OKIGIN OF INTESTINAL WOEMS. 19 



different ; still, the immigration of the Monostomum embryo, 

 which is the principal point, must take place, since the singular 

 relations of the infusorial Monostomum embryos and the young 

 Cercaria-sacs they contain, point distinctly to this conclusion. 



Every one will understand, that the knowledge of even such a 

 small fragment of the history of the development of the 

 Monostomum mutabile as this, was of the utmost value, since it 

 afforded the key to the long inexplicable mode of origin of the 

 Cercaria-sacs. There now only remained the question as to 

 what became of the Cercarice, and in what relation they stood to 

 the Trematoda. It was an old idea that there was great similarity 

 between the bodies of the Cercarice and certain Trematoda, viz., 

 Monostomata and Distomata, and the force of the comparison was 

 strengthened by the fact that the Cercarice cast off their tails 

 after leaving the sacs, and thus become still less different from 

 these Trematoda. Many Distomata whose bodies are encircled 

 with spines at their anterior extremity, for example, Distomum 

 trigonocephalum, echinatum, uncinatum, and militare, are so like 

 certain Cercarice., that when the latter have thrown off their tails, 

 any unprejudiced person would take them for the young of these 

 Distomata. In fact, in their whole organization, the Cercarice 

 are really no other than young Trematoda. The circumstance 

 that one never finds sexual organs in the Cercarice is strongly 

 corroborative of the notion that they are young Trematoda not 

 yet sexually developed. Here again we have to do with parasites 

 destined to emigrate and immigrate, that in some other situation 

 they may arrive at sexual maturity. The course which the 

 Cercarice take in their wanderings is, however, a much longer 

 and more complicated one, than that followed by the sexless 

 Gordiacei. These need only leave the insects they have hitherto 

 infested and withdraw into damp ground, where fully grown as 

 they are, and provided with the necessary store of fat in their 

 bodies, they can quietly await the development of their sexual 

 organs. On the other hand, the emigrating Cercarice are destined 

 to enter vertebrate animals, since it is only in the intestinal canal 

 of certain mammals, birds, reptiles, or fishes, that they can grow 

 and mature their sexual organs. 



Many of my readers may be unable to conceive how it 

 is possible for Cercarice living in water, to enter into the 

 intestines of such mammals and birds as live far away from 

 water, or, at any rate, never come into proximity with the 



