386 



ANNELIDA. 



SC 



fused with the mouth segment. Nais (Stylaria) proboscidea O. Fr. Miill. 

 N. parasita Schm. Both species have a filiform praestomiuin. Chatogaster 

 vermicularis 0. Fr. Miill. 



Sub-class 2. GEPHYREA.* 



Worms with cylindrical body, without external segmentation, with 

 terminal or ventral mouth', ivith cerebral ganglion, cesophageal 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- 

 tifera present traces of seg- 

 mentation (see below, p. 391), 

 while in the Achceta the body 

 cavity remains simple. Of sense 

 organs, eye spots have been 

 observed; these in certain 

 Sipunculidce lie directly upon 

 the brain ; there are also dermal 

 papilla?, into which nerves 

 enter. 



The structure of the integu- 

 ment is similar to that of the 

 Annelida; the thick upper 

 cuticular layer rests upon a 

 cellular matrix, and is not un- 

 FIG. an. Young Eehiurus from the ventral frequently wrinkled. There is 



side (after Hatschek) 0, Mouth at the base no external segmentation. The 

 of the proboscis ; SC, oesophageal 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. Below this is the strongly developed dermal muscular 

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



* Quatrefages," Memoire sur 1'Echiure," Ann. dcs Sc. Nat., 3 Ser., Tom VII. 

 Lacaze-Duthiers, " Recherches sur 1 Bonellia," Ann. des Sc. Nat., 1858. 

 \V. Keferstein, " Beitrage zur anatomischen und systematischen Keimtniss dcr 

 Sipunculiden," Zeitsclir fur miss. Zoologic, Tom XV., 18(55. 1>. Hatschek, 

 " Ueber Entwickelungsgeschichte des Echiurus*," etc. Wien, 1880. J. W. 

 Spengel, "Beitrage zur Kenntniss der Gephyreen. I. Mittheil, aim der zoolo- 

 fjisclicn station w.i Ncapcl, 1879 ; II. ZcitscUr. fur iviss. Zool., Tom XIV., 1881. 



