370 



CHJETOPODA. 



to the internal organization and is, as compared with that of all other 

 Annelida, to a certain extent completely homonomous, inasmuch as 

 the oesophagus is confined to the cephalic segment and does not 

 extend into the anterior segments of the body. Further, the nervous 

 system is connected with the ectoderm along its whole length, and 

 the cerebral ganglion maintains its primitive position at the anterior 

 end, corresponding to the apical plate of the larva ; and the ventral 

 cord is without ganglionic swellings. In all the above points these 



forms seem to 

 have preserved 

 the primitive An- 

 nelidan structure, 

 and they have 

 therefore been 

 united by Hat- 

 schek into a special 

 class, the Archian- 

 nelida. 



In the Pobj- 

 chceta the vascular 

 system is compli- 

 cated by the ap- 

 pearance of bran- 

 chiae, which are 

 provided with 

 blood-vessels. In 

 the forms with 

 dorsal branchiae 

 the branchial 

 blood is derived 

 from the dorsal 

 trunk and re- 

 turned to the ven- 

 tral by special 

 vessels. When, 

 on the other hand, as in the tubicolous capito-branchiate forms, the 

 respiratory apparatus is concentrated on a few segments, the vascular 

 system of that part undergoes greater modifications. In the Tere- 

 bellidce (fig. 305), the dorsal trunk dilates above the pharynx to a 

 branchial heart from which lateral branches are given off to the 

 branchiae. In the same region the transvers loeops connecting the 



FIG. 305. Terebella nebulota, opened from the dorsal side (after 

 M. Edwards). T, Tentacles ; K, Branchiae; Dg, dorsal vessel or 

 heart. 



