86 DEVELOPMENT OF 



extending it, having seen them sometimes as perfect 

 j)up(B in that situation. Hence it necessarily follows, that 

 the Distoma proceeding from one of these pupa will be 

 found within the " nurse" and would be as it were an en- 

 tozoon to it rather than to the snail. Such an occurrence 

 as this is represented in fig. 6 a, where a Distoma-\ik.e 

 individual hes under the creeping multitude of Cercarice, 

 and has already attained such a size as certainly to prevent 

 its exit, except by rupture of the " nurse." I have wit- 

 nessed this interesting sight in only about twenty or thirty 

 individuals, amongst thousands of " nurses" which I have 

 had under the microscope ; an entirely different circum- 

 stance, however, which occurred in some snails is not to 

 be comprehended in this account. These had all their 

 internal organs nearly full of large " nurses" of which 

 every fifteenth or twentieth individual had one, two, or 

 three full-grown Distomata lying under the remaining half 

 or fully developed brood ; two even had more, the one 

 four and the other five ßuhes or Distomata (fig. 6 <5.) In 

 these the mode of arrangement by which they occupied 

 the smallest possible space was very evident; this was 

 efiected by their being placed alternately with the broader 

 and narrower end towards the same side, and this regu- 

 larity of position occurred even when there were only two 

 or three within the " nursed It would not therefore be 

 extraordinary, if we should here and there meet with 

 ''i^2(5m;^y" organisms, containing only full-grown Distomata 

 and no Cercarice at all ; since it is evident from what is 

 stated above, that even the whole progeny may possibly 

 attain their full development within the " nurses!' Thus 

 we are furnished in this case with an instance of the Cer- 

 carice becoming pupce, and of their emerging from that 

 state within the bodies of the " nurses " the Cercarice also 

 frequently enter into \}iMd pupa state within the cavities of 

 the snail, especially in the aqueducts, the walls of which 

 are occasionally seen covered with pupce, and by this 

 means the number of the entozoa infesting one and the 

 same snail is of course very remarkably increased. 



