THE TUEMATODA. 87 



(c) Cercaria ephemera, Nitzsch, and Distoma 

 DUPLiCATUM, Baer. 



It has been shown by the development of Cercaria 

 echinata and C. armata, that both these species enter the 

 snail from mthout, become pu^ce upon and within them, 

 and after quitting thej??/^«a; cases, penetrate, as forms of 

 the genus Distoma, into the internal organs, which they 

 inhabit as true entozoa, so that it is no longer a mere hy- 

 pothesis, but an ascertained fact, that at least some entozoa 

 enter the animals infested by them from without ; w^e have 

 fmllier seen, that both these species of Cercaria are 

 developed only within special organisms, which must be 

 considered as independent creatures, and as individuals of 

 the same species as the Cercarice themselves, but which 

 as regards their form differ very widely from the latter, 

 in consequence of some of their organs having been deve- 

 loped at the cost of others, in order to contribute in the 

 greatest degree possible to the development of the germs 

 of the Cercarice. 



These animalcules, which, on account of the office they 

 fulfil have been termed " nurses," derive their origin, as 

 regards the one species from nearly similar animals, in 

 the bodies of which they were developed from gemma, 

 and which in all probabihty, and according to all analogy 

 had originally proceeded from the ova of a distoma in the 

 form of ciliated embryos, and had afterwards undergone 

 metamorphosis. 



With respect to the second species C. armata, my ob- 

 servations have not yet shown a similar development 

 within " nurses'' of a second generation, or ''parent 

 nurses' as I have called them, although I have occasionally 

 seen several " nurses' of this species which were inclosed 

 as it were in a common membrane, which might be 

 supposed to be perhaps the delicate integument of the 

 ''parent nurse." 



