ABSENCE OF A EHABDITOID STAGE. 



101 



Fig. 64. — Oxyuris ambujua 

 (young). 



animal-body is plastic, and capable of adapting itself to the con 

 ditions of a specific mode of life. Hence we must leave it doubt 

 ful whether the unmistakeable similarity \7hich 

 Oxyuris (Fig. 64) presents in many respects 

 (especially in the form of the body, structure 

 of the alimentary canal, and sexual apparatus) 

 to Ehahditis, is the consequence of such a 

 secondary adaptation ; or whether it may be 

 interpreted as a mark of closer genetic rela- 

 tion. But it is not only the developed animals 

 which present such conditions of adaptation, but 

 also the embryos. Whether these remain where 

 they have become free, or forsake the place of 

 their birth and migrate ; whether in their migra- 

 tion they break through tissues and organs of a 

 particular character ; whether their locomotion be 

 rapid and energetic or not ; — all this finds expres- 

 sion in form and structure, and often expresses 

 itself in forms which, notwithstanding a common 

 type, frequently differ w;idely from each other. 



In this way may also be explained the fact that there are Nema- 

 todes whose embryos exist without a Eliahditis-ioxva. for a time in the 

 free state, until they migrate into their host in some way or other. 

 Such embryos do not lead a true free life, like the Ehabditidee, for they 

 neither feed nor grow, but resemble free-living animals, in so far as 

 they have the power of independent locomotion. It is owing to this 

 circumstance that they are able to escape many of those casualties 

 which otherwise determine the distribution and transference of 

 helminthic germs. There are, then, certain advantages connected 

 with such a larval form, and it may be these which have brought 

 about its existence. It is plain that the form and structure of 

 the embryos change in manifold ways, according to the varying 

 conditions (locality, mode of locomotion, character of the skin to be 

 penetrated) ; and this fact is obvious on even a superficial examination 

 of the embryonic forms, say of CucuUamos or Dracunculus on the one 

 hand, and Stronrjylus filaria on the other (Fig. 65), and may be estab- 

 lished even by a most superficial research. The impossibility of ob- 

 taining nutriment naturally makes it necesssary in aU cases that the 

 duration of such larval stage must be short; and, generally, the 

 shorter the more lively is the locomotion which the embryo exhibits. 



I must of course leave it undetermined whether I have suc- 

 ceeded in the above attempt to develop the phenomena of the 

 parasitic life among tl^/Jjfgg^g^ft^^oSM^*^ ^^'^ natural sequence, 



