THE WORM-LIKE ANIMALS 



partially coiled up, as shown in the lower figure, and firmly attached by their 

 hinder sucker to some rock ; but their muscular strength is so great that they are 

 able to maintain themselves extended in an almost horizontal direction, as repre- 

 sented in the upper figure of the illustration. They feed upon skates and other 

 fish. 



ROCK X,EECH. 



(Natural size.) 



THE GKPHYRKAN WORMS Class Gephyrea 



Gephyreans are marine, cylindrical, worm-like animals, presenting no distinct 

 external segmentation of the body, and possessing nothing of the nature of limbs or 

 gills. The skin is horny, though not calcareous, and often provided with tubercles, 

 hooks, or bristles. The anterior end of the body is furnished with a retractile and 

 sometimes highly flexible proboscis, at the end or at the base of which the mouth is 

 situated; the alimentary canal either traverses the body from end to end, as in 

 Bonellia and Echiurus, or is coiled round a special spindle muscle, and returns upon 

 its course to open in the front half of the body, as in Sipunculus, Phascolosoma, and 

 Phymosoma. In the last-named genus the head is furnished with a circle or half- 

 circle of tentacles. The muscular, vascular, and nervous systems are well developed; 

 the latter consisting of a cerebral ganglion, an cesophageal collar, and a ventral cord, 

 while the most important part of the vascular system is a dorsal vessel which lies 



