ON THE BRITISH ANNELIDA. 187 



lata that the lateral branches designed for the branchiae proceed, the effe- 

 rent vessels of these organs returning the arterialized blood into the dorsal 

 intestinal trunks. Between the longitudinal trunks a complex capillary system 

 of vessels is interposed. Upon this system the glandular functions of the 

 biliary cell-layer of the alimentary canal depend. From the relative con- 

 nection and directions of the primary and secondary trunks, it may be seen 

 that the blood in all the lateral branches connected with the dorsal vessels 

 sets towards the median line, while that in the ventral secondaries sets/ro;w 

 the median line. In this Annelid therefore, as in every other, there are two 

 concentric circular currents, while in many there exist also concentric lon^ 

 gitudinal movements of the blood. 



On the British shores two other species of Sabellce are familiar, of which 

 the circulating system is distinguished in several respects from that of Sa- 

 bella alveolata. In S. a sang vert o{ Milne-Edwards* the dorsal vessel is 

 single, maintaining a median position from one extreme of the body to the 

 other. It is branchial in office. Situated at the cephalic end of the body, 

 the entire blood of the branchial tentacles is derived immediately from this 

 vessel. Its contributory branches proceed from the intestine and integu- 

 ments ; its contained blood is necessarily venous. The sub-neural trunk 

 receives the branchial veins. This vessel in Sabella chhroema ie large, and 

 distributes unmixed arterial blood to the feet, integuments and intestine. 

 There exists in this worm a considerable amount of peritoneal fluid, which, 

 in common with the blood-proper, penetrates into and follows the sub- 

 divisions of the branchial appendages. The blood, bright-green in colour, 

 is perfectly destitute of all morphotic elements ; it is entirely fluid. The 

 peritoneal fluid is colourless and corpusculated. The blood-current in 

 this Annelid observes two leading directions : in the dorsal vessel it moves 

 forwards, in the ventral backwards, in the lateral branches upwards and cir- 

 cularly, in conformity with the law controlling the circulation in all Annelida. 



In the subsequent portion of this Report another species of Sabella will be 

 described, in which the branchial tentacles coexist with the bilateral series 

 of branchias. In this graceful Annelid the features of the circulating system 

 of S. alveolata and S. chlorcema are interfused. The species characterized 

 by Montagu as Sabella vesiculosa, in which the branchial appendages are 

 concentrated around the head, exhibits a blood-system, of which the dorsal 

 vessel is single and branchial, conforming in every detail with that of S. 

 chlorcema. 



The Nereides are elaborately organized ; the blood-system is highly deve- 

 loped ; the peripheral portion is densely subdivided ; the nervous system is 

 numerously ganglionized. Thus is explained the vigorous muscular power 

 of nearly all the species of this genus. 



In Nereis margaritacea of our coasts the system of the blood is double. 

 There exists a primary dorsal vessel and intestinal dorsal, much smaller than 

 the former. This latter vessel is not represented in the diagrams of Prof. Milne- 

 Edwards, but it may be readily exposed to view. The superior or greater 

 dorsal presents its largest diameter about the middle of the body. It receives 

 at every segment considerable venous branches from the intestine, and arte- 

 rial from the bases of the feet. Anteriorly about the commencement of the 

 oesophagus it sends down to the great ventral a large proportion of its blood 

 by means of descending lateral branches, like the moniliform (sic) vessels of 

 the Earth-worm. 



* I have constructed a Greek specific name for this elegant Annelid, namely, Sabella chlo- 

 rcema, in accordance with the French designation applied to it by Milne-Edwards, both sig- 

 nifying the existence oi green blood. 



