218 DADDS veterinary medicine and surgery. 



a round worm, called ths strongylus gigas, measuring from fi\€ 

 inches to three feet in length, and from two to six lines in thick- 

 ness. 



The diseases with which even large intestinal worms are con- 

 nected appear to be sometimes the cause and sometimes the effect 

 of the presence of these parasites. Very often they exist in con- 

 siderable numbers without producing the least disturbance of tho 

 economy, but in other cases they are unquestionably the cause of 

 much suffering and ill-health. How far they are themselves the 

 result of a morbid state of the organs in which they appear is 

 still an undecided question. 



The origin of parasites is extremely obscure, and has long been 

 a mooted point among naturalists. It may not be inappropriate 

 to present a summary of the opinions which are entertained respect- 

 ing a subject of so much interest, but, in doing so, we shall confine 

 our remarks to the parasitic animals which inhabit the interior of 

 the body, or entozoa. 



It is evident that these animals must originate in one of two 

 ways ; that they must be derived directly or indirectly from with- 

 out, or be created out of materials existing within, and furnished 

 by, the body. No other supposition is possible. If an entozoon 

 is in any manner derived from without, it must be admitted that 

 this takes place either through the reception of the animal itself 

 or of its ova. If either opinion be assumed, it follows that the 

 parent animal must exist somewhere external to the body. But 

 the parasites in question have never, in any case whatever, been 

 detected except within the organism. If it is objected that many 

 of these animals are so minute that they might easily elude dis- 

 covery in the elements around us, the argument fails when applied 

 to the giant strongylus, the stout lumbricoid worm, and the taenia, 

 measuring many yards in length. Besides, even admitting for a 

 moment the possibility of the parasites which inhabit the intes- 

 tine, and other mucous cavities, having once existed externally, 

 the insuperable difficulty still remains of explaining the entrance 

 of entozoa into shut cavities and parenchymatous structures — into 

 tin eye, or the muscles, for example, and their presence in the 

 unborn child, and even in the bodies of larger entozoa of a differ- 

 ent species. On the other hand, if it is maintained that the ova 

 are alcne received, it must still be shown that the ova exist exter- 

 nal to the bodv. which has never been done. Nor would th* 



