208 METAMOKPHOSES OE MAN 



viscera of fresh-water mollusks there were developed 

 — how, it was unknown — beings called Sporocysts, 

 species of living envelopes, provided with a well- 

 marked digestive tube, but always devoid of repro- 

 ductive organs. These sporocysts gave rise simul- 

 taneously to new bodies like themselves, and to 

 germs, which became developed into Cercarice, crea- 

 tures in the form of tadpoles, able to inhabit water, 

 but of quite as neuter a character as the others. 

 These Cercarise were the necessary parasites of the 

 Sporocysts. After undergoing their development in 

 the interior of the latter, the Cercarias burst their 

 walls and became encysted, somewhat like the di- 

 pterous insects we spoke of in a preceding chapter, 

 and terminated their short lives in the prison in 

 which they were enclosed. 



We see that, according to this method of explaining 

 observed facts, a sexless animal, of unknown origin, 

 produces simultaneously, by gemmation, creatures like 

 itself, and others of an entirely different character, 

 which could never be directly produced. It would 

 be idle to touch on the vague and evidently incom- 

 plete nature of such ideas. 



Steenstrup, by his theory of alternate generation, 

 threw a light upon these dark clouds, which seemed 

 almost to lower in proportion as men laboured to 

 dispel them. Supported by his own and his prede- 

 cessors' researches, he boldly classed the Distomidce 

 (Helminthes of the trematode group) side by side 

 with the Corynidaa and Medusidge, in regard to their 

 mode of reproduction. It was shown by the Danish 

 savant, that the peculiar bodies then termed Sporocysts 

 were the true nurses of the Trematodes, and that the 



