Transparency Master General Cnidarian Structure

The cnidarians (phylum Cnidaria) or coelenterates are an impressive and interesting group of animals for introductory zoology or biology classes, and frequently pupils have already observed these creatures during educational field trips or family vacations to the seashore. The flower-like sea anemones, colorful colonies of coral, umbrella or bell-shaped jellyfish and minute hydras are perhaps the most familiar cnidarians encountered by the high school student; however, sea fans, sea pansies and sea whips are also examples of the phylum. The accompanying plate illustrates general cnidarian structure, comparing the more basic types of individuals. Two structural types of cnidarians occur: a sessile form or polyp (Figures 1 and 3) and a free-floating form or medusa (figures 2 and 4). The polyp is tubular in shape, possesses an anterior mouth surrounded by tentacles and attaches to the substrate by the adhesive action of the basal or pedal disc. Even though polyps like hydras attach to rocks and aquatic plants they also have the ability to loosen their grip and migrate to a more propitious location (Figure 5). Some species of polyps (e.g., corals and sea fans) are colonial in nature, whereas others (e.g., hydras and sea anemones) are solitary animals. A budding hydra (Figure 3) may give the suggestion of a simple colony but actually such polyps are merely undergoing asexual reproduction. Essentially, the medusa is concave in appearance. The mouth is positioned on the concave side and the tentacles are located on the margin of the umbrella. In some species of jellyfish, these tentacles may extend up to 75 feet in length. Stinging cells or nematocysts, which play a role in protection and securing food, are located on the tentacles and other body surfaces of both the polyp and medusa. Many species of Cnidaria are represented only as polyps (e.g., hydrozoans); others such as the scyphozoans (Figure 4) or true jellyfish lack poly stages and are displayed as medusae; and still others, like Obelia, have both morphological types during their life history. Histological sectioning of the cnidarian body will reveal the epidermis and gastrodermis layers. The epidermis serves as a covering and is involved with muscle contractions and the gastrodermis lines the gastrovascular cavity and functions in digestion. The mesoglea is positioned between the epidermis and gastrodermis and consists basically of a gelatinous substance.