TREMATODES : DEVELOPMENT 



143 



our singing-birds infect themselves or their young with Urogonimus 

 macrostomus by tearing off pieces of the corresponding sporocysts 

 from the snails (Succinea amphibia), which act as the first inter- 

 mediary hosts, and eating, or offering their fledgelings these pieces, 

 which are full of encysted cercaria. 



The miracidia of the digenetic trematodes are comparatively 

 highly organised ; they have a cuticular epithelium (fig. 75) entirely 

 or partly covered with cilia, beneath this a dermo-muscular tube 

 composed of circular and longitudinal muscles ; moreover, a simple 

 intestinal sac with an oesophagus, occasionally also with pharynx, 



FIG. 77. Development of Fasciola hepatica, L. a., The miracidium, optical sec- 

 tion with germinal bodies and cilia. &., Young sporocyst with germinal bodies. 

 .. Sporocysts with redia (magnified). (From Leuckart.) 



salivary glands and boring spine, also a cerebral ganglion on which, 

 in some species, there are eyes (fig. 77, a) ; as to the excretory 

 organs, they are represented by two symmetrically-placed terminal 

 cells joined to vessels that discharge separately ; there is a more 

 or less ample (primary) body cavity between the parietes of the 

 body and the intestine ; from the cellular parietal lining of this 

 cavity single cells become loosened (fig. 77, a, b), and later become 

 cercaria or rediae. 



The SPOROCYSTS, which are produced direct from the miracidia, 

 are very simple, as all the organs of the latter disappear during 

 or after penetration into the intermediary host, even to the muscles 

 and excretory organs, whereas the loosened and still loosening cells 

 of the body continue to develop rapidly. The sporocysts when 

 fully developed have the appearance of tubes or fusiform bodies 



