TEEEBELLA NEBULOSA. 269 



toneal fluid, which is so richly corpusculated, and which freely enters the 

 hollow axes of all these tentacles, is thus brought into contact with the 

 surrounding water. 



(685.) In addition to the two important uses already assigned to the 

 tentacles in the Terebellee, they constitute also the real agents of loco- 

 motion. They are first outstretched by the forcible ejection into them of 

 the peritoneal fluid, a process which is accomplished by the undulatory 

 contraction of the body from behind forwards ; they are then fixed like 

 so many slender cables to a distant surface ; and then, shortening in 

 their lengths, they haul forwards the helpless carcass of the worm. 



(686.) In the Terebellae, in consequence of the concentration of the 

 tentacles and branchise around the head, the blood- system at this extre- 

 mity of the body discovers a great increase of development. The peri- 

 toneal fluid in this genus is very voluminous and densely corpusculated ; 

 the system of the blood-proper is, notwithstanding, elaborate and full- 

 formed. The chamber of the peritoneum is one undivided space, the 

 segmental partitions of the Earthworm and the Leech being here re- 

 placed by limited bands proceeding from the intestine to the integu- 

 ment, tying together these two cylinders so, however, as to permit one 

 to move within the other with remarkable freedom. 



(687.) The great dorsal vessel in Terebdla nebulosa is limited to the 

 anterior of the body (fig. 134, a). It emanates chiefly from a large 

 circular vessel (6) embracing the oesophagus, and which receives all the 

 blood from the intestinal system. In this species, therefore, the primary 

 and intestinal dorsal trunks over the whole intestinal region are united, 

 or the former vessel is superseded by the latter. 



(688.) On the dorsal view of the oesophagus, a large, pulsatile, fusi- 

 form vessel () is displayed on the first laying open of the integument in 

 a longitudinal direction. Slightly attached to the structure on which 

 it rests, it appears as if suspended in the fluid of the peritoneal cavity. 

 Advancing to the occipital ring, it breaks out into six branches (d), of 

 which three proceed to the branchiae of each side, while the reduced 

 continuation of the original trunk furnishes minute ramuscules to the 

 tentacles, in the hollow axes of each of which an afferent and efferent 

 vessel is contained, surrounded by the peritoneal fluid, which penetrates 

 to the remotest ends of these exquisite organs. Both from the ten- 

 tacles and branchiae the blood now returns into the great ventral trunk 

 (c\ which to the posterior extremity of the body is distinct from, and 

 independent of, the intestinal system (f). From this trunk branches 

 are detached on either side of the median line for the supply of the feet 

 and integument. 



(689.) At the point corresponding with the circular vessel (fig. 134, 6), 

 the primary ventral sends off a considerable division for the supply of the 

 intestinal system. The current, therefore, entering the glandular 

 parietes of the intestine is purely arterial in this genus ; for it is un- 



