TYPE PLATYHELMINrHES. 



151 



A still further complexity is found in the Liver Fluke, 

 Distomum hepaticum. In this form the free-swimming embryo 

 makes its way into the tissues of a small snail and there be- 

 comes converted into a Sporocyst. The germ-cells of the 

 Sporocyst give rise by their development, not to Cercarise, 

 as in the usual cases, but to Eedise, and these may give rise 

 under certain conditions to a second brood of Redise (Fig. 79, 

 B, r'). During the summer, however, the Redise produce Cer- 

 carise, which, leaving their host, swim about for a short time, 

 and finally encyst themselves, not necessarily in a second 

 MoUusk, but on grass or any other object with which they 

 may come in contact, the tail at the same time being lost. 

 If, now, these encysted forms are swallowed by a sheep, the 

 young Distome makes its way to the bile-ducts of the host, 

 where it becomes mature. 



The following schema by K. Hertwig will show the relationships of these 

 different methods of development ; 



In a few Distomese a simplification more extensive than that repre- 

 sented in the first columns of the schema occurs, as for instance in the 

 genus Holostomum, whose embryo, after making its way into the body of 

 the first host, seems to be gradually metamorphosed into the immature 

 Distome, without any alternation of generations. 



A very peculiar life-history is found in Distomum mMcrostomum, which 

 is parasitic in insect-eating birds. The Sporocyst is found in a snail, and 

 is peculiar in that it assumes a branching form, the branches forming a 

 network among the tissues of the host. In the ends of filaments of the net- 

 work young Distomes develop without the intervention of a Cercaria stage, 

 and by their development and its own growth the terminal branches be- 

 come of a considerable size, two of them extending into the tentacles of 



