no 



THE ORIGIN OF PARASITES. 



embryos of the T?eniad£e, like those of the Trichocephalidae and other 

 Nematodes, reach their hosts while yet enclosed in the egg-shell. 



A similar form of parasitism is that of the Acanthocephali, which 

 resemble the tape-worms in having no intestine, and are therefore 

 by many zoologists united with the latter into one systematic group 

 (Anenterati). In favour of such a conception, one might adduce 

 the analogies which obtain between the two groups, and are especially 

 noticeable when the structure and mechanism of the proboscidean hook- 

 apparatus (Eig. 71) of the Tseniadfe, with their cylindrical rostellum, and 

 of the TdrarhyncTii are brought into comparison. But all these simi- 

 larities prove scarcely more than a certain agreement in the conditions 

 of life. They represent merely adaptive relationships, and since the 



Fig. 70. — Free-swimming embryo of 

 Bnthrweeplialus latus. 



Fifi. 71. — Echinorhyiirlius spirilla, natural 

 size (after Westrurab). 



morphological structure in the two groups manifests the greatest 

 differences, they by no means permit the conclusion of a genetic 

 relationship to be drawn. The presence of a muscular body-wall 

 separated from the internal organs — not to speak of other peculiarities 

 ■ — prohibits their association with the flat-worms. 



It is indeed useless to seek in other directions for forms with 

 which the Acanthocephali naturally agree. For a time it was 

 supposed that they were allied to the Sipunculids, and might be 

 regarded as parasitic forms of this group. But in this case also it was 

 only a superficial similarity which gave rise to this view, the more so 



Digitized by Microsoft® 



