The Cnidarian Life-cycle

One general let of the Phylum Cnidaria consists of polymorphism. The cnidarian life-cycle may be generally characterized by successive generations of two bole instances: the fixed polyp and the uncommitted medusa. For example, the polyp may give rise asexually to the medusae; this body type may then reproduce sexually to lay down zygotes and develop back into polyps. Among the more narrow down phylum members, however, body type alternation is completely suppressed (Clarkson, 1986, p. 80).

Cnidarian have no specialized excretory, respiratory, or circulatory structures. In fact, their single body cavity, or enteron, has only a single opening, which is normally surrounded by a ring of tentacles. Within the enteron, infolded endoderm typically forms give out vertical partitions, or mesenteries. These are attached both to the smother of the polyp and to the oral disc. Their primary function is to increase the theater over which digestion occurs (Clarkson, 1986, p. 80).

more or less cnidarians are carnivorous. Their normal prey accommodate fish, crustaceans, and zooplankton. Some advanced species, however, have become nutritionally myrmecophilous upon photosynthetic algae harbored inside their tissues (Oliver & Coates, 1987, p. 167).

Various subdivisions of Phylum Cnidaria embarrass the following: Class Hydrozoa, Class Scyphozoa, and Class Anthozoa. Of these, Anthozo

Clarkson, E. N. K. (1986). Invertebrate paleontology and evolution. Second edition. Boston, MA: Allen & Unwin.

Most hermatypic corals are colonial. In addition, the organisms basis grow rather large. unfortunately though, there is no direct point of algal mutualism in the fogy record: no symbionts have been engraft embedded in the hard tissues of a host (Cowen, N.d., p. 449). In addition, there are no definitive morphological characteristics that can be used to identify corals as having zooxanthellae. Although rapid harvest-feast might provide indirect evidence of symbiosis, interpretations of such characteristics, for example, as skeleton-to-body size ratio are highly subjective.

There are, however, original morphological patterns that can be used to "separate an collecting of zooxanthellate corals from fauna which is non-zooxanthellate" (Oliver & Coates, 1987, p. 162). Essentially, living zooxanthellate coral species tend to have a smaller average corallite size and high levels of interdependence, or integration, within their colonies. Moreover, zooxanthellate corals also generally adopt the following growth forms: colonial sheets, mounds, and trees with multiple series of corallites. Stanley (cited in Cowen, N.d., p. 453) discusses the origin of symbiosis among fossil scleractinian corals during the Middle Triassic. The zooxanthellate pattern often appears among Mesozoic and Tertiary " take down" corals (Oliver & Coates, 1987, p. 162). It should also be noted though that certain zooxanthellate-pattern cretaceous corals have been found that did not build reefs.

Such paleontologic evidence suggests that evolution is progressing toward increased cooperation between separate genomes and phenotypes. Although Rosen (cited in Cowen, N.d., p. 436) makes a tentative suggestion that a few Late Paleozoic corals may have had symbionts, the Paleozoic corals did not typically scupper zooxanthellate morphology. It seems more likely that any algae associated with
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