28 STUDIES IN ANIMAL LIFE. 



beside this embryo you will find many eggs, which 

 would in time become insects ! 



Or take that lazy water-snail (Paludina vivipara), 

 first made known to science by the great Swam- 

 merdamm, the incarnation of patience and exact- 

 ness, and you will find, as he found, forty or fifty 

 young snails in various stages of development ; and 

 you will also find, as he found, some tiny worms, 

 which, if you cut them open, will suffer three or 

 four infusoria to escape from the opening.* In your 

 astonishment you will ask, Where is this to end ? 



The observation recorded by Swammerdamm, 

 like so many others of this noble worker, fell into 

 neglect, but modern investigators have made it the 

 starting-point of a very curious inquiry. The worms 

 he found within the snail are now called Cercaria 

 sacs, because they contain the Cercarice, once classed 

 as Infusoria, and which are now known to be the 

 early forms of parasitic worms inhabiting the di- 

 gestive tube and other cavities of higher animals. 

 These Cercarice have vigorous tails, with which they 

 swim through the water like tadpoles, and, like tad- 

 poles, they lose their tails in after life. But how, 

 think you, did these sacs containing Cercarice get 

 into the water-snails? "By spontaneous genera- 

 tion," formerly said the upholders of that hypothe- 

 sis, and those who condemned the hypothesis were 

 forced to admit they had no better explanation. It 

 was a mystery which they preferred leaving unex- 



* SWAMMERDAMM. Bibel der Natur, p. 75-77. 



