208 THE FLUKES 



is the almost cosmopolitan liver fluke, Fasciola (or Distomum) 

 hepatica, of sheep, goats and other ruminants. This species 

 occasionally establishes itself in man also, but it can be looked 

 upon only as an accidental parasite as far as man is concerned. 

 Its life history (shown diagrammatically in Fig. 63) will be 

 described in some det-ail since it is more thoroughly known than 

 is that of most of the flukes, and because it is typical of the group. 



The adult of the liver fluke (Fig. 63A) lives normally in the 

 bile passages and liver tissue of its host. About three weeks 

 after the flukes have reached their destination in or near the 

 liver, reproduction commences. Eggs (Fig.-63B) begin to pass 

 out through the uterus of the fluke, and are carried by the bile 

 of the host to the intestine and thence out of the body by the 

 faeces, a single fluke producing as many as 50,000 eggs. Since 

 there may be over 200 flukes in a single host, the number of 

 eggs voided may amount to many millions. These eggs, if they 

 chance to fall into water of moderate temperature, hatch out 

 little ciliated embryos known as miracidia (Fig. 63D), which 

 resemble ciliated protozoans. They are about 100 ju (^Jg^ of an 

 inch) in length. Each of the embryos swims about for a day or 

 two, by means of its cilia, in an effort to find a suitable interme- 

 diate host, in this case certain species of snails of the genus 

 LimnoBa (Fig. 63E), and if successful it bores into the snail by 

 means of a little pimple-like projection at the anterior end of 

 the body. It is obvious that only a small per cent of the embryos 

 are likely to survive the double risk of not reaching water, and 

 if safely in water of not reaching a suitable snail to bore into. 

 However, once safely within the tissues of the snail, the embryo 

 begins the second phase of its existence, during which it reproduces 

 to make up for the enormous mortality encountered in the trans- 

 fer from sheep to snail. 



In the course of some days the ciliated embryo transforms into 

 a saclike body or "sporocyst" (Fig. 63F), the inner " germina- 

 tive " cells of which act as parthenogenetic eggs (i.e., eggs which 

 do not need fertilization), each developing into a larva of a new 

 type, known as a redia (Fig. 63G). The latter, when nearly 

 mature, burst the wall of the mother sporocyst and migrate into 

 other tissues of the snail. The redite are very simple organisms 

 with a sucker and an imbranched blind pouch for a digestive 

 tract. Like the sporocyst they contain germinative cells within 



