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Phylum Ctenophora Ctenophore, from Fitzinger, 1862.

Because Ctenophores have transparent gelatinous bodies that are biradially symmetrical, and commonly lead a planktonic lifestyle, they superficially resemble pelagic Cnidarians such as jellyfish . However, they have many features that are distinctly different from the Cnidarians , such as a mesenchymal musculature, a more developed digestive system, monomorphic life history (only one body type), a cydippid larval stage, and the fact that they are never colonial. Ctenophores move through the water by the coordinated beating of cilia on eight rows of ciliated "combs" (known as ctenes ). This is in contrast to Cnidarians , where the motile medusoid form moves through the water by muscular contractions and pulsing of the medusa 's bell . All Ctenophores are marine, unlike Cnidarians which have some freshwater forms. Ctenophores can be found in all seas and oceans, and are occasionally extremely abundant and a dominant organism in some plankton communities. There are only about 100 extant species of Ctenophores , which can be divided into just two classes, Class Nuda and Class Tentaculata . As their names suggest, members of the class Nuda have no tentacles, whereas those of the class Tentaculata do. They may be found in a wide variety of sizes and shapes, - from spherical to ribbonlike, and from less than 1 cm. diameter to over one meter in length for some ribbonlike forms. They are usually planktonic , living in waters from the surface to as deep as 3,000 meters, and a few species may even be found crawling about on the bottom.

Ctenophore, modified from various sources

Pleurobrachia sp. from BIODIDAC, Univ. Ottawa

The general Characteristics of the Phylum Ctenophora are: Biradially symmetrical, with an oral - aboral body axis . Eight rows of ciliated "combs" or ctenes , controlled by an apical sense organ at the aboral pole. Incomplete gut (mouth but no anus ). No excretory, respiratory, or circulatory systems. Diploblastic , with ectoderm and entoderm , and a cellular mesenchyme , which gives, rise to the musculature (some researchers feel that Ctenophores are triploblastic , with a true mesoderm ). Adhesive structures called coloblasts (but no cnidoblasts or stinging cells as in Cnidaria ). A diffuse nervous system consists of a nerve net, somewhat more specialized than that found in the Cnidaria . A cydippid larval stage, monomorphic life cycle, never with a sessile or benthic stage. REFERENCES Barnes, 1980 Brusca & Brusca, 1990 Barnes, Calow, and Olive, 1993 Meglitsch & Schram, 1991

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