CHARACTERISTICS OF GEPHYREANS. 225 



covered with slender, firm, calcareous spines. It lias no 

 tentacles, a straight digestive canal, the vent being terminal, 

 and two internal gill-sacs, with external lamellate gills. 



Instead of a single nervous cord, as usual in the Gephyrea, 

 in ClicBtoderma there are two separate nerve-cords, one on 

 each side of the body. The Gephyrea were formerly asso- 

 ciated with the Echinoderms, but the resemblance is only a 

 superficial one. 



CLASS IX. GEPHYREA. 



Body long, cylindrical, smooth, or spiny, or provided with bristles, not 

 segmented; usually a large proboscis, but none in Phascolosoma; vent 

 either terminal or situated dorsally on the anterior end of the body. A 

 true blood-system homologous loith that of the Annulata. Bisexual or 

 hermaphroditic ; young of the Annelid type, undergoing a metamorpho- 

 sis. (Chsetoderma, Phascolosoma, Sipunculus, Bonellia, Ecliiurus, and 

 Pkoronis.) 



Laboratory Work. The common star-worm, Phascolosoma, is one 

 of the easiest worms to dissect, as it can be readily laid open with 

 the scissors, and the skin pinned down on the bottom of the dissecting 

 trough, when the parts can be readily distinguished, its structure being 

 unusually simple. 



CLASS X. ANNULATA (Leeches, Earth-ivorms, and 

 Sea-ivorms). 



General Characters of the Annulata This group, rep- 

 resented by the leeches, earth-worms, and nereids or bristled 

 sea-worms, tops the series of the classes of worms, and in 

 the highly specialized, regularly segmented bodies, with their 

 sense-organs and highly differentiated appendages, stand 

 nearer the Crustacea and Insecta than any other class of in- 

 vertebrate animals, their internal organization on the whole 

 being nearly as complicated. 



Reference to the accompanying diagram (Fig. 148) will 

 show the general relation of the organs of an Annelid to the 

 body-walls, as compared with corresponding parts, when seen 

 in sections of Amphioxus and a fish. 



