Hydrozoa
Hydrozoa, class of invertebrates belonging to the cnidarian phylum. The hydrozoans make up six orders. Most species of hydrozoans show alternation of generations. One generation consists of individuals called polyps and is almost always colonial; the colony is known as a hydroid because of the resemblance of its individual polyps to the noncolonial hydra, the simplest of all hydrozoans. The other generation, members of which are known as medusae, consists of free-swimming, sexual individuals that produce either eggs or sperm; the familiar jellyfish is a typical medusa. Each fertilized egg develops into a ciliated larva (planula), from which the hydroid develops and then gives rise to medusae by budding.
Hydrozoans differ in their conformity to the typical alternation of generations. The order to which the hydra belongs has small medusae and sometimes no free-swimming forms at all. The siphonophoran order, containing the Portuguese man-of-war, has a short-lived medusa stage, the larvae of which develop into medusalike organisms from which the colony arises. The individual members of a siphonophoran colony are extremely specialized; the polyps specializing in defense produce a venom strong enough to kill a human being. Another order has a short-lived polyp stage; in some species no polyp stage has yet been observed. Its members spend most of their lives as free-swimming medusae. A fourth order combines traits of polyps and medusae, somewhat resembling free-swimming, ciliated hydras. The two primarily tropical orders contain a number of interesting forms that secrete a large, hard, calcareous exoskeleton and often form corals.
Scientific classification: The class Hydrozoa belongs to the phylum Cnidaria. Hydra belong to the order Hydroida, siphonophores to the order Siphonophora. Species with a short-lived or nonexistent polyp stage belong to the order Trachylina. Species resembling free-swimming polyps belong to the order Actinulida. The two primarily tropical orders are Stylasterina and Milleporina; the latter is sometimes considered a suborder.
Hydrozoans differ in their conformity to the typical alternation of generations. The order to which the hydra belongs has small medusae and sometimes no free-swimming forms at all. The siphonophoran order, containing the Portuguese man-of-war, has a short-lived medusa stage, the larvae of which develop into medusalike organisms from which the colony arises. The individual members of a siphonophoran colony are extremely specialized; the polyps specializing in defense produce a venom strong enough to kill a human being. Another order has a short-lived polyp stage; in some species no polyp stage has yet been observed. Its members spend most of their lives as free-swimming medusae. A fourth order combines traits of polyps and medusae, somewhat resembling free-swimming, ciliated hydras. The two primarily tropical orders contain a number of interesting forms that secrete a large, hard, calcareous exoskeleton and often form corals.
Scientific classification: The class Hydrozoa belongs to the phylum Cnidaria. Hydra belong to the order Hydroida, siphonophores to the order Siphonophora. Species with a short-lived or nonexistent polyp stage belong to the order Trachylina. Species resembling free-swimming polyps belong to the order Actinulida. The two primarily tropical orders are Stylasterina and Milleporina; the latter is sometimes considered a suborder.
Comments