ANNULOIDA: SCOLECIDA. 



151 



ORDER I. ACANTHOCEPHALA. The Acanthocephala are en- 

 tirely parasitic, vermiform in shape, and devoid of any mouth 

 or alimentary canal. They are provided with a kind of snout 

 or proboscis armed with recurved hooks, which is continued 

 backwards into a bandlike structure (lig amentum suspensorium) , 

 to which the reproductive organs are attached. ' Immediately 

 beneath the integument lies a series of reticulated canals con- 

 taining a clear fluid, and it is difficult to see with what these 

 can correspond if not with some modification of the water- 

 vascular system.' (Huxley.) 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 be- 

 tween the Acanthocephala and the Tceniada 

 has recently been established by the dis- 

 covery that the adult worm is developed 

 within a hooked embryo, from which it 

 is secondarily produced. 



The ' Thorn-headed worms ' include 

 some of the most formidable parasites 

 with which we are acquainted. The jEchi- 

 norhyn<;hus (Jig. 42) is found in the intes- 

 tinal 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 exis- 

 tence inhabit the bodies of various in- 

 sects, chiefly of beetles and grasshoppers. 

 They possess a mouth and alimentary 

 canal, but they are not provided with a 

 distinct anal aperture. The sexes are dis- 

 tinct, 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. 



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. 42. Acanthoce- 

 phala. a. Echinorhyn- 

 chus gigas. nat. size. 

 a'. The head of the 

 same magnified. 



