DIRECTIONS FOR OBTAINING MATERIAL 93 



confined exclusively, or almost exclusively, to man. The others are 

 normally parasitic in the domestic or wild animals ; they may, how- 

 ever, adopt man as a host, in which case they are described as 

 11 occasional " parasites. In many cases, the normal hosts of parasites 

 which are occasional in man have not as yet been discovered. 



It is not possible, nor is it very necessary, to examine all these 

 many varieties of Helminthes, some of which are exceedingly rare. 

 It will suffice for the student to recognize from personal observation 

 those species which are of frequent occurrence ; while the rarer sorts, 

 should they come under his notice, may be identified by the aid of 

 the text-books of the subject. 



CHAPTEK I. 

 DIRECTIONS FOB OBTAINING MATERIAL. 



It is probable that every vertebrate species may, under conditions, 

 harbour Helminthes. Naturally, they are not to be found in every 

 individual, nor will all the parasitic varieties to which it is subject 

 be found in a single member of the host-species. Certain host-species 

 are less frequently attacked than others, while the percentage of 

 affected individuals of the same species is dependent upon locality and 

 age. In other words, it is dependent upon the food, the nature of 

 which changes at different stages of growth. As long as food is 

 taken in a form which precludes the transmission of Helminthes^ for 

 so long will the individual be preserved from infection at least, from 

 infection by food. As a general rule, the sucking young of mammals 

 do not harbour Helminthes, although infection with certain varieties 

 may occur as a result of external conditions ; or the young mammal 

 may inadvertently swallow some small animal containing parasites. 

 But, speaking generally, material for demonstration purposes should 

 be sought in animals which have passed the stage of early youth. 1 



We will now assume that a cat is to be examined for Helminthes. 

 We know, from the earlier text-books, that the cat harbours full-grown 

 worms, as well as certain of their developmental stages. The former 

 inhabit the intestine, the gall-bladder, the gall-ducts, and, occasionally, 

 the urinary bladder; while the latter are found encysted on and in 

 different internal organs (bowel-wall, liver, muscular structure), or free 

 in the great body-cavities. The frequency of occurrence varies with 

 the species. Some varieties are found in every individual ; such are 



1 O. von Linstow's " Compendium der Helminthologie " (Hanover, 1878, 1889) 

 will be found very useful for reference. Although out of date in certain particulars, 

 it is, as far as it goes, an exceedingly reliable work. 



