The Hydrozoa are divided into five sub-classes - viz., the Hydroida, the Siphonophora, the Lucernarida, the Graptolitidae, and the Hydrocorallinae.

Sub-Class I. Hydroida. - This sub-class comprises those Hydrozoa which consist of an alimentary region or "polypite" which is, typically, provided with an adherent disc, or "hydro-rhiza," and prehensile tentacles.

In some few cases the hydrosoma is composed of a single polypite only, as in the Hydrida and in some of the Corynida; but usually there are several polypites united together by means of a common trunk or "coenosarc," as in most of the Corynida and in the orders Sertnlarida and Campanularida. Further, in the great majority of cases, the "hydrorhiza" is permanently attached to some foreign object.

The Hydroida comprise six orders - viz., the Hydrida, the Corynida, the Sertularida, the Campanularida, the Thecomedusae, and the Medusidae.

* For full details as to the morphology and physiology of the Hydroid Zoophytes, the student should refer to the magnificent 'Monograph of the Gymnoblastic Hydroids,' by Professor Allman (Ray Society). The student may also consult the excellent 'History of British Hydroid Zoophytes,' by the Rev. Thomas Hincks.