Hydrozoa

Hydrozoa hydrozoans; from taxonomic class of individually very small, predatory animals, some solitary in addition to some colonial, most of which inhabit saline water. a colonies of the colonial manner can be large, & in some cases the specialized individual animals cannot constitute outside the colony. A few genera within this class equal in freshwater habitats. Hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria.

Some examples of hydrozoans are the freshwater jelly Portuguese man o' war Physalia physalis, chondrophores Porpitidae, "air fern" Sertularia argentea, and pink-hearted hydroids Tubularia.

Systematics and evolution

The earliest hydrozoans may be from the Vendian slow Precambrian, more than 540 million years ago.

Hydrozoan systematics are highly complex. Several approaches for expressing their interrelationships were presents 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 multiple is apparently paraphyletic, united by plesiomorphic ancestral traits. Other such(a) 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 shared 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 regarded and sent separately. subclass is still in need of verification.

In any case, according to this classification, the hydrozoans can be subdivided as follows, with taxon designation emended to end in "-ae":

Class Hydrozoa

ITIS uses the same system, but unlike here, does not usage the oldest usable names for many groups.

In addition, there exists a cnidarian parasite, Polypodium hydriforme, which lives inside its host's cells. it is for sometimes placed in the Hydrozoa, though its relationships are currently unresolved—a somewhat controversial 18S rRNA sequence analysis found it to be closer to the also parasitic Myxozoan. It was traditionally placed in its own class, Polypodiozoa, and this belief is often seen to reflect the uncertainties surrounding this highly distinct animal.

Some of the more widespread family systems for the Hydrozoa are sent below. Though they are often found in seemingly authoritative Internet direction and databases, they pretend not agree with the usable data. especially the presumed phylogenetic distinctness of the Siphonophorae is a major flaw that was corrected only recently.

The obsolete classification intended above was:

A very old classification that is sometimes still seen is:

Catalogue of Life uses:

Animal Diversity Web uses: