THE TREMATOD A. 77 



body, through which a fluid containing numerous glo- 

 bules is expressed. I must suppose with Siebold, that 

 this is the proper excretory organ of the Distoma, which 

 thus obtains an exit. For it cannot be doubted that 

 the fluid secreted and contained in this organ is of an 

 entnely different nature in the Distoma to that in the 

 Cercaria or its " nurses" and "parent-nurses'' since in 

 the former it is rejected, and in the latter retained in the 

 body. 



When the Cercaria is buried in the skin of the snail, 

 it apparently secretes from its whole surface a very large 

 quantity of slime, in which the animal, which we recog- 

 nize as a distoma-larva, is in continual motion around the 

 same point ; and as the mucus graduaUy hardens, a case 

 is as it were formed around it (fig. 4 b). The walls of 

 this pupa case appear to be thickened by a delicate 

 membrane, which is shed doubtlessly from the body of 

 the animal itself, since the spiculum mentioned above is 

 always found to be cast off", and to be stationary at some 

 part or other of the walls of the cyst, although during the 

 circular movements of the larva within the case it had 

 been carried round from side to side together with it. 

 The detached spiculum is afterwards covered over, and 

 waUed off by a distinct layer of mucus. {Compare ö in 

 figs. 4 Ö?, 4 ^, and 4/.) 



In a few hours after the larva or CercaricB have af- 

 fixed themselves to the skin of the snaü, nearly all of 

 them have assmned the pupa state ; and the oval pupae 

 may be placed so close to each other on (the back \nacken\ 

 and edge of the mantle of the snail) that a portion of the 

 skin viewed under the microscope resembles, in miniature, 

 a pavement composed of oval pebbles ; and this is the 

 case not only in snails which have been kept in glasses 

 and in a smaU quantity of water, and consequently ex- 

 posed to great numbers of their enemies, but also in 

 those taken from pools of water in the natural state. 

 After entering the pupa state, the animal appears to 

 shrink away from the sides of the case, so as to leave a 



