PLATHELMINTHES 



187 



motion. The tailless Cercarise of Dlstomnm maerostomum 

 (produced in LeucocMoridium paradoxum), together with 

 parts of the germ tubes, arrive in the intestine of the final 

 host [birds], in the cloaca of which they become sexually 

 mature (Zeller). As a rule the Cercaria passes, by an 

 active mig-ration, from its first intermediate host into a 

 second, which naturally is also an aquatic animal, either 

 another snail or a worm, crustacean, insect, mollusc, fish, or 

 amphibian. In this second intermediate host it casts off the 

 tail and becomes encysted. The young worm awakens to 

 new life only after its host has been taken as food and 

 digested by some other, usually higher, animal. In this 

 way the cyst is dissolved, and the young 

 Distomum now reaches the stage of the 

 sexually mature animal. But we have 

 seen that in Distomum hepaticum the 

 second intermediate host may be omitted, 

 and that the Cercaria, after becoming 

 encysted in the free condition, passes 

 directly into the final host. The state- 

 ment, often made, that tailed Cercarise 

 could migrate directly into the final host 

 (for example, the Cercaria macrocerca of 

 Distomum cygnoides into the urinary 

 bladder of the fi'og), has not been sub- 

 stantiated. On the contrary, these Cer- 

 carise appear to he obliged to pass 

 thixDugh the encysted stage. 



Fig. 92.— Embryo of 

 UoiiostoiauDi mutabile, 

 shortly after hatching 

 (after v. Siebold). R, 

 Redia. 



A most remarkable condition is presented by 

 the embryos of Monostomum viuiabile and 31. 

 flavum, two Distomidce, which are found in the 



thoracic and orbital cavities of various aquatic birds. The embryos 

 abandon the egg-membi-ane when still in the uterus of the parent. These 

 Distomids are therefore viviparous. In each embryo a Eedia-like 

 creature is ah-eady present (Fig. 92). In this case, therefore, the 

 embryo produces the new generation even before it has time to find an 

 intermediate host, within which to develop into a sporocyst. There is 

 scarcely a doubt but that the bud is formed from the germ cells of the 

 embryo. 



