ORIGIN OF INTESTINAL WORMS. 23 



Now, if these Cercarice can only attain their sexual maturity 

 in the warm-blooded vertebrate animals, which devour in- 

 sects, and therefore seek their food in the air or on land alone, 

 the Cercaria, that had established themselves in the Aselli and 

 Gammari, would wait in vain for the time to arrive when they 

 should be transported into the air, since the animals in which 

 they were domiciled would never quit the water. Again, many 

 Cercarice, in their haste, become encysted incautiously at so early 

 a period, that the purpose of the process is defeated. 



I have already shown that the emigrated Cercaria ephemera 

 attaches itself to water-plants, or any other objects in the water, 

 by means of the cyst which it elaborates ; other Cercarite even be- 

 come encysted before they quit the body of the aquatic snail in which 

 they were generated ; whilst some, again, have even been found 

 encysted within the Cere aria-sacs. 1 Steenstrup takes this to be 

 a normal phenomenon ; I should only consider it such, provided 

 that the encysted Cercarice in the snails are intended to attain 

 their sexual maturity, in the intestines of fishes or of water-birds 

 feeding on snails. 



Although the various facts I have communicated can only be 

 regarded as fragments of the natural history of certain Trematoda, 

 they are yet capable of being connected into a whole, if the 

 theory of the Alternation of Generations be extended to them. 

 For instance, from the foregoing statements, we perceive that certain 

 sexually matured Trematoda (Monostomum, Distomum) generate 

 young within their sexual organs, which are not developed into 

 sexual individuals similar to their parents in form and structure ; 

 but that, on the contrary, each embryo is converted into an 

 animal of remarkably different form, viz., into a Cercaria-sac, 

 which has the import of a sexless nurse, since without possessing 

 sexual organs, it nevertheless generates young Cercarice. These 

 Cercarice again differ from their parents, but gradually become 

 sexually perfect, and in form and structure take the likeness of 

 their grandparents. The several embryos of these Trematoda, 

 therefore, do not pass into an equal number of new and separate 

 sexual Trematoda, but each embryo produces a nurse, which, by 

 asexual generation, brings forth a greater or less number of 

 sexual Trematoda. 



1 Steenstrup (1. c., p. 85, pi. iii [English translation]), has more particularly described 

 and figured such Cemzn'a-sacs containing encysted Cercariae. 



