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XLI] feature of certain other fork-tailed cercariæ, but absent in bilharzia; (4) the presence of two sets of glands communicating with the mouth, and lying on both sides of the posterior end of the body. Leiper has shown further, to use his own words, "that those animals infected with cercariæ from Bulinus dybowski, etc., always produce adult worms which give rise to terminal-spined eggs only, while those infected with cercariæ from Planorbis boissyi give rise equally constantly to lateral-spined eggs. In no case do both varieties arise from the same intermediate host. Moreover, the adult worms reared from these two sources show constant ..... morphological differences." In S. hæmatobium, reared from Bulinus, there are four or five large testes and the gut

Fig. 137.—Bilharzia daughter-sporocysts teased from liver. (Leiper.)

branches do not unite until late, resulting in a short cæcum. In S. mansoni, reared from Planorbis boissyi, there are eight or nine testes, and the gut branches unite early, forming a long cæcum. In S. hæmatobium the uterus contains many eggs, whereas in S. mansoni it is short and contains only one or two eggs. S. mansoni, unlike S. hæmatobium, always causes a heavy deposition of black granules in the liver of the human host. "Lastly, as regards habitat. The males of S. hæmatobium appear to leave the liver early and to pass down into the finer branches of the mesenteric veins before they attain maturity. The females found in the gynæcophoric canal are diminutive. The males of S. mansoni remain in the liver until the females in copula begin to lay eggs, and large numbers of lateral-spined eggs are frequently laid in series by coupled worms in the veins even of the edge of the liver."