THE TREMATODA. 65 



moved at all angles with the latter ; on which account 

 the more the tail is developed, the more lively are the 

 motions of the embryo within its foster-parent ; at the 

 same time the body has become more flexible, contrac- 

 tile, and extensible, and the abdominal acetabulum which 

 was indeed visible as a depression anterior to the root of 

 the tail, assumes now the distinct appearance of a sucto- 

 rial disc, but this organ like the tail does not attain its 

 full activity until the embryo has left the " nurse,'' 

 although it is frequently very prominent and expanded, 

 as shown in fig. 5 m, in which it is also seen how much 

 the abdomen is hollowed out, before the animal begins to 

 move about upon it. None of the internal organs are 

 evident before the embryos are fully grown ; but at that 

 period, some organs are more distinct than in the free 

 swimming individuals which have escaped from the 

 " nurses,'' viz., the organs filled with minute cells, which 

 in several instances of embryos taken from the " nurses" 

 themselves, were clearly seen to be the two branches of a 

 canal which ran the whole length of the body between 

 the branches of the digestive organ, and appeared to open 

 into the cavity within the tail. The collar itself is formed 

 the last. Some doubt exists as to the mode in which the 

 CercarifE quit their " nurses," since it has been observed 

 under the microscope that there are two places at which 

 they come away, viz., from each side of the body in the 

 depression under the collar, and from the abdominal 

 surface between the two oblique processes, but they 

 escape from the latter situation only when the animal 

 has been slightly compressed between the glasses ; and 

 from the former on the contrary when no pressure at all 

 has been employed. It very often appeared to me that 

 there were two openings in the cervical depression, which 

 were smTounded as it were with elevated borders. That 

 the " nurses" are able to discharge their brood of Cer- 

 carice without external aid, is shown by the great number 

 of those creatures swimming about free in the internal 

 cavities of the snails ; but that the contraction of the 



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