356 COELENTER ATA H VDROZOA 



ill its normal position. In the other case the foot is not detached, 

 but glides along the support until it stands close to the tentacles, 

 which now loosen their hold." ^ 



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 ^ of the endoderm may possibly assist in the nourish- 

 ment 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.^ 



H. viridis. — Colour, grass -green. Average number of 

 tentacles, eight. Tentacles shorter than the body. Embryonic 

 chitinous membrane spherical and almost smooth. 



H. vidgaris, 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. fiiscci, 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 Frotohydra (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. 



Protoh.ydra^ was found in the oyster-beds off Osteiid, and 

 resembles Microhydra in the absence of tentacles. It multiplies 

 by transverse fission, but neither gemmation nor sexual repro- 

 duction has been observed. 



Halercmita is a minute hydriform zooid which is also marine. 

 ^ G. Wagner, Quart. Journ. Micr. Sci. xlviii. 1905, p. 589. - See p. 12C. 



^ Hydra 2)allida, Beardsley, has been found to be very destructive to the fry of 

 the Black-spotted Trout in Colorado, U.S. Fish. Mep. Bull. 1902, p. 158. 



■* For figures of Frotohydra see Chun, Bronn's Thier-Reich, " Coelenterata," 

 1894, Bd. ii. pi. ii. 



