SCOLECIDA. 



121 



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

 ■' cnidfe " of the Coelenterata. The intestine may be either straight 

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

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

 municates with the exterior. The nervous system consists of two 

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

 are generally rudimentary eyes or pigment-spots, which vary in 

 number from two to sixteen. 



The remaining members of the Turhellaria are known as Ribbon- 

 worms {N emertidce), and are not uncommonly found in the sea, 

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

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

 moist earth. They differ from the Planarians in being worm-like 

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

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



SECTION B. NEMATELMIA. 

 Order I. Acanthooephala. 



The "Thorn-headedWorms" 

 included in this order are all 

 internal parasites. They are 

 worm-like in shape, marked 

 with transverse wrinkles, and 

 destitute of any mouth or ali- 

 mentary canal. The anterior 

 extremity of the body (fig. 76) 

 forms a kind of proboscis or 

 snout, which is aiTned with 

 recui'ved hooks, and has placed 

 at its base a, single nervous 

 ganglion. Beneath the skin 

 is a network of canals, con- 

 taining a. clear fluid, and be- 

 lieved to represent the water- 

 vascular system. The Thorn- 

 headed Worms include some 

 of the most formidable para- 

 sites with which we are as yet 

 acquainted, the best known 

 being the various forms of 

 Echinorhynchus, which are 

 found inhabiting the alimen- 

 1 



Fig. 76.— Acaiithocephala. A, Echinorhynchus 

 gigas, slightly enlarged. B, Head of the 

 same, still filrther enlarged. 



