PREVENTION OP VEEMICEOUS DISEASES. 483 



Nematode or round- worms are little afifeoted by the above, at least 

 the majority of species. The roimd-worms may or may not have 

 two distinct hosts. Such groups as the Trichinae live in two 

 different animals or in different parts of the same animal. The 

 asexual forms live in the connective-tissue organs, and in the blood- 

 vessels, &c. ; the sexual forms in the intestines, the air-passages, and 

 a few beneath the skin. The majority of Nematodes pass their eggs 

 out in the host's dung, the worms coming away when their full 

 complement of eggs are laid. These ova lie about upon the ground, 

 get carried into the water, and are thus taken up again with food 

 and drink. It seems that some undergo a slight development out- 

 side the host upon the damp ground and vegetation ; and possibly 

 some few live in a secondary host, such as earthworms, snails, 

 insects, &c. 



The well-known disease, trichinosis in pigs, rats, and men, is 

 distributed chiefly by rats, and through them it is given to pigs, and 

 from the latter to man. Here again the knowledge of the life- 

 history helps us, for by stopping the common practice of giving 

 dead rats to the pigs, we shall tend to check a disease which in 

 human beings may be attended with fatal results. 



Some pastures are kno\yn to be impregnated with certain 

 diseases, such, for instance, as lung-worm or husk. When this is 

 Icnown to be the case, it is well to keep the animal subject to that 

 disease off the land for some time, feeding it down in the mean- 

 while with other stock that are not invaded by the particular kind 

 of parasite we wish to destroy. In the red intestinal worms, the 

 armed sclerostomes, in horses, we can employ this way of clearing 

 the paddocks by grazing sheep on them for some time : these 

 animals, feeding close and making the land obnoxious by their 

 excreta, destroy the majority of the eggs passed out in the horses' 

 dving, and are not themselves invaded by the equine species. A 

 large number of round- worms live in the intestines of all animals, 

 and as the ova are passed out in the dung, it is very essential in 

 an outbreak of these nematodes to see that the dise^ised animals 

 are boxed and all the excrement burnt, whilst the meadows should 

 be kept free from the hosts, and if the disease is not an ovine one, 

 dressed with salt and fed down by them. Other diseases, such as 

 husk, are spread by the embryos being brought up in the mucus 

 from the air-passages ; these germs are scattered about upon the 

 ground, and thus sow the seeds for numbers of other lambs and 

 sheep to obtain. When that spasmodic cough so characteristic of 



