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Increase the general level of organization cnidarians compared with sponges.
The Cnidarians and Ctenophores
The phylum Cnidana, or Coelenterata, includes the familiar hydras, jellyfish, sea anemones, and corals. The bright coloring of many species, combined with a radial symmetry, creates a beauty that is surpassed by few other animals. The radial symmetry is commonly considered justification for uniting the cnidarians and the ctenophores within a division of phyla of the Animal Kingdom called the Radiata.
The cnidarians possess two basic metazoan structural features. There is an internal space for digestion, called in cnidarians a gastrovascular cavity (Fig. 5-1). This cavity lies along the polar axis of the animal and opens to the outside at one end to form a mouth. The presence of a mouth and digestive cavity permits the use of a much greater range of food sizes than is possible in the protozoa and sponges. In cnidarians a circle of tentacles, rep¬resenting extensions of the body wall, surrounds the mouth to aid in the capture and ingestion of food.
The cnidarian body wall consists of three basic layers (Fig. 5-1): an outer layer of epidermis, an inner layer of cells lining the gastrovascular cavity, and between these a layer called mesoglea. Thei soglea ranges from a thin, noncellular membranetl a thick, fibrous, jelly-like, mucoid material with without wandering cells. The mesoglea probably evolved from a basement membrane, and in forms like Hydra it has hardly progressed beyond thJ level. In others it is more like connective tissue] but the cells appear to be derived from the epidel mis. Thus, cnidarians are diploblastic; i.e., tbl body is constructed from only two germ layers, eel toderm and endoderm.
Histologically, the cnidarians have remained I rather primitive, although they anticipate some el the specializations that are found in higher meta¬zoans. A considerable number of cell types conJ pose the epidermis and gastrodermis, but there J only a limited degree of organ development.
Although all cnidarians are basically tentaculate and radially symmetrical, two structural types are encountered within the phylum. One type, which is sessile, is known as a polyp. The other form is free swimming and is called a medusa. Typically, the body of a polyp is a tube or cylinder, in which the oral end, bearing the mouth and tentacles, is directed upward, and the opposite, or aboral, end is attached (Fig. 5-L4).
The medusoid body resembles a bell or an um¬brella, with the convex side upward and the mouth located in the center of the concave undersurface (Fig. 5-IB).
The tentacles hang down from the margin of the bell. In contrast to the polypoid mesoglea (middle layer), which is more or less thin, the medusoid mesoglea is extremely thick and constitutes the bulk of the animal. Because of this mass of jelly-like mesogleal material, these cnidarian forms arc commonly known as jellyfish. Some cnidarians exhibit only the polypoid form, some only the medusoid form, and others pass through both in their life cycle. Colonial organization has evolved numerous times within the phylum, especially in polypoid forms.
Except for the hydras and a few other freshwater hydrozoans, cnidarians are marine. Most are inhabitants of shallow water; sessile forms abound on rocky coasts or on coral formations in tropical waters. The phylum is composed of approximately 9000 living species, and a rich fossil record dates from the Cambrian period.