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758 In careful comparative examination Flu has shown that there are several structural differences between the adult S. hœmatobium and S. mansoni. He finds that whereas in the male S. hœmatobium the anterior border of the lateral folds forms almost a right angle with the long axis of the body, in S. mansoni the folds are gradually sloped off. In the female S. mansoni the ovaries run a tortuous course unlike those in other schistosomes, the ootype is asymmetrical, and the oviduct opens laterally on the ventral side where the shell gland is situated. In sixty females he found only lateral-spined ova.* Leiper has made out several additional points of difference in anatomical detail, particularly as regards the testes and alimentary canal (see p. 747); these, together with his discovery that S. hœmatobium makes use of a Bulinus and S. mansoni of a Planorbis as intermediary host, are conclusive evidence that these trematodes belong to different species.

Apparently S. mansoni, like Necator americanus, is a West African species which had been introduced into the western hemisphere by the African negro; and just as N. americanus and Ankylostomum duodenalefrequently concur in the same country and even the same individual, so it is with S. hœmatobium and S. mansoni.

S. mansoni inhabits chiefly the mesenteric veins, and its ova deposited within the submucous layer of the rectum give rise to dysenteric-like symptoms, mucus with blood being passed from time to time, the ova-laden stools becoming frequent and their passage perhaps being attended with tenesmus. As in the case of the schistosomum next to be referred to, the initial stages of the infection are apt to be attended with urticarial eruption, eosinophilia, and fever apt to be regarded as enteric. In certain well-established cases small, sometimes large, branching, soft growths are to be felt inside the sphincter ani. They