i66 ZOOLOGY FOR MEDICAL STUDENTS chap. 



ventral sucker (v.s). If we ignore the tail it is clear that the structure 

 of the cercaria is merely that of the adult fluke in a comparatively simple 

 and undeveloped condition. The cercariae^ whether produced from the 

 first generation of rediae as they sometimes are or from a later generation 

 as they more usually are, make their way out of the body of the parent 

 redia and finally out of the snail, swimming away with a characteristic 

 jerky motion. 



Presently they drop off their tails and creep about in leech-like 

 fashion by means of their suckers, shooting out the body to a considerable 

 length and then shortening it. Under normal circumstances the cercaria 

 creeps up on to a blade of grass and adhering to this by its ventral 

 sucker proceeds to encyst, surrounding itself with a clear secretion 

 produced by the conspicuous gland on each side of its body. 



Within the cyst the cercaria goes on slowly developing, the features 

 of the adult fluke becoming more and more distinctly recognizable (Fig. 

 74, G), but the development is not completed unless the blade of grass 

 with its adherent cyst is cropped by a sheep. In this event the cyst is 

 digested and the young fluke set free in the sheep's alimentary canal : 

 it wanders up into the bile-duct and there in due course attains to its 

 adult form and sexual maturity. 



The Trematoda are essentially parasites, and the group is charac- 

 terized by the following combination of structural features — the un- 

 segmented body with usually at least a ventral sucker for attachment 

 to the host, the thick cuticle, the forked blindly-ending intestine, and the 

 hermaphrodite reproductive organs. 



The group is divided into two sub-groups, the Monogenea and the 

 Digenea. 



The Digenea, or digenetic Trematodes, are given this name from the 

 fact that the parasitic portion of their life-history is divided between 

 two distinct host-animals as is the case with Fasciola — the sexual 

 phase inhabiting usually the alimentary canal of a Vertebrate, while 

 the asexual generations (rediae, cercariae) infest Molluscs. In some cases 

 a second Vertebrate host may be introduced into the life-cycle, the 

 cercaria making its way into the body of a fresh-water fish and there 

 encysting. 



There exist a great variety of Trematodes in the group Digenea, 

 inhabiting various Mammalian hosts, and many of these are liable, as 

 is the case with Fasciola hepaiica, to find their way occasionally into 

 the bodies of human beings. A few are normal and dangerous parasites 

 of man. 



