_Hydra_ appears to be purely carnivorous. It will seize and swallow Entomostraca of relatively great size, so that the body-wall bulges to more than twice its normal diameter. But smaller Crustacea, Annelid worms, and pieces of flesh are readily seized and swallowed by a hungry _Hydra_. In _H. viridis_ the chlorophyll corpuscles[291] of the endoderm may possibly assist in the nourishment of the body by the formation of starch in direct sunlight. Three species of _Hydra_ are usually recognised, but others which may be merely local varieties or are comparatively rare have been named.[292] _H. viridis._—Colour, grass-green. Average number of tentacles, eight. Tentacles shorter than the body. Embryonic chitinous membrane spherical and almost smooth. _H. vulgaris_, Pallas (_H. grisea_, Linn.).—Colour, orange-brown. Tentacles rather longer than the body, average number, six. Embryonic chitinous membrane spherical, and covered with numerous pointed branched spines. _H. oligactis_, Pallas (_H. fusca_, Linn.).—Colour, brown. Tentacles capable of great extension; sometimes, when fully expanded, several times the length of the body. Average number, six. Embryonic chitinous membrane plano-convex, its convex side only covered with spines. The genera _Microhydra_ (Ryder) and _Protohydra_ (Greeff) are probably allied to _Hydra_, but as their sexual organs have not been observed their real affinities are not yet determined. _Microhydra_ resembles _Hydra_ in its general form and habits, and in its method of reproduction by gemmation, but it has no tentacles. It was found in fresh water in North America. _Protohydra_[293] was found in the oyster-beds off Ostend, and resembles _Microhydra_ in the absence of tentacles. It multiplies by transverse fission, but neither gemmation nor sexual reproduction has been observed. _Haleremita_ is a minute hydriform zooid which is also marine. {257}It was found by Schaudinn[294] in the marine aquarium at Berlin in water from Rovigno, on the Adriatic. It reproduces by gemmation, but sexual organs have not been found. Another very remarkable genus usually associated with the Eleutheroblastea is _Polypodium_. At one stage of its life-history it has the form of a spiral ribbon or stolon which is parasitic on the eggs of the sturgeon (_Acipenser ruthenus_) in the river Volga.[295] This stolon gives rise to a number of small _Hydra_-like zooids with twenty tentacles, of which sixteen are filamentous and eight club-shaped. These zooids multiply by longitudinal fission, and feed independently on Infusoria, Rotifers, and other minute organisms. The stages between these hydriform individuals and the parasitic stolon have not been discovered. ORDER II. MILLEPORINA. _Millepora_ was formerly united with the Stylasterina to form the order Hydrocorallina; but the increase of our knowledge of these Hydroid corals tends rather to emphasise than to minimise the distinction of _Millepora_ from the Stylasterina.