186 REPOET 1851. 



same portion of the vessel becomes densely distended with blood ; and this 

 is the true mechanism of the circulation in those species even in which a 

 central propulsive organ exists, for example, in Nais and Arenicola. In no 

 part of the system, therefore, is the superadded contractile bulb required as 

 an agent of circulation, since this contractile power resides in every part of 

 evei-i/ vessel, in virtue of the muscularity of its parietes. The truth of these 

 observations, opposed as they are to the statements of Milne-Edwards, may 

 be established beyond doubt, and easily, by a scrutiny of the circulating 

 system of Arenicola Piscatorum *. 



A general survey of the circulation in Eunice will suffice to satisfy the 

 physiologist that no part of the system contains pure arterial and no part 

 pure venous blood. Into the double dorsal trunk arterial blood is poured 

 from the branchiae, but into the same trunk the intestinal branches contri- 

 bute venous blood ; the mingling of these two classes of currents in the same 

 trunk must result in blood of an intermediate quality. It is then manifest 

 that the great subneural trunk, which in this worm is both systemic and 

 branchial, must distribute blood of composition intermediate between venous 

 and arterial. No part of the circulatory apparatus therefore contains pure 

 arterial blood but the efferent branches of the branchiae. 



The SabellcB, in the number and general disposition of the primary blood- 

 vessels, do not very materially differ from those Annelids of which the circu- 

 latory apparatus has been already described. The evidence of " centraliza- 

 tion" is less complete in this genus than in the genera i'zmice and Terebella, 

 The dorsal vessels preserve a uniform diameter from origin to termination. In 

 Sabella alveolata, the dorsal vessels, whicii repose on, and belong to, the ali- 

 mentary tube, commence at the caudal extreme of the body as a small single 

 trunk. Where the true intestine begins, as indicated by the segmental saccu- 

 lations of the canal, this single vessel divides into two trunks perceptible on the 

 dorsum of the intestine, which on either side of the median line proceed for- 

 wards in parallel directions. At the crop-like dilatation, which occurs at the 

 commencement of the oesophagus, these two vessels are united by a large 

 transverse branch, and advancing round the sides of the crop-like bulge, be- 

 come again united into a single trunk, which follows the oesophagus as far as 

 the occiput, where it resolves itself into numerous minute branches for the 

 supply of the cephalic tentacles. These latter organs are penetrated by the 

 peritoneal fluid which moves to and fro in a liollow axis, along which a 

 single delicate blood-vessel reaches the extreme end of the tentacle and then 

 returns upon itself. The functions of these tentacular ramusculi have refer- 

 ence more to an absorbent than a respiratory process. In this worm the 

 sub-ganglionic trunk is comparatively small, while the sub-intestinal is more 

 developed. It is from the latter and not the former vessel in Sabella alveo- 



* Speaking of the circulating system in Eunice, this anatomist thus expounds the mecha- 

 nism of the circulation : — " Les vaisseaux sanguins, consideres d'une maniere absolue, se dis- 

 tribuent done a-peu-pres de la menie maniere chez les Eunices et les Terebelles, mais, si on 

 les considere dans leur fonctious et dans leur rapports avec I'appareil respiratoire, on y voit, 

 dans ces deux genres, des differences tres grandes. Dans les Eunices, le cours du sang n'est 

 pas determine par les contractions des branehies ni meme du vaisseau dorsal, dont Taction 

 perd presque toute son importance ; mais par les battemens de bulbes contractiles formes par 

 la dilatation de la base de chacune des brancbes transversales du vaisseau ventral. Ces bulbes 

 au nouibre de deux dans cbacun des anneaux du c6rps, excepte les six ou sept premiers, en- 

 voient le sang aux branehies en meme temps qu'a Tintestin, aux muscles, a la peau, etc., et 

 par consequent, sous le rapport physiologique, ils representent autant de coeurs. On en 

 compte quelquefois plusieurs centaines ; et cette multiphcite des organs moteurs du sang, iu- 

 dependans les uns des autres, est probablement une des circonstances qui donnent aux 

 tronfons du corps de ces Annelides la faculte de vivre pendant fort long-temps apres avoir 

 ete separes du reste de I'animal." 



