192 ANIMAL PARASITES AND MESSMATES. 



port where their progeny may prosper. This first ena- 

 bryonic state is that in which there are the greatest 

 perils. When stripped of their swimming tunic, these 

 young distomes have the form of a bag, which for a long 

 time was called a sporocyst. From these sporocysts we 

 see hundreds and thousands of young ones proceed, 

 resembling in no respect the mother, which has brought 

 them into the world. These, in their turn, will resume 

 a free and independent life. They are colonists whom 

 the distome has left on a foreign land. This simple 

 multiplication is often not sufficient for the preservation 

 of the species ; the first sporocyst produces other similar 

 sporocysts, and these bring into the world a rich pro- 

 geny of tadpoles, which after a certain metamorphosis 

 will become sexual distomes. These tadpoles are often 

 well armed, and devour occasionally even the last scrap 

 of flesh belonging to their host. They have long been 

 known under the name of Cercariae, which was given to 

 them at a time when their genealogy was unknown. 

 They are not very unlike the tadpoles of the frog (Fig. 45). 

 The mother was only a bag with ciliae, and sometimes 

 with eyes. The tadpole has a distinct body, with a 

 movable deciduous tail ; and after this falls off they 

 have sexual organs. 



The cercarise often abandon their first host in which 

 they have been developed, and live at liberty in the 

 water while waiting for their final host. They are taken 

 sometimes in the open sea. In 1849, J. Miiller wrote to 

 me from Marseilles that he had just discovered cercariae 

 and distomes living at liberty in the Mediterranean. 

 Since then this illustrious naturalist has observed them 

 again at Trieste; while pursuing his studies on the 



