4 ENTOZOA. 



animals. To tliis rule there are a few exceptions, and in some 

 instances tlie helminths remain parasitic externally, so that, in one 

 sense, they become ectozoa rather than entozoa. Of this kind the 

 genera Gyrodactylus, DiplozooUy and Tristoma afford characteristic 

 examples. 



I have already hinted that the entozoa or helminths display a 

 considerable variety of character. So much, indeed, is this the case 

 that many naturalists protest against the common habit of placing 

 all the internal parasites in a separate class. Doubtless there are 

 legitimate grounds for this protest ; but, after a prolonged exami- 

 nation of the question, I have satisfied myself that the method of 

 retaining the entozoa as a distinct group is fraught with advantages 

 more than counterbalancing the apparent orthodoxy which a rather 

 more exact and systematic treatment of the subject might involve. 



The happiest, and perhaps after all, the most truly philosophic 

 way of studying the entozoa is to regard them as a peculiar fauna, 

 destined to occupy an equally peculiar territory. That territory is 

 the widespread domain of the interior of the bodies of man and 

 animals. Each animal or "host " maybe regarded as a continent, 

 and each part or viscus of his body may be noted as a district. 

 Each district has its special attractions for particular parasitic 

 forms ; yet, at the same time, neither the district nor the continent 

 are suitable localities as a permanent resting-place for the invader. 

 None of the internal parasites " continue in one stay ;" all have a 

 tendency to roam ; migration is the very soul of their prosperity ; 

 change of residence the sine qua non of their existence, whilst a 

 blockade in the interior, prolonged beyond the proper period, ter- 

 minates only in cretification and death. 



The above-mentioned phenomena will scarcely fail to suggest, 

 also, several peculiarities respecting the distribution of these 

 creatures ; for it is obvious that their disposition throughout 

 space will be co-extensive with the geographical range of the 

 " hosts" in which they dwell, and it therefore probably follows 

 that they must also acquire a corresponding distribution in time. 



