68 DEVELOPMENT OF 



snails, which had been exposed for several months to a 

 degree of temperature unusual to them, and not been 

 furnished with their natural food. The individuals which 

 remain within the aqueous reservoirs, and undergo their 

 transformation in the interior of the snails, are specially 

 destined for the multiplication of the Bistomata witliin 

 those animals ; and on the contrary, those Cercarice 

 which are expelled through the water-passages, serve to 

 propagate the species on other snails, although some of 

 them may enter into the pupa state on the surface of the 

 same snail from which they had been expelled, and in 

 the interior of which their foster-parents had lived. 



Let us now leave the consideration of the destiny of 

 the Cercaria which are not expelled from the animals 

 infested by them, and return to the " nurses,'' in order 

 through them to come at the origin of the Bistomata. 

 For this purpose, we must seek for the smallest indivi- 

 duals of that class, and endeavour to discover their primi- 

 tive form, so that we may be the better able to obtain a 

 glimpse at their origin. Here, however, we are met by 

 the difficulty, that although they may be found of a cer- 

 tain degree of smallness, yet that we can never get 

 beyond this, and find them still smaller. However, they 

 may always be met with of such small size and imperfect 

 form, that it is impossible to suppose they could have 

 originated in the transformation of Cercarice or full-grown 

 Bistomata. Eig. 4 d, represents a very young individual, 

 about the youngest form I have been able to meet with 

 in the viscera of the snail, in the large masses of " nurses' 

 which so often occur there, and which have received from 

 Siebold the name of ''Cercaria nests." These younger 

 individuals are distinguished from the older ones by the 

 proportionately larger size of the head and collar, and the 

 greater length of the ventricular cavity, which reaches 

 almost as far as the oblique processes. When visible 

 germs of embryos are present, they occupy only the pos- 

 terior portion of the cavity of the body, and do not 

 surround the stomach. It may also be remarked, that in 



