HYDRA (Lat., from Gk. 1.4a, water-snake. Skt., Lith. udra, OChurch Slay. vydra, OHG. attar, Ger. Otter, Eng. otter, with Gk. ifitdp, Ryder, water). A minute polyp or hydrozoan, common in still. fresh waters. It possesses a gelatinous, subcylindrieal body, which, from its contractility, undergoes various alteration,: of form. One end expands into a disk or foot. which adheres to a leaf or twig: while a mouth sur rounded by a circlet. of tentacles, varying from five to twelve or more in number, is at the op posite end. These tentacles are exceedingly contractile: at one moment they are thrown out as long, delicate threads, at the next. drawn up into minute wart-like knobs. Numerous thread cells project from their surface. the larger ones possessing a sheath and three recurred darts or barbs, and terminating in a large and extremely slender filament. The mouth leads into a capa cious ea \ i t y. excavated throughout the entire length of the animal, which, ex clusive of its tentacles, sel dom exceeds three-fourths of an inch. The food of the hydra eonsists of such mi nute living organisms come within the reach of its tentacles, and by these ap parently fragile threads, which the animal projects like a lasso, crustaceans, worms and the like are seized, whiell would be deemed at first sight supe rior to their captor in strength and activity. The tentacles, however. exert through the action of the thread-cells a powerful benumbing or para l•zing influence. The prey when mastered. but often \Olen still alive, is thrust into the internal cavity, where the nutritive parts are absorbed by the hydra, while the indigestible parts are ex pelled through the mouth.
Although the hydra is usually found adhering to submerged supports. it is not permanently fixed. It often moves on surfaces under water somewhat after the manner of a leech, and oeca sionally the disk is nrotnided above the water, and thus acts as a float. Sometimes, especially in the aulumn, true reproductive organs may ho obsened, both male and female organs being usually situated on the same animal. Propaga tion by gemination is, however. the most common mode of increase. :Minute tubercles appear on the body of the parent animal, which, as they increase in size, gradually become perforated at their free extremity, and acquire tentacles. The pedicel by which they originate by degrees be comes thinner, and finally gives way, leaNing the ?oting hydra perfectly independent. One of the most remarkable points in the history of this animal is its power of tieing multiplied by me chanical division. If a hydra be cut into two or even more pieces, every one will in time assume the form and functions of the original animal. Several sleeks of hydra have been deseribed, which differ in size, color, and respects. Two species are common in America. in still water in nearly all parts of the country. One of these (hydra riridis) is bright green in color, while the other (Hydra fuser) is grayish-brown. The green hydra is notable because its color is due to the presence of chlorophyll. the coloring matter characteristic of plants. Both may eas ily is reared in aquariums.