Linton. — Trematode Parasites of Fish 247 



their cilia and are transformed into a more or less irregular 

 mass called a sporocyst. In the interior of a sporocyst there 

 develop, from masses of cells, small worm-like structures 

 provided with a mouth, a pharynx, and a straight intestine. 

 These structures, called redise, leave the parent sporocyst 

 and invade the liver of the snail. There they develop fur- 

 ther and there is differentiated within them a more or less 

 considerable number of young" flukes. These differ from the 

 adult form in the rudimentary character of their reproduc- 

 tive organs and, usually, in having a long tail. They agree 

 with the adult form in having both an oral and ventral 

 sucker and a forked intestine. 



These young flukes are called cercaria?. They are ex- 

 ceedingly active, and in form and movement suggest tad- 

 poles. In the tails of cercaria? which I found at Woods 

 Hole this summer (1911) striated muscle fibres were very 

 distinctly shown. Upon escaping from the parent redia the 

 cercaria of the liver fluke lives for a time in water. After 

 it has abandoned its host the cercaria creeps upon the sur- 

 face of vegetation, where it secretes around itself a trans- 

 parent cyst, losing its tail during the process of encystment. 

 Sheep or cattle, or, accidentally, man, eating vegetation on 

 which these cysts occur, may thus become the final host in 

 which the young distome develops to maturity after finding 

 its way into the bile ducts of the liver, where it produces 

 eggs, and the round of life is completed. 



In passing, it may be w r orth while to note the enormous 

 number of individual flukes that might develop from a sin- 

 gle egg. Thus the egg gives rise to one ciliated larva, and 

 the larva to one sporocyst. The number of redias which may 

 develop within a single sporocyst varies considerably within 

 the same species. Furthermore, more than one generation 

 of sporocysts or of redias may appear before the cercaria. 1 

 are produced. On the other hand the redia stage may be 

 omitted altogether. Sporocysts which I have found thus 

 far in marine invertebrates produce cercaria; directly with- 

 out the intervention of a redia stage, and, in one case, the 



