BIOL 1101 Lecture 90: Diversity of the Cnidarians
27 Jun 2016
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Document Summary
Hydrozoans alternate between polyp and medusa forms, but the polyp form is the dominant organism. The common hydra is unusual in that it is one of the few freshwater cnidarians. Many hydrozoans live as colonial polyps, and much of the growth on docks that is dismissed as seaweed is actually colonial hydrozoans. These colonies may consist of polyps specialized for feeding (gastrozooids), defense (dactylozooids) and reproduction (gonozooids). The familiar portuguese man-of-war is a hydrozoan, but it is not a medusa. Instead, it is a colony of specialized polyps suspended from a float derived from a medusa. Scyphozoans, which are all marine, include the jellyfish. In this class, the medusa is dominant, and tends to be rather substantial because of a thick, firm mesoglea. The inconspicuous sexually-produced polyp form produces the medusae by asexual budding. In some pelagic (open ocean) schphozoans, the polyp has been eliminated entirely.
