THE TREMATODA. 91 



oval bodies into the adherent, sluggish Paramacia, so 

 that I have been unable to distinguish betv^^een them, and 

 I consequently see in the whole series of changes, only 

 the passage towards the metamorphosis of the ciliated 

 animalculse into motionless parenchymatous bodies. Al- 

 though motionless, they must not be considered as life- 

 less, since their continued growth may be readily fol- 

 lowed, and is evidently the effect of nutrition, and not 

 an expansion caused by the substance distending them 

 from within. From a length of 7% '" they frequently 

 attain that of A '" ; and the size of a transparent spot 

 indicating a cavity, increases in the same proportion. 

 Wlien the animalcules have attained the size of -io '" their 

 internal cavity is evidently filled with minute oval or 

 spherical corpuscles, which are germs, and which now 

 commence a remarkable development. As the animal- 

 cules increase in size, they become more and more paren- 

 chymatous and friable, and their external substance is 

 broken by the slightest pressure, nothing remaining then 

 but a strong internal membrane which incloses the cavity 

 in which the germs are contained. This rupture occurs 

 especially when they have attained their full size of | '" or 

 even of 1 '", at which time the largest of the successively 

 developed embryos have acquired the length of tV '" and 

 He inclosed merely by a saccular integument, in which the 

 metamorphosed Para7n(Bcium-\^kQ animalcule is scarcely 

 any longer recognizable. 



The included embryos, which, like the Cercarice or 

 their " nurses,'' occur in all stages of development within 

 the sac, present usually a short neck-like prolongation at 

 one end, and on the other a distinct depression, which 

 may probably be an oral orifice or suctorial organ, by 

 which they can affix themselves when they have in some 

 way or other, most likely by bursting it, escaped from 

 the sac ; they are then found of the size mentioned above 

 (u% '") in the internal organs of the muscles. If some of 

 these embryos extracted from the sac or newly escaped 

 from it are allowed to rotate on their axis, under the mi- 



