The bulk of the larva is occupied by a number of sarcode globules. A careful description of the miracidium is given by Brock (Lancet, Sept. 9, 1893, p. 625), to which the reader is referred for further details.
Life-history.—Beyond its first stage of free-swimming ciliated larva, the extracorporeal life of Schistosomum hæmatobium was until quite recently unknown. Leiper's remarkable work in Egypt, however, has completely filled in this hiatus in our knowledge of the life-history of this important parasite. It is therefore no longer necessary to describe, as in previous editions of this manual, the many attempts that had been made previously to trace the progress of the miracidium. Leiper has shown (1916) that the miracidia of S. hæmatobium and of S. mansoni, after escape from the egg, enter a fresh-water snail: in the case of S. hæmatobium, a Bulinus (B. dybowski, B. innesi, B. contortus) (Figs. 133, 134, 135); in the case of S. mansoni, Planorbis boissyi (Fig. 136)—small molluscs abounding in the irrigation canals of Egypt. The miracidia, after penetrating the integument of the mollusc, pass to the liver, wherein they develop into sporocysts and daughter sporocysts (Fig. 137), multiplying to such an extent that the entire liver becomes permeated with the long, delicate, transparent tube-like bodies. Presently numberless bifid-tailed cercariæ develop within the sporocysts (Figs. 138, 139), and on maturing escape spontaneously into the surrounding water. Opportunity occurring, the now free cercariæ penetrate the skin of some suitable vertebrate—man, mouse, rat, monkey—dropping their tails in the passage. Entering lymphatics or blood-vessels, they proceed to the liver of the definitive host, wherein, in the course of about six weeks, they attain sexual maturity and produce terminal- or lateral-spined eggs according to species. To obtain these results in the laboratory, all that is necessary is to place the living experimental animal, or a part—tail, limb, etc.—of such animal, in the water into which cercariæ have escaped from the snail, care being taken that the dose of cercariæ is not too large, as in such case the excessive invasion of the liver may prove rapidly fatal. The experimenter should be very careful to avoid bringing cercariæ-containing water into contact with his own skin. As molluscs are liable to invasion by the miracidia, sporocysts, and cercariæ of many species of trematodes, it is necessary for those who would confirm Leiper's discoveries that they should be able to recognize the specific
