i 9 4 



MANUAL OF ZOOLOGY. 



can correspond if not with some modification of the water- 

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

 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 affinity 

 between the Acanthocephala and the Tceniada has recently 

 been established by the discovery that the adult worm is 

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

 darily produced. 



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

 formidable parasites with which we are acquainted. The 

 Echinorhynchus (fig. 71) 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, chiefly 

 of beetles and grasshoppers. They possess 

 a mouth and alimentary canal, but they 

 are not provided with a distinct anal aper- 

 ture. 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 open- 

 ing posteriorly into a cloaca. The sexes 

 are distinct, and they leave the bodies of 

 the insects 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 migra- 



Fig. 71. Acanthoce- . . _ , , . . 



phaia. a Echino- tion the mouth ot the adult Gordius 

 SeTl'' Vh'f head na i appears to be obliterated, and the anterior 

 the same magnified, portion of the alimentary canal becomes 

 atrophied. The embryo of Gordius pos- 

 sesses a snout armed with booklets, and retractile, thus present- 

 ing a close resemblance to the proboscis si Echinorhynchus. 

 It at first leads a free existence in water, but soon penetrates 

 the tissues of the larva of some aquatic insect, by means of the 

 cephalic armature, and becomes encysted. 



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. 



