ON THE BRITISH ANNELIDA. 183 



toneal fluid. They constitute the special branchial system of vessels (internal 

 branchiae), while they discharge incidentally an absorbent function. In the 

 movement of the blood, then, in Nais as in Lumbricus, there are discernible 

 only two leading directions, — one forward, in the primary and intestinal dor- 

 sal vessels (fig. 8, a, b), the other backward, in the primary and intestinal 

 ventral (fig. 8, d, e,f). It is not possible to trace the blood into the capil- 

 lary parietal system of the intestine, in consequence of the transparency of 

 the stream when thus minutely subdivided. In Nais there is also an integu- 

 mentary system which intervenes between the two primary (dorsal and abdo- 

 minal) trunks (a,/), ramifying on the substance of the integuments, upon 

 which in part a respiratory function may devolve. 



In the Terebellcs, in consequence of the concentration of the tentacles and 

 branchige around the head, the blood-system at this extremity of the body 

 discovers a great increase of development. The peritoneal 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 

 Earth-worm and the Leech being here replaced by limited bands proceeding 

 from the intestine to the integument, tying together these two cylinders, such 

 as to permit one to move longitudinally within the other with remarkable 

 freedom. 



The great dorsal vessel in Terebella nebulosa is limited to the anterior 

 extremity of the body (fig. 9, a). It emanates chiefly from a large circular 

 vessel (6), embracing the base of the oesophagus which receives the whole 

 blood of 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 former vessel is superseded by the latter. 



On the dorsal view of the oesophagus, a large, pulsatile, fusiform vessel (a) 

 is displayed on the first opening of the integument in a loqgitudinal directian. 

 Little 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 (rf), 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 axis of each of which an afi'erent 

 and efi"erent vessel is contained, surrounded by the peritoneal fluid, which 

 penetrates to the remotest ends of these exquisite organs. Both from the 

 tentacles and branchiee, the blood now returns into the great ventral trunk (c), 

 which to the posterior extremity of the body is distinct from and independ- 

 ent of the intestinal system (/). From this trunk branches are detached on 

 either side of the median line, for the supply of the feet and integument. 



At the point corresponding with the circular vessel (fig. 9, b), 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 unmixedly composed of blood returning from 

 the tentacles and the branchiae, by both which the function of respiration is 

 performed. Here again there exist but two principal directions in which 

 the blood circulates, viz. longitudinally and transversely, or circularly, the 

 former currents being connected by the latter. The circular vessel (fig. 8, 6) 

 acts like an auricle ; it receives the blood from the intestinal system and de- 

 livers it into the great dorsal (a). The alimentary canal is embraced in this 

 genus, as in all Annelids, by a framework of longitudinal and transverse 

 vessels, in which the blood moves backwards below and forwards above (/). 

 In Terebella conchilega the circulating system is planned on the same 

 type with that of the former species. Here, as in the former case, the main 



