no 



THE ORIGIN OF PARASITES. 



embryos of the Tamiadre, like those of the Trichocephalidre 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 (Fig. 71) of the Tseniadre, with their cylindrical rostellum, and 

 of the Tetrarhynchi 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 

 Botltriocephalux latits. 



FIG. 71. Echinorhynfltus sp!rula, natural 

 size (after Westrumb). 



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 



