392 ON THE ORIGIN OF ENTOZOA 



should be thought that the arguments as yet brought forward 

 may be easily overthrown, there is still one which sets all 

 controversy at defiance ; and this is, that a large proportion of 

 intestinal worms are viviparous, therefore we must cease to as- 

 sume that they are procreated from ovules derived from without, 

 and it would be too great a tax upon our powers of speculation 

 to conceive any mode by which living embryones could be 

 carried into the bodies of fcetal animals enclosed in the uterus ; 

 and no one who has duly considered the preceding arguments 

 will contend that these embryones are capable of maintaining 

 life out of the animal body. 



The fact that Entozoa reside, and are propagated in the 

 healthy bodies of animals, is one which proves that animal 

 bodies are best adapted to the habits and nature of intestinal 

 worms; for foreign worms quickly die* in them, nor have they 

 the remotest tendency to propagate. We have indeed observed 

 the \a.Yvse of Dipterous insects, and of certain species ofthegenus 

 CEstrus, in various parts of domestic animals, but they leave 

 the body, after a stated time, to undergo a metamorphosis out 

 of it ; the eggs, therefore, are deposited on the surface of the 

 animal by the perfect insect, and the generative process is not 

 conducted within it : and this is certainly an important dis- 

 tinction ; nevertheless it is still doubtful whether some species 

 of Coleoptera, of the hardier kinds, may not undergo a meta- 

 morphosis within an animal body. But these are carried into 

 the body from without by accident, and their young are evident 

 to the senses. 



They are generated too in all parts of the animal body. 



This is an argument of great weight, for animals (insects) 

 carried into the body from without, are only found in those 

 parts of it which have extensive external communication, viz. 

 the intestinal canal, under the skin, frontal sinuses, &c. ; 

 whereas there is no part of the body, however far removed 

 from external communication, which is free from parasitical 

 inhabitants, and it is impossible to conceive any method by 

 which they can make their way to the brain and other excluded 

 parts. 



But besides, parasitical worms residing in an animal not 

 only do not inflict any injury on their host, but frequently 

 excite no suspicion of their presence. On the contrary, when 

 extrinsic animals take up their abode in the bodies of animals, 



