The exact causes are usually obvious, practically always so in human medicine, but a number of cases escape adequate explanation. From a practical standpoint two origins are important to zoological collections, trauma and intestinal perforations by sharp objects. Ungulata frequently suffer abdominal injuries in fighting, as do rodents. Peritonitis sometimes supervenes even in the absence of penetrating wounds, probably by reason of damage to the intestine whereby its permeability is increased. Pointed objects are frequently swallowed by animals and perforation occurs. The danger of feeding split bone to carnivores is well known; some years ago we lost two tigers and a lion in this manner. Ileus, in one of the several forms, has been an occasional cause of peritonitis in primates and ungulates. The extension of purulent inflammation, abscesses and the like is easy to understand, but we have seen several cases of apparent extension from enteritis without perforation. The reason for this is probably in the kind of enteritis. Monkeys with amœbiasis and gallinaceous birds with enterohepatitis have supplied most of the cases, these infestations of the gut wall being deep and spreading so that a chance is afforded to penetrate the serosa along blood and lymph vessels. One case in a deer seemed to originate from a simple catarrhal colitis; trichocephalus in the colon may have helped. Parasites are not very potent in causing an acute peritonitis, but aggravate the action of other agents. Septicemic states are at the bottom of 24 per cent. of our cases of peritonitis. This is particularly true of birds, it being recognized that their acute general infections frequently have such an effect, but the primates and carnivores also have a vulnerable peritoneum when septicemia exists. The principal outstanding visceral lesions in the mammalian cases is pneumonia; in birds it is cholera and plague. The rupture of eggs in birds lays the foundation of a peritonitis, while bacteria from the oviduct or cloaca complete the process. Chronic peritonitis is not common. It is usually due to parasites or to tuberculosis. There has been observed, however, no complete general involvement of the peritoneal cavity including the liver and spleen, sometimes called “sugar-icing,” and believed to be tuberculous in origin. The only tumor found in very close association with the peritoneum occurred in a Chapman’s Zebra (_Equus burchelli chapmani_) in which animal a fibromyoma seemed to spring from a loop of intestine and grow away from the gut wall. It was undergoing myxoid change. SECTION VIII THE URINARY TRACT THE KIDNEY.