THE SCOLECIDA. 



49 



the body (Fig. 23), and were originally observed by Von 

 Siebol 1, and since by myself and others, are to be regarded as 

 homologous with the water-vessels of the Trematoda, this ques- 

 tion must, I think, be answered in the affirmative. It is almost 

 the only system of organs in the Nematoidea which gives us a 

 definite zoological criterion, the condition of the nervous system 

 in these animals being still, notwithstanding the many inquiries 

 which have been made into the subject, a matter of great 

 doubt. 



Fig. 23. 



Fig. 23. Oxyuris. .7. Mouth, b. Pharynx, c. Commencement of intestine, and d its 

 termination ; the intermediate portion is not figured, e. Genital aperture, f. Open- 

 ing of vessels, g. Their receptacle, h. One of the vessels, i. Cellular matter 

 enveloping them. A portion of one of the contractile vessels is represented above, 

 more highly magnified. 



In habit and feature, the Gordiacea, filiform parasites which 

 inhabit the bodies of insects, and leave their hosts only to breed, 

 resemble the Nematoidea so much that there can be no doubt 

 that their systematic place must be close to that of the latter. 



The structure of the Acanthocephala, comprising the formid- 

 able Echinorhynchus (Fig. 24) and its allies, is pretty clearly 

 made out. They are vermiform parasites, like the T&niada, 

 devoid of any mouth or alimentary canal, but provided with a 

 proboscis armed with recurved hooks. The proboscis is sup- 

 ported within by a sort of rod-like handle, whence a cord is 

 continued, to which the reproductive organs are attached. A 

 single ganglion is seated in the " handle " of the proboscis. 

 Immediately beneath the integument lies a series of reticulated 

 canals containing a clear fluid, and it is difficult to see with 



