ON THE BRITISH ANNELIDA. 195 



but the view propounded in this memoir, with reference to the share taken 

 by the peritoneal fluid in the function of respiration, will enable the phy- 

 siologist to reconcile this apparent incongruity. 



The dorsibranchiate order comprehends a considerable proportion of the 

 class Annelida. Of them, Cuvier remarks, "Ont leur organs et surtout leur 

 branchies distribues a-peu-pres egalement le long de tout leur corps, ou au 

 moins de sa partie moyenne." In the Cuvierian arrangement, at the head 

 of this order stands the genus Arenicola. Respiration is performed in 

 Arenicola Piscatorwn by means of naked blood-vessels, projecting at the 

 root of the setiferous process upwards and outwards one-fourth of an inch 

 in the adult worm from the surface of the body (fig. 13). They are limited 

 in number and distribution to the fourteen or sixteen middle annuli of the 

 body. They are commonly described as forming an arborescent tuft ; the 

 division of the vessels is however regulated by a fixed principle. When fully 

 injected with blood, the vessels of each branchia form a single plane (fig. 13), 

 rising obliquely above and across the body, and immediately behind each 

 brush of setae. In the adult animal each gill is composed of from twelve to 

 sixteen primary branches (fig. 12, a, a), proceeding from a single trunk, which 

 arises from the great dorsal vessel ; the vessels in the branchial tuft describe 

 zigzag outlines ; the secondary branches project from the salient point, or the 

 outside of each angle of the zigzags ; and the tertiary from similar points on 

 the secondary branches. This mode of division, occurring in one plane, and 

 in all the smaller branches, results in a plexus of vessels of extreme beauty of 

 pattern or design. Each branchial tuft, and each individual vessel possess an in- 

 dependent power of contraction ; in the contracted state the tuftalmost entirely 

 disappears, so completely effected is the emptying of the vessels. The con- 

 traction, or systole, in any given tuft occurs at frequent but irregular inter- 

 vals ; this movement does not take place simultaneously in all the branchiae, 

 but at different periods in different tufts. As there is no heart-like dilata- 

 tion in the afferent vessels of the branchiae (fig. 10,/,/), the contractile 

 power with which the exposed branches are endowed, becomes an important 

 means of reinforcing the branchial circulation. The vessels appear quite 

 naked, and if examined in the living state, each ramuscule seems to consist 

 only of a single trunklet ; if this were really the case, it would of course re- 

 solve itself into a tube ending in a cul-de-sac, and the blood movement would 

 be a flux and reflux ; but by injection it is easy to show that the finest divi- 

 sion of the branchial arbuscle contains a double vessel (fig. 13, B), enveloped 

 in a common muscular, although extremely diaphanous sheath. That these 

 vascular sheaths, which are only fine productions of the integuments, are 

 furnished with voluntary muscular fibres, is proved by the rapid and simultane- 

 ous retraction of all the branchiae into the interior of the body, which follows 

 when the animal is touched. This sheathing of the blood-vessels with true 

 muscular coats is a frequent character of the circulating system of the Anne- 

 lida; it is a power which compensates the absence of a heart. It is extremely 

 interesting to watch in the young Arenicola, the manner in which one little 

 blood-vessel after another, in the progress of growth, shoots slowly from its 

 stock-branch. In Arenicola, as in all Annelida in which the vessels of these 

 organs are naked, the branchiae are destitute of vibratile cilia. It will be 

 found that under such circumstances, viz. when the branchial vessels occur as 

 naked projections from the external surface, the description now given of 

 these organs in Arenicola will apply in every minute respect of structure to 

 all other Annelida. It will prove exact in relation to the structure of the 

 branchiae in the several species of the beautiful genus Euphrosyne of Savigny. 

 In E.foliosa (M. E.) the branchial vessels form larger and richer tufts, having 



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