PARASITES AND PARASITISMUS. ' 201 



and this is most marked after a warm wet season with 

 frequent floods ; and those animals pastured on lowland 

 soil are most liable to become affected. These parasitic 

 living beings^ endowed with a power of motion, and to a 

 certain extent of selection of their special abodes, in their 

 migrations through the system cause irritation, and this 

 occurs as well with the movements of the larval as of the 

 mature forms. The former sometimes bore through the 

 tissues until they come to a desirable place to stay at, the 

 latter generally limit their movements to those which 

 suffice to enable them to traverse the canal or organ in 

 which they are found. The irritation causes slow local 

 changes, condensations, sometimes abscesses, and so on. 

 And it often gives rise to a debilitated state of the con- 

 stitution which kills animals, either being induced by 

 some impediment to function, by drain on the constitu- 

 tion, or by excess of nervous irritation. 



Thus it is that debility (and constitutional deficiency of 

 energy, as seen in young or old animals) is a marked 

 phenomenon in disease of a parasitic nature; it supple- 

 ments that caused by the parasites, and the two together 

 suffice for the production of death, which would not have 

 resulted from either of them operating by itself. In this 

 sense we must take the statement that debility is a cause 

 of parasitismus, and not infer that a weak state of the 

 patient will tend to the generation of parasites de novo. 

 According as parasites live in or upon their hearers or 

 hosts, as the affected animals are termed, they are 

 classified as Entozoa and Ectozoa. Whereas the former 

 are Anumloida, such as tapeworms, fluke-worms, and 

 round worms, the latter are generally allied to spiders, 

 being truly Annulose ; insecta, or arachnida. There are, 

 of course, exceptions to this rule ; thus, the larval form of 

 the insect oestrus, and the spider-like pentastoma, are 

 found internally in the horse and dog respectively, but in 

 the ox the oestrus is an external parasite, and the penta- 

 stoma in its larval form only is harboured by this animal. 



The phases of development of the various parasites are 

 of the highest interest and practical importance. Thus, 



