j66 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



.,-). If we ignore the tail it is clear that the structure 



merely that of the adult fluke in a comparatively simple 



and undeveloped .-(.million. The cercariae, whether produced from the 



nerati n of mliae as they sometimes are or from a later generation 



mure usually are, make their way out of the body of the parent 



:id finally out of the snail, swimming away with a characteristic 



jerky motion. 



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



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



,ind 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 



prodiu <(! l>y 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. 



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 



1 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 TKKMATODA are essentially parasites, and the group is charac- 

 teri/ed by the following combination of structural features the un- 

 sr- merited 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 uroup is divided into two sub-groups, the Monogenea and the 



iea. 



The 1 >I<,I.M;A, or digenetic Trematodes, are given this name from the 



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



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



inhabiting usually the alimentary canal of a Vertebrate, while 



rations (rediae, cercariae) infest Molluscs. In some cases 



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



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



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



inhabit in- various Mammalian hosts, and many of these are liable, as 



is the cast- with I<asciola hepatica, to find their way occasionally into 



I human beings. A few are normal and dangerous parasites 



o| man. 



