28.3 Superphylum Lophotrochozoa: Flatworms, Rotifers, and Nemerteans Cnidarian species include individual or colonial polypoid forms, floating colonies, or large individual medusa forms (sea jellies). In species with both forms, the medusa is the sexual, gamete-producing stage and the polyp is the asexual stage. Cnidarians have separate sexes and many have a lifecycle that involves two distinct morphological forms-medusoid and polypoid-at various stages in their life cycles. The mouth is surrounded by tentacles that contain large numbers of cnidocytes-specialized cells bearing nematocysts used for stinging and capturing prey as well as discouraging predators. Cnidarians possess a well-formed digestive system and carry out extracellular digestion in a digestive cavity that extends through much of the animal. They possess outer and inner tissue layers that sandwich a noncellular mesoglea between them. 28.2 Phylum CnidariaĬnidarians represent a more complex level of organization than Porifera. Although sponges are very simple in organization, they perform most of the physiological functions typical of more complex animals. Glass sponge cells are connected together in a multinucleated syncytium. These organisms show very simple organization, with a rudimentary endoskeleton of spicules and spongin fibers. Animals included in phylum Porifera are parazoans because they do not show the formation of true embryonically derived tissues, although they have a number of specific cell types and “functional” tissues such as pinacoderm.
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