The New Student's Reference Work/Hydrozoa
Hy′drozo′a. Sometimes casual visitors at the seacoast gather certain animal forms from rocks and submerged objects, and press them on paper under the name of seaplant or seamoss. This shows how much some of the hydrozoa resemble delicately branching plants. The general stem is usually horny and hollow. At various points are small, tubular polyps resembling the fresh-water hydra in form and structure. While it is solitary, they are colonial. In this group also the polyps have become modified into several kinds of persons. The feeding persons—with tubular body disc and tentacles—are the most typical. The reproductive persons are often shaped like small jellyfish, and are set free from the colony when fully formed. They are called medusoids. There are in some colonies, in addition to the above, protective persons, swimming persons and stinging persons. These are all modified polyps, and the variety of form gives rise to what is called polymorphism. The hydrozoa make a natural division of the subkingdom Cœlenterata (which see). See Hydra.