THE TREMATOUA. 83 



and which, as it were, filled every snail with their thou- 

 sands, such a depression was never wanting at one end, 

 and at the opposite extremity T could always distinguish 

 another smaller depression or opening, the skin over which 

 was sometimes elevated in a vesicular form ; in these I 

 was certainly unable to detect the germs of the future 

 progeny ; but before they had doubled their size, and 

 while still not so large as the bodies of the Cercaria 

 afterwards to be developed in them, I could very plainly 

 distinguish the germs in the form of rounded granules. 

 The whole alteration undergone by these sacculi diu-ing 

 then* growth (figs. 1 a — /) may perhaps be conceived in 

 this way; that the supposed sacculi are not hollow in their 

 earliest state, but consist throughout of a mass of glo- 

 bules and vesicles ; that while they are still extremely 

 small, a minute cavity is formed within them, in which 

 the germs of the future Cercarice are situated ; that, nor- 

 mally a simultaneous increase goes on of the germinal 

 cavity, and of the supposed sacculus itself, the walls of 

 which evidently consist of numerous round glandular ve- 

 sicles. The independent motions which are observable in 

 them in this condition cease altogether when the embryos 

 approach the cercaria form, and the whole integument is 

 then distended into a thin membrane in which any or- 

 ganized structure can with difficulty be distinguished. 



Now, as these presumed sacculi, at least at one period 

 of their existence, are capable of spontaneous motion, and 

 are only in appearance destitute of organization, since they 

 are seen to grow or increase in bulk by organic means, 

 we must certainly consider them as animated beings, and 

 according to the observations adduced in the preceding 

 pages, even as animals which stand in such intimate 

 relation with the Cercaria, that these latter occur only 

 in the sacciform animals, and on the other hand the 

 saccifonn animals alivays contain Cercarm more or less 

 developed. They must consequently both be necessary 

 parts or links of one and the same unity, and I have no 

 hesitation in placing the sacciform individuals in the same 



