ON THE BRITISH AMNELIDA. 189 



any other known Annelid. A large dorsal trunk (a) at the anterior three- 

 fourths of the body, receiving exclusively the efferent vessels of the branchiae, 

 proceeds forwards from the tail and empties itself into the cardiac cavities, of 

 which one is situated on either side of the oesophagus (b, b). Another ves- 

 sel, proceedingyrom the head towards the heart, empties itself into the same 

 cavity with the former. The blood then enters a second cavity (c', c') more 

 ventrally situated, by which it is propelled forwards into the subcesophageal 

 trunk, but principally backwards into the great longitudinal trunks of the ali- 

 mentary canal. The blood, returning from the intestinal system of vessels, 

 reaches the dorsal intestinal (^) (lying in the median line, underneath the 

 great dorsal trunk), from which the current diverges laterally at right angles 

 into the branchiae (/,/)• This conform' 'i:on differs from that prevalent in 

 all other dorsibranchiate Annelids, in which the great ventral trunk is the 

 source of the branchial arteries. But the typical plan of the circulation is 

 observed in the system of Arenicola, at the posterior half of the branchial 

 division of the body, whereas the afferent vessels of branchiae emanate from 

 the ventral trunk. It may be necessary to explain that the motion of the 

 blood in that part of the circulating system which is anterior to the heart is 

 the reverse of that in that posterior to this centre. The ventral oesophageal 

 carries the blood forwards and the dorsal backwards towards the heart. 



The independent contractile (ergo circulating) power of each individual 

 vessel may be very completely proved by an examination of the branchiae of 

 a living Arenicola (see fig. 12). A single ramuscule in the branchial tuft 

 may contract and empty itself, while the surrounding branches are expanding 

 diastolically. There is no synchroneity in the circulatory movements of these 

 vessels. Both the afferent and eff'erent vessels of the branchiae are long and 

 tortuous, but discover no cardiac ampullce in any part of their course. In 

 fact such formations exist in no known Annelid, and this conclusion has now 

 been substantiated by anatomical demonstration. 



Over the parietes of the stomach in this worm a very dense reticulation 

 of capillary vessels may be observed with the naked eye, from the bright yel- 

 low colour of the biliary gland-layer. In Arenicola the peritoneal chamber 

 is filled with a highly corpusculated fluid, the basis of which consists of sea- 

 water, and the presence and movements of which are indispensable to the 

 circulation of the blood-proper. By this remarkable mass of fluid, the slender, 

 tortuous vessels are shielded from injurious pressure. 



In the BorlasicE, a genus of Annelids, of which the true organization is ex- 

 plained for the first time in this Report, the central organ of the circulation 

 occurs as a bilocular heart, which is situated on the dorsal surface of the 

 proboscis and near the occiput. 



This organ, in every species oiBorlasia hitherto examined by the author, 

 consists of two chambers, between which, by means of a large transverse 

 channel, a free communication exists ; into one of these cavities the blood of 

 the dorsal vessel is poured. This blood is derived from the cutaneous system 

 of capillaries, which in these worms are superficially situated, and only pro- 

 tected from the surrounding element by a coating of vibratile epidermis. 

 This vibratile epidermis is limited to the dorsal half of the body, which may 

 be therefore assigned as the true area of the respiratory process. Vibratile 

 epidermis in all other Annelids is restricted to special localities, wherein the 

 function of respiration is performed. In the Borlasice and Liniadm it is co- 

 extensive with the whole dorsal region of the body, and becomes a distinct- 

 ive anatomical character of these unfamiliar genera. From the dorsal cham- 

 ber of the heart, the blood through the connecting channel is directed into 

 the ventral cavity, and thence distributed over the integumentary and intes- 



