HYDRA, polypes, in natural history, a genus of the Vermes Zoophyta class and order. Animal fixing itself by the base, linear, gelatinous, naked, contractile, and furnished with setaceous tentacula or feelers; inhabiting fresh waters, and pro ducing its deciduous offspring or eggs from the sides. There are five species. H. gelatinosa, minute, gelatinous, milk. white, cylindrical, with twelve tentacula shorter than the body: it inhabits Den mark in clusters on the under side of Fuci. But on the viridis, the fusca, and the grisca, the greater number of experi ments have been made by naturalists, to ascertain their true nature and very won derful habits. They are generally found in ditches. "Whoever has carefully ex examined these when the sun is very pow erful, will find many little transparent lumps, of the appearance of jelly, and size of a pea, and flatted upon one side. The same kind of substances are likewise to be met with on the under side of the leaves of plants that grow in These are the polypes in a quiescent state, and apparently inanimate. They are generally fixed by one end to some solid substance, with a large opening, which is the mouth; at the other, having several arms fixed round it, projecting as rays from the centre. They are slender, pellucid, and capable of contracting themselves into a very small compass, or of extending to a considerable length. The arms are capable of the same con traction and expansion as the body, and with these they lay hold of minute worms and insects, bringing them to the mouth, and swallowing them. The indigestible parts are again thrown out by the mouth. The green polype was that first discover ed by M. Thembley : and the first appear ances of spontaneous motion were per ceived in its arms, which it can contract, expand, and twist about in various direc tions. On the first appearance of danger they contract to such a degree, that they appear little longer than a grain of sand, of a fine green colour, the arms disap pearing entirely. Soon afterwards, he found the grisca, and afterwards the fusca. The bodies of the viridis and grisca di minish almost insensibly from the ante rior to the posterior extremity ; but the fusca is for the most part of an equal size, for two-thirds of its length, from the an terior to the posterior extremities, from which it becomes abruptly smaller, and then continues of a regular size to the end. These three kinds have at least six, and at most twelve or thirteen arms. They can contract themselves till their bodies do not exceed one fourth of an inch in length, and they can stop at any intermediate degree of expansion or con traction. They are of various sizes, from
an inch to an inch and a half long. Their arms are seldom longer than their bodies, though some have them an inch, and some even eight inches long. The thick ness of their bodies decreases as they extend themselves, and vice versa; and they may be made to contract them selves, either by agitating the water in which they are contained, or by touch ing the animals themselves. When taken out of the water they all contract so much, that they appear only like a little lump of jelly. They can contract or expand one arm, or any number of arms, inde pendently of the rest; and they can like wise bend their bodies or arms in all possible directions. They can also dilate or contract their bodies in various places, and sometimes appear thick set with folds, which, when carelessly viewed, ap pear like rings. Their progressive mo tion is performed by that power which they have, of contracting and dilating their bodies. When about to move, they bend down their heads and arms, lay hold by means of them on some other substance to which they design to fasten themselves ; then they loosen their tail, and draw it towards the head ; then either fix it in that place, or stretching forward their head as before, repeat the same operation. They ascend or descend at pleasure in this manner upon aquatic plants, or upon the sides of the vessel in which they are kept; they sometimes hang by the tail from the surface of the water, or sometimes by one of the arms ; and they can walk with ease upon the surface of the water. On examining the tail with a microscope, a small part of it will be foundto be dry above the surface of the water ; and, as it were, in a little concave space, of which the tail forms the bottom ; so that it seems to be sus pended on the surface of the water on the same principle that a small pin or needle is made to swim. When a polype, there fore, means to pass from the sides of the glass to the surface of the water, it has only to put that part out of the water by which it is to be supported, and to give it time to dry, which it always does upon these occasions ; and they attach them selves so firmly by the tail to aquatic plants, stones, &c. that they cannot be easily disengaged : they often further strengthen these attachments by means of one or two of their arms, which serve as a kind of anchors for fixing them to the adjacent substances.