THE TREMATODA. 81 



I have, however, satisfied myself that very far from all 

 the mdividiials reach the organs mentioned above, viz., 

 the liver and generative organs ; for I have observed 

 gi'eat numbers of them full-grown which could not be 

 distinguished from those in the liver, occupying the ca- 

 vity in the head and anterior part of the body which is 

 filled with a slimy fluid, and in which the nervous ring 

 of the pharynx is situated, in the cavity around the heart, 

 and in several other situations, so that this Distoma is 

 not one of those species whose full development is re- 

 stricted to one organ of the animal infested by them. 



We have thus again traced a free swimming Cercaria 

 or JDistoma-larva, until it is deposited as a true entozoon 

 Distoma or ßiike, deep in the organs of the snail ; we 

 shall now see that it has previously inhabited, also, the 

 interior of a snail, although not of the same one. The 

 observations already referred to of Bojanus, Baer, and 

 Siebold, have proved that all Cercaria occur in sacciform 

 bodies in the interior of snails, and the sacs in which 

 these animals are situate have been described by Siebold, 

 in Bm-dach's ' Physiologie,' 1. c. The figures 1 / and 

 1 ff represent sacs of this kind, containing numbers of 

 developed and undeveloped Cercarice ; and if one of these 

 sacs is opened, the whole series of forms through which 

 the Cercaria pass can be readily followed (figs. 2 a — -y,) 

 from the globular gemmules to the perfect forms, which 

 as soon as they have quitted the sac, can no longer be 

 distingidshed from those swimming free in the water. 

 It may be soon ascertained that the whole course of de- 

 velopment entirely corresponds with that of the former 

 species. The parts which are completed last are the 

 frontal spiculum and the double tortuous organ, which 

 passes down on each side of the body. 



By the first inquiries it was soon determined that all 

 Cercarice originate in the sacciform bodies, and that 

 these always contain Cercaria, and grow and are de- 



6 



