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ZOO-MJ-T-101 (Taxonomy and Introduction to Non-chordates) Module 4: Cnidaria (L2) > Definition of polymorphism: Polymorphism is the phenomenon where existence of individuals (zooids) of the same species in more than one forms with different functions. Example: It is a characteristic feature of Coelenterates (Cnidaria + Ctenophora). In coelenterates different individuals get united in the form of a colony and hence polymorphism is a very important feature of this phylum. » Basic units of polymorphism (stages of metagenesis): All forms of zooids can be divided into two fundamental forms which can be derived from each other; these are polyp form and medusoid form. These two forms alternate with each other on the life history of a typical cnidarian i.e. polyp form produces to medusa asexually and medusoid form produces to polyp sexually. Generally polyps are sessile and medusoid forms are motile. But a homology exists between the two in their basic features, tron (stomach) Eeumbrella Polyp form Medusoid form A] Polyp form (Asexual form) or hydroid generation: This form is tubular and the mouth is surrounded by tentacles only at one end (preoral end) while the other end (aboral end) is usually attached by a pedal disc to the substratum. These forms have following characteristics: 1. Sedentary tubular form with one end closed. 2. Free conical end (preoral end) bearing mouth and tentacles. 3. Aboral end fixed. 4. Un-branched elongated tentacles surrounding the mouth, 5. The polyp may be encased by a transparent covering, the hydrotheca (e.g. //ydra sp.). 
