72 DEVELOPMENT OF 



cilia, by aid of which it moves readily in the water. In 

 the anterior portion of the body are two quadrangular 

 spots which are inclined obliquely towards each other, 

 and can scarcely be regarded as anything but eyes. The 

 posterior two thirds of the trunk are occupied by a slightly 

 transparent, whitish body (y), which it might be supposed 

 was an organ as Siebold thought it to be, if it were not 

 that after a time, and some rather vigorous motions, it 

 becomes detached, ruptures the body of its parent, and 

 presents itself as an independent animal of entirely 

 different appearance to that in which it lay concealed, and 

 was already developed (fig. 1 a y,) whilst the latter was 

 still within the ovum. 



Now, since this inclosed animalcule is constantly 

 present in the ciliated young Monostomum, and as there 

 is always hut one animalcule in each individual, and 

 always in the same situation, there must necessarily be 

 some organic connexion between the two, and one 

 entirely different from that which it has been supposed 

 could be explained by styling the one a necessary parasite 

 of the other. If one animal is, organically speaking, 

 necessarily connected with another, so that each can be 

 developed only in or around the other, they must belong 

 to one and the same unity, or constitute such a unity, 

 and this is doubtless the case with the animals we are 

 now considering. 



Eig. 1 h, represents the embryo Monostonmm recently 

 born, and possessed of sufficient locomotive powers to 

 enable it to seek out a fit habitation ; when it has met 

 with this, the ciliated integument, and the eyes now use- 

 less, are thrown off by the animal stripping itself of the 

 whole of its skin, together with which it loses also its 

 external form, and becomes the sluggish creatm^e (fig. 1 c 

 and 1 d,) whose motions are confined to a vermicular 

 wriggling. The first form of this embryo is not unlike 

 that of the common ciliated progeny of the Trematoda, as 

 they have been known to us in many species for a long 

 time, from the observations of Mehlis, Nordmann, and 



