[Illustration: FIG. 34.—_Bilharzia haematobia_ Cobb. × 10. The female (♀) lying in the gynaecophoric canal of the male (♂). _d_, Alimentary canal; _ms_, oral sucker of male; _vs_, ventral suckers. (After Leuckart.)] The worm is found usually in couples, which have been proved to be male and female individuals (Fig. 34), often in considerable numbers in the veins of the pelvic region, chiefly the veins of the bladder and of the large intestine, and it is tolerably certain that _Bilharzia_ enter these vessels from the portal vein. Their long slender bodies enable them to penetrate into the finer vessels, which get partially or entirely choked up, and the circulation accordingly impeded. But the most serious consequences are observed in the urinary bladder. The mucous membrane is swollen and inflamed here and there, chiefly on the dorsal surface, the capillaries appear varicose and covered with mucus, mixed with blood-extravasations in which _Bilharzia_-eggs are noticeable. The eggs also cause numerous swollen knots in the submucous tissue. Should the disease not pass beyond this stage (and such is usually the case, especially in South Africa), a temporary haematuria ensues. The urine, which is only expelled with great effort, accompanied by intense pain, is mixed with blood, mucous clots, and masses of _Bilharzia_-eggs, from which some of the embryos have already hatched out. The symptoms, however, may gradually pass away, and a more or less complete recovery accomplished. The disease may indeed be of a far less severe character, and may not interfere with the usual occupations of the patient; but, on {70}the other hand, a far more extensive thickening of the wall of the bladder sometimes occurs; hard masses of eggs, uric acid crystals, and other deposits, may lead to the formation of stones, degeneration of the substance of the ureter, and eventually to that of the kidney itself. The stone, indeed, has long been known to be a prevalent disease in Egypt, and it is now known to arise from concretions formed round masses of _Bilharzia_ eggs. From the portal vein, again, other _Bilharzia_ may gain access to the rectum, or the liver, and it has also been found in the lungs, and may give rise to most serious complications, if indeed the patient lives. How infection occurs is a question to which at present no satisfactory answer can be made. The attempt to introduce embryos of _Bilharzia_ into the common fresh-water animals of Alexandria has hitherto proved fruitless (Looss[88]), although there seems little doubt that the comparative immunity of Europeans from the disease is in some way owing to their drinking purer water than the natives. Possibly, as Leuckart suggests, the embryo becomes a sporocyst in man himself, somewhat as _Taenia murina_ is known to develop in the rat without an intermediate host.[89] The immense numbers of the parasite in one host would then readily receive an explanation.