that exhibit both forms, such as members of the cosmopolitan genus Obelia, the polyp is the asexual stage and the medusa the sexual stage. In such organisms the polyp, by budding, gives rise to medusae, which either detach themselves and swim away or remain permanently attached to the polyp. The medusae then produce new polyps by sexual reproduction. A medusa produces eggs or sperm, which are usually shed into the water; when an egg is fertilized, it develops into a swimming larva, which eventually settles and grows into a polyp. In addition to this elaborate means of reproduction, the polyp can form new polyps by budding. In some groups of coelenterates either the polyp or the medusa has become highly developed, with the reduction or complete loss of the other form. Where only the medusa occurs, as in many jellyfish, the larva never settles, and grows directly into a medusa. Where only the polyp exists, as in the hydra and the sea anemone, the organism has the ability to produce new polyps sexually, as well as by budding.

Colonial Species

In many species the polyp, or hydroid, stage is colonial: as new polyps are created by budding, they remain attached to a branching common stalk, often hardened with nonliving material, forming a plantlike structure called a hydroid colony. The branching, hydroid colonies of Obelia are commonly found on North American seashores; the individual polyps are microscopic, but the feathery white or yellow colony grows up to 12 in. (30 cm) tall. Individual polyps project from the branches of such a colony; they produce the tiny, free-swimming Obelia medusae, barely visible to the naked eye. Other hydroid colonies are colonial coral and the sea pens; these have no medusa stage. All hydroid colonies have at least two types of individual polyps, specialized for feeding and reproduction respectively; some have additional specialized types. The purple sail, genus Vellela, and the Portuguese man-of-war, genus Physalia, are elaborate floating colonies composed of many types of specialized individuals, both polyplike and medusalike in structure; the entire colony is equipped with a float.

The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press. Licensed from Lernout & Hauspie Speech Products N.V. All rights reserved.

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