Many species are known, of which three are common in European waters. It has been shown by C. F. Jickeli (28) that the species are distinguishable by the characters of their nematocysts. They also show characteristic differences in the egg (Brauer [2]). In _Hydra viridis_ the polyp is of a green colour and produces a spherical egg with a smooth shell which is dropped into the mud. _H. grisea_ is greyish in tint and produces a spherical egg with a spiky shell, which also is dropped into the mud. _H. fusca_ (= _H. vulgaris_) is brown in colour, and produces a bun-shaped egg, spiky on the convex surface, and attached to a water-weed or some object by its flattened side. Brauer found a fourth species, similar in appearance to _H. fusca_, but differing from the three other species in being of separate sexes, and in producing a spherical egg with a knobby shell, which is attached like that of _H. fusca_. The fact already noted that the species of _Hydra_ can be distinguished by the characters of their nematocysts is a point of great interest. In each species, two or three kinds of nematocysts occur, some large, some small, and for specific identification the nematocysts must be studied collectively in each species. It is very remarkable that this method of characterizing and diagnozing species has never been extended to the marine hydroids. It is quite possible that the characters of the nematocysts might afford data as useful to the systematist in this group as do the spicules of sponges, for instance. It would be particularly interesting to ascertain how the nematocysts of a polyp are related to those possessed by the medusa budded from it, and it is possible that in this manner obscure questions of relationship might be cleared up. _Protohydra_ is a marine genus characterized by the absence of tentacles, by a great similarity to _Hydra_ in histological structure, and by reproduction by transverse fission. It was found originally in an oyster-farm at Ostend. The sexual reproduction is unknown. For further information see C. Chun (HYDROZOA [1]. Pl. I.). [Illustration: FIG. 49.--Diagram showing possible modifications of persons of a gymnoblastic _Hydromedusa_. (After Allman.) a, Hydrocaulus (stem). b, Hydrorhiza (root). c, Enteric cavity. d, Endoderm. e, Ectoderm. f, Perisarc, (horny case). g, Hydranth (hydriform person) expanded. g', Hydranth (hydriform person) contracted. h, Hypostome, bearing mouth at its extremity. k, Sporosac springing from the hydrocaulus. k', Sporosac springing from m, a modified hydriform person (blastostyle): the genitalia are seen surrounding the spadix or manubrium. l, Medusiform person or medusa. m, Blastostyle.]