Гидрозои: Класс кишечнополостных

Class of cnidarians

Hydrozoa (hydrozoans; ) is a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water. The colonies of the colonial species can be large, and in some cases the specialized individual animals cannot survive outside the colony. A few genera within this class live in freshwater habitats. Hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria. Some examples of hydrozoans are the freshwater jelly (Craspedacusta sowerbyi), freshwater polyps (Hydra), Obelia, Portuguese man o' war (Physalia physalis), chondrophores (Porpitidae), and pink hearted hydroids (Tubularia).

Most hydrozoan species include both a polypoid and a medusoid stage in their life cycles, although a number of them have only one or the other. For example, Hydra has no medusoid stage, while Liriope lacks the polypoid stage.

The hydroid form is usually colonial, with multiple polyps connected by tubelike hydrocauli. The hollow cavity in the middle of the polyp extends into the associated hydrocaulus, so that all the individuals of the colony are intimately connected. Where the hydrocaulus runs along the substrate, it forms a horizontal root like stolon that anchors the colony to the bottom. The colonies are generally small, no more than a few centimeters across, but some in Siphonophorae can reach sizes of several meters. They may have a tree like or fan like appearance, depending on species. The polyps themselves are usually tiny, although some noncolonial species are much larger, reaching , or, in the case of the deep sea Branchiocerianthus, a remarkable 2 m (6.6 ft). In hydrozoan species with both polyp and medusa generations, the medusa stage is the sexually reproductive phase. Medusae of these species of Hydrozoa are known as "hydromedusae". Most hydromedusae have shorter lifespans than the larger scyphozoan jellyfish. Some species of hydromedusae release gametes shortly after they are themselves released from the hydroids (as in the case of fire corals), living only a few hours, while other species of hydromedusae grow and feed in the plankton for months, spawning daily for many days before their supply of food or other water conditions deteriorate and cause their demise. Additionally, some hydrozoan species (particularly in Turritopsis genus) share an unusual life cycle among the animals they can transform themselves from sexually mature medusae stage back to their juvenile hydroid stage.

The earliest hydrozoans may be from the Vendian (late Precambrian), more than 540 million years ago. Hydrozoan systematics are highly complex. Several approaches for expressing their interrelationships were proposed and heavily contested since the late 19th century, but in more recent times a consensus seems to be emerging. Historically, the hydrozoans were divided into a number of orders, according to their mode of growth and reproduction. Most famous among these was probably the assemblage called "Hydroida", but this group is apparently paraphyletic, united by plesiomorphic (ancestral) traits. Other such orders were the Anthoathecatae, Actinulidae, Laingiomedusae, Polypodiozoa, Siphonophorae and Trachylina. As far as can be told from the molecular and morphological data at hand, the Siphonophora for example were just highly specialized "hydroids," whereas the Limnomedusae—presumed to be a "hydroid" suborder—were simply very primitive hydrozoans and not closely related to the other "hydroids." So, the hydrozoans now are at least tentatively divided into two subclasses, the Leptolinae (containing the bulk of the former "Hydroida" and the Siphonophora) and the Trachylinae, containing the others (including the Limnomedusae). The monophyly of several of the presumed orders in each subclass is still in need of verification.