CHARACTERISTICS OF GEPHYREANS. — 163 
covered with slender, firm, calcareous spines. It has- 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 Chetoderma 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. 
Cuass VIII.—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 with that of the Annulata. Bisexual or 
hermaphroditic ; young of the Annelid type, undergoing a metamorpho- 
sis, (Chetoderma, Phascolosoma, Sipunculus, Bonellia, Echiurus, and 
Phoronis.) 
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. 
Cuass [X.—Annutata (Leeches, Harth-worms, and 
Sea-worms). 
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. 112) 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. 
