66 DEVELOPMENT OF 



snail itself, probably also exerts a considerable influence 

 in this respect is evident, from the swarms of Cercarice 

 which appear in the water when the snail is compelled to 

 contract itself forcibly within its shell. It would appear 

 that the Cercarice ^are expelled from the internal cavities 

 of the snail (the aqueous chambers, which extend be- 

 tween the viscera and are partly bounded by them, and to 

 the walls of which the " nurses" especially attach them- 

 selves) through the same minute canals, by which the 

 water enters those cavities. It is very rare, however, 

 that all the Cercarice come away ; some of them are re- 

 tained and swim about, as has been said, in the aqueous 

 chambers, and assume the pupa state on their walls when 

 they have found a fit spot, and there undergo the same 

 transformations as the individuals which have made their 

 escape into the v/ater outside. 



Siebold frequently found pupse in the snails, as I have 

 often done myself; Baer has also seen them, but not 

 understood what they really were, for he did not conceive 

 that the phenomena observed by Nitzsch in connexion 

 with the supposed death of the Cercarice was their en- 

 tering into the pupa state, but assumed that a single cer- 

 caria was placed in each of the cases under the same 

 conditions as those to which a number of cercarice are 

 subjected, in the particular forms to which I have above 

 given the name of " nurses." 



There is one organ especially in which above all others 

 the pupse of the Cercarice are met with, viz., the amicle, 

 in which if there are any pupae in the animal they are 

 sure to be found, and usually in considerable numbers. 



In Planorbis corneus and Limnceus stagnalis I found 

 pupse even in the month of August, and afterwards 

 throughout the autumn. 



In Paludina vivipara, in the auricle of which Baer so 

 constantly found the pupse, both of the Cercaria ecJdnata 

 (Sieb. ?) and of another species, the Cercaria ephemera 

 (Nitzsch,) especially the latter, that he states them to be 

 peculiar to that part, I have, in the winter and beginning 



