386 AITNELIDA. 



fused with the mouth segment. Nais (Stylaria) proboscidca O. Fr. Mull. 

 N. parasita Schm. Both species have a filiform prastotniurn. Chat ay aster 

 vermicularis 0. Fr. Mull. 



Sub-class 2. GEPHYREA.* 



Worms with cylindrical body, without external segmentation, ^v^th 

 terminal or ventral mouth ; icith cerebral ganglion, ozsopliageal ring 

 and ventral cord. Setce are sometimes present. 



The Gephyrea possess an elongated cylindrical body and live, as do 

 the Holothuria, in sand and ooze in the sea. The characters which 

 distinguish them as Annelids are the possession of an cesophageal 

 ring connected with a cerebral ganglion and of a ventral cord par- 



tially surrounded by ganglion 



cells. The larvae of the Chce- 



., 

 tijera present traces ot seg- 



mentation (see below, p. 391), 

 while in the Achxta the body- 

 cavity remains simple. Of sense 

 organs, eye spots have been 

 observed ; these in certain 

 Sipuncididce lie directly upon 

 the brain ; there are also dermal 

 papillae, into which nerves 

 enter. 



The structure of the integu- 

 ment is similar to that of the 

 A nnelida ; the thick upper 

 cuticular layer rests upon a 



_ cellular matrix, and is not un- 



PIG. 3ii. Young Eehiurus from the ventral frequently wrinkled. There is 



side (after Hatschek) 0, Mouth at the base nQ externa l segmentation. The 

 of the proboscis ; SC, cesophngeal commis. 



sure ; ss, ventral cord ; A, anus ; H, hooks, connective tissue dermis is of 



considerable thickness and en- 



closes numerous glandular tubes, which open to the exterior by pores 

 in the epidermis. Belosv this is the strongly developed dermal muscular 

 tunic, which is regularly composed of an outer layer of circular fibres 



* Quatrefnges," Mc-moire sur 1'Echiure," Ann. des Sc. 3W.. 3 S<'T.. Tom VII. 

 Lacaze-Duthiers, " Rccherches sur 1 Bonellia," Ann. <!rx Sr. Nat., 1858. 

 \V. Keferstein, " Beitriige zur anatomischen uncl systematischen Kenntniss dcr 

 Sipimculidcn," Zcilxrlir fiir w/.v.v. Zoologlr, Tom XV., ISC,.". B. Hatschek, 

 " Ueber Entwickclung?geschichte des Echiurus," etc. \Virn, 1880. J. \V. 

 Spcngel, "Beitriige zur Kenntniss der fiephyreen. I. Mittlril, av.t fli-r zoolti- 

 gisclicn station zn frcaj'tl, 1879; II. Zi'ituchr.fur n-iaa. Zool., Toni XIV., 1881. 



