NEMATODA. 771 



plisliment of their larval growth. The addition of horse-dung 

 to the soft wet mud in one case, and of cow-dung in another, 

 neither appeared to advance nor retard the process of embryonal 

 growth so long as the embryos remained in their shells. On the 

 other hand, when the embryos were reared in simple horse- 

 dung, purposely kept moist, they attained a higher degree of 

 organization than did those which were reared in water and wet 

 mud. Having watched hundreds of these larvse under varying 

 conditions, Cobbold came to the conclusion that after their 

 escape from the egg, their growth, strength, and activity are 

 favoured, if they happen to have gained access to fluid media 

 containing impurities. Ditch or muddy pond water would 

 appear to be eminently favourable to the development of the 

 escaped larvse up to a certain stage of growth. 



That warmth is eminently favourable to the development of 

 all kinds of parasites is a well-established truth ; and in the case 

 of most nematodes it appears to be absolutely essential to the 

 formation and hatching of the embryos. Take the case of 

 Oxyuris, for example. As Leuckart observes {Die MenscldicJien 

 Parasiten, Bd. ii. s. 326), " One only needs to expose the eggs of 

 the human thread-worm to the action of the sun's rays in a 

 moistened paper envelope, when in f|ye or six hours the tadpole- 

 shaped embryos become slender elongated worms, which are not 

 unlike the sexually mature oxyurides in form, exhibiting rather 

 lively movements under the influence of the warmth." The 

 power of warmth is thus very obvious in the case of oxyuris, 

 since without a certain degree of temperature the earliest em- 

 bryonal change cannot be accomplished. As in the oxyuris of 

 man, these early changes are sometimes accomplished whilst the 

 e"f!S, discharged from the maternal worm, still lie in the faeces 

 or rectum of the bearer; so also it is probable that similar 

 changes occur in the eggs of Oxyuris curvula whilst they still 

 remain in the rectum of the horse. According to Leuckart, tlie 

 escape of the embryos of the human oxyuris ordinarily takes 

 place when the eggs are swallowed by a new human bearer ; but 

 from the observations of Heller, it is also quite certain that any 

 person may infest himself by swallowing the eggs which have 

 come from oxyurides dwelling in his own person. In either 

 case the escape of the embryos from the egg is brought about by 

 the action of the gastric juice acting upon the egg-shell. The 



