(From the same). See Aquula. Htdatis. A hydatid. (See Phlyctaenae.) Hy-datis, considered as a genus of disease, has been placed by Dr. Cullen among the local diseases in the class tumo-ret. It has been found, however, to possess an independent life, and has been referred to the vermes intes-tinalcs, characterised as a vesicular body, at least posteriorly, and terminated anteriorly by a head furnished with three or four antennae, with or without fangs. These lymphatic vesicles have been long known, and observedon the liver, the brain, the ovaria, and the other viscera; but it is lately discovered that they resemble in nature and almost in form the taenia.

This genus differs from taenia in the membranous vesicular expansion, which appears to constitute its whole form, and in which the head is generally concealed. It differs also in its habitation, for it is never found in the intestines, but on the surface of the viscera, on the membranes, and sometimes enveloped in the-fat. In every other respect they are the same; and we remark in them four suckers, and in their centre either a corona of fangs or a depression, apparently the mouth of the animal. It is not surprising that its nature has so long been overlooked, since many trials must be made to discover the head, often the only organic part of its system.

Dr. Tyson first discovered hydatids in the livers of sheep; Bartholine observed them in the livers of goats, and Pyerus in hogs. To Pallas, however, we are indebted for the first correct and connected account of these animals, which he has described and figured in his Miscellanea Zoologica. Since the period of his publication, Goeze, Batsch, Bloch, and others, have added to the stock of facts, though much remains to be known. Mougeot, a young physician, has collected in the form of a thesis, entitled "A Zoological and Medical Essay," what has been hitherto published; to which he has added some new facts, and the treatment necessary in one of the species, viz. the hydatids of the uterus. This essay, unfortunately, we have not been able to obtain. Dr. Adams also has endeavoured, with some success, to show that cancer is owing to the introduction of an animal of this kind. But if cancer is owing to an animal, its structure forms the septa so often described in such ulcers. These are, in fact, the animal, not the residence of its formation.

Hydatids are, in general, superficial; that is, some portion is enveloped in the substance of the liver, for instance, and a part rises above it. Yet this is not an universal rule; for, as they are very numerous, some must be wholly concealed. Those species which burrow in the fat are entirely covered, and can never change their place. Their size differs according to the pecies, the age and temperament of the animal at whose expense they live. Pallas speaks of some as large as the fist, and others are mentioned still more bulky. Such we have ourselves seen. Their figure is infinitely varied] but very generally approaches that of a flatted sphere. Their colour is generally white, or semitransparent, sometimes of an amber colour; their substance composed of different membranes, conglutinated and formed of circular fibres, visible by means of a lens; but in many animals, particularly in sheep, susceptible of an evident contractile power. Internally, on the part opposite to the head, we can observe a disc, somewhat thicker than the membrane, with often a number of fatty tubercles, which have been supposed eggs. They are filled, though not wholly, with lymph, usually transparent, of an oily, salt taste, which becomes cloudy by heat. These vesicles have a motion of their own, which may be styled peristaltic, and is often very lively. The head is not always at the termination, but often concealed by the reduplication of its skin; and from this part the hydatid acts on the viscus in which it lives, and sucks its lymph. The animal is best seen by separating the vesicle, and placing it between two panes of glass, taking care that they press very gently on the anterior part. The head, in that case, pressed forward by the lymph, projects; and even with a lens, the suckers and fangs, if it has any, are observable. This part may be preserved dry for a considerable time.

Hydatids, in the human race, are found chiefly in the liver, the spleen, the uterus, the ovaria, the kidneys, the placenta, the lungs, and even the muscles. Those in the accumulated fluid of dropsies are accused as the cause of the disease; and those in the head are supposed to occasion insanity. Hydatids exist occasionally in the human viscera, without occasioning any inconvenience; but acute pains, either continual or temporary, are supposed to be sometimes owing to them. Their existence may, it is said, be suspected by weakness, emaciation, and oppression at the stomach; but these symptoms are owing to many other causes, and were the existence of hydatids ascertained, there is, we fear, no remedy that would reach them. Hydatids of the liver are often found in a cyst of a cartilaginous firmness, composed of different laminae, and thicker, in some instances, than in others. The laminae are white, and apparently lined with coagulable lymph. In one body, Dr. Baillie saw the cyst divided by a pulpy substance. A cyst often contains various hydatids, some floating loosely in the fluid, and some attached to its sides; of very different sizes, from a pin's head to that of a walnut. The largest are generally found floating in the cavity of the abdomen when distended by dropsy.

In animals they are more common than in man. They are generally found in the livers of hares, especially those which have fed in marshy ground. In rats they are equally numerous; and in-sheep they occasion vertigo when in the brain, and the rot when they burrow in the liver. In the hog they produce the appearance sometimes called measles; and they are found also in the rein-deer, the goat, and the ox. The remedy in sheep is to change the pasture from the marshes, where the disease is caught, to high gravelly soil, where it seldom appears, and to give the animal a portion of salt; for in salt marshes the hydatids are not found. An author whom we have mentioned, but whose work we have not seen, recommends, we are told, injections of salt water into the uterus when hydatids exist in that organ.