774 PARASITIC DISEASES. 



velopment either within soft mud alone, or within the bodies 

 of small slugs and other minute denizens of herbage, or possibly 

 within the bodies of larval insects and minute entomostracans, 

 inhabiting ponds, ditches, and running streams. It thus seems 

 that whilst some nematodes can accomplish their develop- 

 mental processes without any lengthened sojourn outside their 

 final bearer, others, on the contrary, require particular, varied, 

 and prolonged conditions which shall enable them to undergo 

 certain preliminary changes altogether exterior to and apart from 

 the bodies of their ultimate hosts. In short, as Leuckart points 

 out, we have two distinct groups of strongyles : those which lead 

 a free life in their larval state, undergoing a certain grade of 

 development in mud and water ; and those which pass through 

 certain larval changes of growth within the bodies of insects 

 and other intermediary bearers. 



So much for the natural history of the nematoda, or order of 

 thread-worms and round-worms, on which much more might be 

 said, were we not limited to a general view of all the groups of 

 internal parasites. As it is, the reader will not fail to perceive 

 that, speaking generally, it is now clearly understood how cattle 

 and sheep and other animals obtain one frequent form of lung 

 disease. It is almost needless to add that the labours of hel- 

 minthologists have thus contributed largely towards the forma- 

 tion of rational principles on which to base successfully both a 

 radical and prophylactic method of treatment. 



PLATYHELMINTHES. 



The flukes and tape-worms which belong to this class have 

 flattened bodies, hence the name Platylielminthes or flat- worms. 

 They are hermaphrodite, and are usually furnished with organs 

 of attachment, such as suckers and hooks. Usually there is an 

 alternation of generations, i.e., the young one is not like its 

 parent, but must pass through various metamorphoses in order 

 to reach the adult form. 



TREMATODA. 



This order comprises the flukes ; and it is of great interest 

 to the veterinarian, inasmuch as he is occasionally consulted iu 



