HY'DATID (from the Greek hydatis, a watery vesicle), a tern indefinitely applied to several distinct objects of a vesicular or cyst-like character, which. arc found in the bodies of men and certain mammals. True hydatids were formerly regarded as cystic entozoa (q.v.), such as cystieercus, ccenurus, and echinococcus, but ail_ these animal forms are now discovered to be larval stages of twnia or tape-worm (q v.). These hydatids may occur in alnaost any part Of the body, and they have been observed in man, the ape, the ox, the sheep, the horse, the camel, the pig, the kangaroo, and some other vegetable feeders, but they apparently do not occur in carnivorous animals or in the rodents. They are generally inclosed in an external sac, which is attached to the i tissue of the organ in which it is situated, and which is frequently common to many hydatids, each of which has a distinct envelope. The fluid in the interior of the hydatid itself is almost always colorless and limpid, but the fluid in the common cyst in which the hydatids float is often of a yellow color. The ewnitrus cerebralis is found in the brain of various ruminants, and gives rise to the disease in sheep known as " the stag gers." When the hydatid occurs in the fourth ventricle, the animal, instead of turning round and round in one direction, springs in the air, and this variety of the affection is hence distinguished by German veterinarians as dos Springen. Whenever any of the above forms of hydatids are swallowed by man or the lower animals, they may proceed, under favorable circumstances, to be developed into the higher stages of tape-worm. Two specieS of echinococcus are usually noticed, namely, the E. liominis, wide]) has been occasionally met with in the brain and abdomen of man, and the E. veterinoriem, which is of common occurrence in-various parts of the body of the pig, and several other mammals, but it is by no means certain that they are really distinct. These echino cocci do not become developed into tape-worms unless they reach the intestinal canal of some animal, by being taken as food; and in ordinary cases of hydatids, consisting of echinococci, the cysts and their contents undergo a kind of degeneration, becoming in some cases converted into fatty or calcareous matter, while in other cases the contents become granular, the peculiar booklets (which will be described in the article TAPE Wolof) which occur in them, and which remain unaltered for a long time, revealing their true origin.
The so-called acephalocyst, or common globular hydatid, which sometimes attains the size of a child's head, is probably a degenerated or abnormally developed echino coccus.
Hydatids sometimes occasion so little inconvenience, that persons, in whom they are discovered after death, have not suspected any disease in the organ in which they are found. On other occasions, they grow rapidly, and cause so much irritation that suppuration is excited in or around the common sac, which may either burst externally, or into a mucous canal or a serous cavity. In the first or second case, the hydatids will be discharged, and recovery may take place; in the third case, fatal inflammation will ensue. Little can be done for the treatment of this affection, except that occasionally, if the cyst is near the surface, it may carefully be punctured. The means of preventing the affection will be noticed in the article TAPE-WORM.
False hydatids are simple serous cyst, either occurring alone or in clusters, whose mode of origin is not distinctly understood. Structures of this kind, on a small scale, are common in the choroid plexus of the brain, while on a large scale they are found containing the fluid in ovarian dropsy. These false hydatids are also of a comparatively common occurrence in the uterus, which they may distend to such a size as to simulate pregnancy.