{66} It is hardly necessary to describe here the structure or contents of the hydatid cyst which forms the home of the parasite, nor its etiology, since these topics belong to general pathology, and the cyst is the same in whatever organ it may be seated. When it affects the kidney, it is usually the left--more frequently that of a man between thirty and forty years of age. A hydatid cyst may be situated upon any part of the kidney. If small, it may never make its presence known. A larger one may give rise to those vague pains in the back found with so many diseases of the kidney and characteristic of none of them. A cyst may open in any direction, but is more likely to empty into the pelvis of the kidney. When this happens, the smaller cysts or pieces of the larger ones often enter the ureter and give rise to renal colic, and possibly, later, to a pyelitis. Other points of discharge are the intestines, the lungs, or the abdominal walls. After a hydatid cyst has reached a certain size its presence may be recognized by palpation, but the diagnosis between it and other tumors of the kidney must be very difficult unless characteristic fragments make their appearance in the urine at the same time that the tumor diminishes in size, or unless they can be obtained by puncture. The hydatid thrill, if it can be obtained, will be an important factor in diagnosis. The TREATMENT of this affection in the kidney presents no special points of difference from that of similar cysts in the liver; with this important exception, that besides punctures with large and small trocars, incisions, electrolysis, etc., the resource of complete extirpation still remains. Cures have been obtained by repeated punctures and subsequent suppuration, and by partial removal through the abdominal walls and subsequent drainage. Among the parasites of the kidney it is customary to mention the Strongylus gigas, which is a worm somewhat resembling the ascaris and inhabiting the pelvis. It is not very infrequent among the Carnivora, but since only seven cases have been described in the human subject since the seventeenth century, and only a part of these are admitted as genuine by certain authors, its diagnosis, prognosis, and treatment must depend more upon theory than upon experience. The diagnosis is to be made, if at all, on the basis of a pyelitis and the discovery of the eggs of the parasite in the urine. The Distoma hæmatobium is a parasite found chiefly in the blood-vessels, and especially those of the portal system. It is occasionally, however, met with in the veins of the kidney and also in the urinary passages. Its eggs pass into the pelvis and ureters, and there begin their development, which, however, is soon arrested, as they rapidly perish in the urine.