This development, though probably representing the primitive sequence of events, is never actually found in its full extent, but is always abbreviated by omission or elimination of one or more of the stages. We have already seen that the parenchymula stage is passed over when the gastrulation is of the invaginate type. On the other hand, the parenchymula may develop directly into the actinula or even into the polyp, with suppression of the intervening steps. Great apparent differences may also be brought about by variations in the period at which the embryo is set free as a larva, and since two free-swimming stages, planula and actinula, are unnecessary, one or other of them is always suppressed. A good example of this is seen in two common genera of British hydroids, _Cordylophora_ and _Tabularia_. In _Cordylophora_ the embryo is set free at the parenchymula stage as a planula which fixes itself and develops into a polyp, both gastrula and actinula stages being suppressed. In _Tubularia_, on the other hand, the parenchymula develops into an actinula within the maternal tissues, and is then set free, creeps about for a time, and after fixing itself, changes into a polyp; hence in this case the planula-stage, as a free larva, is entirely suppressed. The Hydrozoa may be defined, therefore, as Cnidaria in which two types of individual, the polyp and the medusa, may be present, each type developed along divergent lines from the primitive actinula form. The polyp (hydropolyp) is of simple structure and never has an ectodernal oesophagus or mesenteries.[1] The general ectoderm loses its cilia, which persist only in the sensory cells, and it frequently secretes external protective or supporting structures. An internal mesogloeal skeleton is not found. The class is divisible into two main divisions or sub-classes, Hydromedusae and Scyphomedusae, of which definitions and detailed systematic accounts will be found under these headings. GENERAL WORKS ON HYDROZOA.--C. Chun, "Coelenterata (Hohlthiere)," _Bronn's Klassen und Ordnungen des Thier-Reichs_ ii. 2 (1889 et seq.); Y. Delage, and E. Hérouard, _Traité de zoologie concrète_, ii. part 2, _Les Coelentérés_ (1901); G. H. Fowler, "The Hydromedusae and Scyphomedusae" in E. R. Lankester's _Treatise on Zoology_, ii. chapters iv. and v. (1900); S. J. Hickson, "Coelenterata and Ctenophora," _Cambridge Natural History_, i. chapters x.-xv. (1906). (E. A. M.) FOOTNOTE: [1] See further under SCYPHOMEDUSAE.