Editing Hydrozoa (section)

==Anatomy==
Most hydrozoan species include both a [[polyp (zoology)|polyp]]oid and a [[medusa (biology)|medusoid]] stage in their [[Biological life cycle|life cycles]], although a number of them have only one or the other. For example, ''Hydra'' has no medusoid stage, while ''[[Liriope tetraphylla|Liriope]]'' lacks the [[polyp (zoology)|polyp]]oid stage.<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia |pages= 122–139|isbn= 978-0-03-056747-6}}</ref>

===Polyps===
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.

[[File:Tubularia-indivisa.jpg|left|thumb|250px|The hydroid ''[[Tubularia indivisa]]'', fertile]]
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 {{convert|6|to|9|cm|in|abbr=on}}, or, in the case of the deep-sea ''[[Branchiocerianthus]]'', a remarkable 2&nbsp;m (6.6&nbsp;ft).<ref name=IZ/>

The hydrocaulus is usually surrounded by a sheath of [[chitin]] and proteins called the perisarc. In some species, this extends upwards to also enclose part of the polyps, in some cases including a closeable lid through which the polyp may extend its tentacles.<ref name=IZ/>

In any given colony, the majority of polyps are specialized for feeding. These have a more or less cylindrical body with a terminal mouth on a raised protuberance called the [[wikt:hypostome|hypostome]], surrounded by a number of tentacles. The polyp contains a central cavity, in which initial digestion takes place. Partially digested food may then be passed into the hydrocaulus for distribution around the colony and completion of the digestion process. Unlike some other cnidarian groups, the lining of the central cavity lacks stinging [[nematocyst]]s, which are found only on the tentacles and outer surface.

All colonial hydrozoans also include some polyps specialized for reproduction. These lack tentacles and contain numerous buds from which the medusoid stage of the life cycle is produced. The arrangement and type of these reproductive polyps varies considerably between different groups.

In addition to these two basic types of polyps, a few colonial species have other specialized forms. In some, defensive polyps are found, armed with large numbers of stinging cells. In others, one polyp may develop as a large float, from which the other polyps hang down, allowing the colony to drift in open water instead of being anchored to a solid surface.<ref name=IZ/>

===Medusae===
The medusae of hydrozoans are smaller than those of typical jellyfish, ranging from {{convert|0.5|to|6|cm|abbr=on}} in diameter. Although most hydrozoans have a medusoid stage, this is not always free-living and in many species exists solely as a sexually reproducing bud on the surface of the hydroid colony. Sometimes, these medusoid buds may be so degenerated as to entirely lack tentacles or mouths, essentially consisting of an isolated [[gonad]].<ref name=IZ/>

The body consists of a dome-like umbrella ringed by tentacles. A tube-like structure hangs down from the centre of the umbrella and includes the mouth at its tip. Most hydrozoan medusae have just four tentacles, although a number of exceptions exist. Stinging cells are found on the tentacles and around the mouth.

The mouth leads into a central stomach cavity. Four radial canals connect the stomach to an additional, circular canal running around the base of the bell, just above the tentacles. Striated muscle fibres also line the rim of the bell, allowing the animal to move along by alternately contracting and relaxing its body. An additional shelf of tissue lies just inside the rim, narrowing the aperture at the base of the umbrella, and thereby increasing the force of the expelled jet of water.<ref name=IZ/>

The nervous system is unusually advanced for cnidarians. Two nerve rings lie close to the margin of the bell, and send fibres into the muscles and tentacles. The genus ''[[Sarsia]]'' has even been reported to possess organised [[ganglion|ganglia]]. Numerous sense organs are closely associated with the nerve rings. Mostly these are simple sensory nerve endings, but they also include [[statocyst]]s and primitive light-sensitive [[ocelli]].<ref name=IZ/>