Zoology 10th Edition by Stephen Miller,John Harley

Exercise 9

Members of the phylum Cnidaria include the jellyfish, anemones, corals, and their relatives. They are diploblastic, radially or biradially symmetrical, and possess a unique cell type, the cnidocyte. Contractile cells in the epidermis and gastrodermis of their body walls act on water confined within the gastro-vascular cavity to create a hydrostatic compartment used to accomplish support and body movements. Their life histories include an attached polyp stage, which is often asexual, and/or a medusa stage, which is dioecious and free swimming. Hydrozoans often have life histories in which polyp and medusa stages alternate. Scphozoans have life histories in which the medusa stage predominates. Anthozoans have life histories in which the polyp stage predominates.