SCOLECIDA. 



121 



numerous cells which have been compared to the " nettle-cells " or 

 "cniche" of the Ccelenterata. The intestine may be either straight 

 or branched, buf always terminates behind in blind pouches, and 

 is never provided with an anus. The water-vascular system com- 

 municates with the exteiior. The nervous s)'stem consists of two 

 ganglia, placed in front of the mouth, and united hy a cord. There 

 ai-e generally rudimentary eyes or pigment-spots, which vary in 

 number from two to sixteen. 



The remaining members of the Turhdhcria are known as Ribliun- 

 woi'ins {Xeinertidce), and are not uncommonly found in the sea, 

 sometimes between tide-marks, sometimes at great dejiths, some- 

 times near the surface of the open ocean: occasionally they occur in 

 moist earth. Tliey differ from the Planarians in being woi-m-like 

 in shape, by the fact that the alimentary canal is furnished with a 

 distinct anus, and by the pjossession of a definite body-cavity. 



SECTION B. NEMATELMIA. 

 Order I. Acanthocephala. 



The "Thorn-headed Woi'ms" 

 included in this order are all 

 internal parasites. They are 

 worm-like in shape, marked 

 with transverse wiinkles, and 

 destitute of any mouth or ali- 

 mentary canal. The anterior 

 extremity of the body (fig. 7G) 

 forms a kind of proboscis or 

 snout, which is armed with 

 recurved hooks, and has placed 

 at its base a single nervous 

 ganglion. Beneath the skin 

 is a network of canals, con- 

 taining a clear fluid, aiid be- 

 lieveil to represent the water- 

 vascular sj'stem. The Thorn- 

 headed Worms include some 

 of the most formidable para- 

 sites with which we are as yet 

 accjuainted, the best known 

 being the various forms of 

 Echinorhynchvx, which are 

 found inhabiting the alimcn- 



Fig. 76.— Acanthocephala. A, EchinorTiynchus 

 rjirjas, .sli^^litly eniargerl. B, Head of the 

 yanie, .still further enlyrgi'il. 



