IN THE BODIES OF ANIMALS. 393 



they soon prove themselves very unwelcome guests, by tor- 

 menting with severe pain, and by producing great emaciation ; 

 unless indeed they are speedily ejected, or perish under the 

 intolerable heat of their new residence, or yield to the digestive 

 powers of the stomach and intestines : for whether they be 

 under the skin or in the stomach, they are constantly restless, 

 excepting perhaps the larvae of the CEstrus, which, as they are 

 more congenial, and exist in fewer numbers, are sometimes 

 nourished with impunity. Entozoa, on the contrary, frequently 

 never excite a knowledge of their presence, and are productive 

 of but little mischief, and that little accidentally. I think that 

 those worms which spring up in the body and have become 

 habituated to it, lead a tranquil life, and bear the continual 

 but moderate motions of all the parts, and the accustomed 

 stimulus of the chyme, faeces, &c. without inconvenience; and 

 that it is not till the motions of the body have become un- 

 usually violent (perhaps spasmodic), the food poor or insufficient, 

 the secretions of the chyme acrid, as in typhus, that they 

 become irritable, and attempt an exit. Hence, as I have 

 before stated, no sooner have they been dejected, or taken 

 flight, than they as quickly perish. Some authors, opposed to 

 the innate doctrine, have laid great stress upon the fact that 

 some species of worms are peculiar to certain species of 

 animals ; but how they would wish us to apply the fact in their 

 favour I am at a loss to discover. 



In having pointed out the extravagance of some writers on 

 this subject, it must not be understood that I deny their 

 position in toto, for I am aware that many worms are peculiar 

 to certain animals, and that there is less distinction between 

 the Entozoa of similar kinds of animals, or of similar parts of 

 the same, than between those of either, in which there is a 

 striking dissimilarity. Now this could hardly take place, sup- 

 posing the parasite germs to be carried in from without; for 

 those animalcules which are diffused through the same region, 

 and occupy the same situation promiscuously, are all equally 

 exposed to the chance of indiscriminate transportation into the 

 same animal body. It was the opinion of Retzius that the 

 ovules found a congenial habitation in those animals only, or 

 in those situations within the animal in which we discover 

 them, and that they either perished or were not developed 

 when carried into others less congenial, or positively noxious, 



NO. IV. VOL. III. 3 E 



