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. In some, defensive polyps are found, armed with large numbers of stinging cells. In addition to these two basic types of polyps, a few colonial species have other specialized forms. The arrangement and type of these reproductive polyps varies considerably between different groups. These lack tentacles and contain numerous buds from which the medusoid stage of the lifecycle is produced. Unlike some other cnidarian groups, the lining of the central cavity lacks stinging nematocysts, which are found only on the tentacles and outer surface.Īll colonial hydrozoans also include some polyps specialized for reproduction. Partially digested food may then be passed into the hydrocaulus for distribution around the colony and completion of the digestion process. The polyp contains a central cavity, in which initial digestion takes place. These have a more or less cylindrical body with a terminal mouth on a raised protuberance called the hypostome, surrounded by a number of tentacles. In any given colony, the majority of polyps are specialized for feeding. 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. The hydrocaulus is usually surrounded by a sheath of chitin and proteins called the perisarc. The polyps themselves are usually tiny, although some noncolonial species are much larger, reaching 6 to 9 cm (2.4 to 3.5 in), or, in the case of the deep-sea Branchiocerianthus, a remarkable 2 m (6.6 ft). They may have a tree-like or fan-like appearance, depending on species. The colonies are generally small, no more than a few centimeters across, but some in Siphonophorae can reach sizes of several meters. The hydroid Tubularia indivisa, fertile, Gulen Dive resort, Norway
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