314 SOME COMMON INTERNAL PARASITES [CH. IX 



In some species the bladder-worm may surpass even these 

 dimensions and become as large as a swan's egg or even as 

 a man's head (Echinococcus polymorphous). 



It is most remarkable how these parasites have 

 succeeded in adapting themselves to the conditions 

 necessary to their perpetuation. Indeed a special pro- 

 vidence seems to have cared for them. It will doubtless 

 have been noted that in the above-mentioned instances 

 the intermediate host is one that in the ordinary course 

 of wild nature is likely to fall a victim to the carnivorous 

 animal which is the final host of the tapeworm. One of 

 the most striking examples of the singular adaptation is 

 found in the life-history of the tapeworm Bothriocephalus 

 cordatus. The bladder- worm stage is here passed in fish ; 

 Icelanders are in the habit of giving their dogs fish to eat 

 and in the dogs the worm reaches maturity. It is probable 

 that the natural host of this worm is some aquatic carni- 

 vore, such as the seal. 



It may not be out of place, seeing how frequently 

 domestic dogs are affected with these unpleasant parasites, 

 to emphasise the importance of never allowing a dog to 

 eat mice or rats, or the entrails of rabbits, sheep, and 

 other animals in which the intermediate stages of the 

 worm are passed. It is only by eating food of this 

 character that it is possible for a dog to "catch" tape- 

 worm. It is also important to keep the animals well 

 groomed, free from external parasites and, in the case of 

 long-haired dogs especially, to cut away hair that may 

 have got soiled circum anum. 



Upwards of two dozen different species of tapeworms 



