176 



MANUAL OF ZOOLOGY. 



can correspond if not with some modification of the water- 

 vascular system." — (Huxley.) This system of water-vasculai 

 canals, however, does not communicate, so far as is known, 

 in any way with the exterior. At the base of the proboscis 

 is placed a single nervous ganglion, which gives off radiating 

 filaments in all directions. 



Besides the presence of a water-vascular system and the 

 absence of any alimentary canal, another point of aflinity 

 between the Acanthocephala and the Tcsniada has recently 

 been established by the discovery that the adult worm is 

 developed within a hooked embryo, from which it is se- 

 condarily produced. 



The " Thorn-headed worms " include some of the most 

 formidable parasites with which we are acquainted. The 

 Echinorhynchus (fig. 51) is found in the intestinal canal of 

 many vertebrate animals, especially of birds and fishes. 



Order II. Gordiacea. — The Gordiacea, 

 or " Hair-worms," are thread-like parasites, 

 which in the earlier stages of their existence 

 inhabit the bodies of various insects, cliiefly 

 of beetles and grasshoppers. They possess 

 a mouth and alimentary canal, but they are 

 not provided with a distinct anal aperture. 

 In Gordius itself the gullet is said to open 

 directly into the body-cavity ; but it is more 

 probable that this is an error, and that 

 there is a complete intestine opening pos- 

 teriorly into a cloaca. The sexes are dis- 

 tinct, and they leave the bodies of the in- 

 sects which they infest in order to breed ; 

 subsequently depositing their ova in long 

 chains, either in water or in some moist 

 situation. At the time of its migration the 

 mouth of the adult Gordius appears to be 

 obliterated, and the anterior portion of the 

 alimentary canal becomes atrophied. 

 In form the Gordiacea are singularly like hairs, and they 

 often attain a length many times greater than that of the insect 

 which harbours them. 



Order III. Nematoda (or Nematoidea). — The Nematoda 



"Thread-worms" or " Round-worms "— are of an elongated 

 and cylindrical shape; and are often, though by no means 

 always, parasitic in the interior of other animals. They possess 

 a distinct mouth and an alimentary canal which is freely sus- 

 pended in an abdominal cavity, and terminates posteriorly in a 



Fig. 51. — Acanthoce- 

 phala ; a. Echittor- 

 kynchits gtgas, nat. 

 size ; a' The head of 

 the same magnified. 



