HYDRA, THE, or fresh-water polype, is the type of the class EvnnozoA, which, with the ANTHOZOA, form the sub-kingdom ealenterata of recent zoologists. See ZOOPHYTE.
The hydra possesses a gelatinous, sub-cylindrical body, which, from its contractility, undergoes various alterations of form. One end expands into a disk or foot, which adheres to a leaf, twigs, etc.; while a mouth, surrounded by a circlet of tentacles, vary ing from five to twelve or more in number, is situated at the opposite end. These ten tacles are exceedingly contractile, at one moment thrown out as long delicate threads, at the next, drawn up into minute wartlike knobs. Numerous thread-cells project from their surface, the larger ones possessing a sheath and three recurved darts or barbs, and terminating in a long and extremely slender filament. The mouth leads into a capa cious cavity. excavated throughout the whole length of the animal, which, exclusive of its tentacles, seldom exceeds three-fourths of an inch. On minutely examining the hydra, or any member of the class hydrozoa, the body is found to be composed of two membranes, an ectoderm and an endoderm, the former constituting the outer layer of the animal, and having one side always in contact with the water, while the other side is in close contact with the endoderm, whose free surface forms the lining of the great internal cavity. The food of the hydra consists of such minute living organisms as come within the reach of its tentacles, and by these apparently fragile threads, which the animal projects like a lasso, crustaceans, worms, etc., are seized, which would be 'deemed at first sight superior to their captor in strength and activity. The tentacles .appear, however, to possess, through the action of the thread-cells, a powerful benumbing .or paralyzing influence, for it has been observed that soft-bodied animals which have suc ceeded in escaping from the grasp of the hydra frequently die very shortly. The prey,
when mastered, but often when still alive, is thrust into the internal cavity, where the nutritive parts are absorbed by the hydra, while the indigestible portions are expelled through the mouth.
Although the hydra is usually found adhering by its circular foot or disk to sub merged leaves, twigs, etc., it is not permanently fixed. It often moves on surfaces under water somewhat after the manner of a leech, both ends taking a part in the movement, and occasionally the disk is protruded above the water, and thus acts as a float.
Sometimes, especially in the autumn, true reproductive organs may be observed, both male and female organs being usually situated on the same animal. Propagation thy gemmation 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 resem ble it; becoming perforated at their free extremity, and tentacles gradually being formed. The pedical by which they originate by degrees becomes thinner, and finally gives way, leaving the young hydra perfectly independent. One of the most remarkable points in the history of this animal is its power of being multiplied by mechanical 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 species of hydra such as ./7. viridis, IT. vulgaris, /T. fusca, etc., have been described, which differ in size, color, etc. When living hydrm are removed from the water, they appear like minute specks of jelly, which quickly recover their true form on being restored to their proper element. The great authority on these singular animals is Trembley, whose ..41gmoires pour sersir d efristoire d'un Genre de Polypes douce was published in 1744.