The =mature sclerostomata= are found attached to the mucosa of the large intestine into which the head is sunk for the purpose of sucking the blood, and they may be gray, brown or red according to the quantity of blood which they have imbibed. The author has found them in little hernial sacs of the mucosa hanging from the peritoneal surface. The =sexually immature sclerostomata= are found in little pill-like masses of ingesta in the large intestines and from which they project part of the body through a narrow opening. Another habitat is in cysts of the mucosa of the cæcum and colon and less frequently of the small intestine, individual cysts varying in size from a pin’s head to a hazel nut, and containing the young worm rolled upon itself, and varying in size but always less than the intestinal worm and always asexual. In some cases the cyst is found empty but with a small opening toward the lumen of the bowel showing the means of escape of the parasite. A third habitat of the immature worm is in the blood-vessels, especially the posterior aorta and its divisions, and still more constantly the anterior and other mesenteric arteries. =Biology.= The ova of the sclerostoma are segmented in the oviduct but are hatched out after they have been laid. The hatching may be effected in the intestine or in manure or water external to the body. When hatched out in the intestine they may pass out at once with the manure or they may envelop themselves in pellets of the finer ingesta and remain for a time in the bowel and finally pass out in this condition. Baillet has traced their development out of the body. In a watery or damp medium they are hatched out in a few days as a cylindroid worm ¼ to ⅓ mm. long, thick in front and with a filiform tail. In moist environment but especially in damp manure they grow to 1 mm. or 1.5 mm. and continue for months in this condition, but remain small and asexual, until taken in, in the drink or green food of the soliped. Reaching the intestine and especially the cæcum and colon they bore their way into the mucosa and encyst themselves, or if they happen to perforate a blood-vessel they make a habitat of that. In the cyst, development proceeds and when it has reached a certain stage the worm once more bores its way through the mucosa and reaching the intestine becomes sexually mature. In this last migration the young worm is liable to perforate a blood-vessel in which case it is destined to a period of existence in the blood. It may, however, have blundered upon a blood-vessel at an earlier stage when seeking a temporary home in the mucous membrane, so that the sclerostomata of aneurisms may be derived from two separate sources. In the blood-vessels the parasite attains a length of 1 to 8 lines, whereas in the mucous cysts it does not exceed 3½ lines. Yet Neumann holds that after leaving the blood-vessels they may again encyst themselves in the mucosa before escaping into the intestine.