BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 355 



The course of the circulation, so far as I can see, is as follows : — Posteriorly 

 a gentle contraction from behind forwards drives the contained fluid along the 

 great central vessel to the front, where it is forced through the anastomotic into the 

 lateral vessels and the cephalic arch. The lateral trunk may be seen to swell 

 with the wave, and the fluid then passes to the posterior end to enter the median 

 as before-mentioned. In addition to the stream poured into the lateral trunks, 

 another passes into the cephalic arch by the vessel on each side, and the counter- 

 currents must meet and commingle, returning again during the diastole of the 

 central vessel. I have not made out any branches in the British species except 

 in 0. purpurea ; but this is a somewhat difficult task, on account of the trans- 

 parency of the circulating medium and channels. 



In many species the fluid contained in these vessels is transparent and homo- 

 geneous. M. de Quatrefages, however, found corpuscles in his Polia bembix, 

 Prof. Keferstein small oval discs in the reddish blood of his Borlasia splendida* 

 and I have seen in Ommatoplea purpurea minute granular corpuscules, but both 

 they and the fluid are colourless. Minute colourless globules also occur in the 

 blood of 0. pulchra. 



Such, in the Ommatopleans, is a brief outline of the circulation, which, 

 although resembling that of M. de Quatrefages, in so far as each describes three 

 main trunks, differs considerably in detail. The first point to be noticed in the 

 descriptions of this author is the statement that the lateral trunks pass through 

 the cephalic diaphragm — a structure which has not been seen. He is slightly in 

 error also when he states that the median vessel lies immediately under the sub- 

 cutaneous muscles. The arrangement shown in his two sections of Borlasia 

 angliw cannot apply to this group. I have not been able to verify the elaborate 

 curves which this author givesf each anastomotic division of the central vessel 

 anteriorly, and which may be described as first forming a loop behind the gan- 

 glion, with its curve directed outwards, and a second inversely curved round its 

 anterior border — in its passage outwards to join the lateral, which is scarcely 

 bent inwards at all, but occupies a space where no vessel occurs in the British 

 forms. The mere shortening of the anastomotic will not retrieve this anatomical 

 error. The cephalic arch is also placed otherwise than " immediatement au- 

 dessous des couches sous-cutanees," as already described (Plate IV. fig. 1). 

 He mentions the presence of distinct walls to these vessels, which, however, he 

 learned from Borlasia angliw, and in this I concur (Plate IV. figs. 1 and 6). 

 The walls are highly contractile, and in the latter figure the vessels have been 

 cut across just before they complete the cephalic arch; they are observed to be 

 surrounded by a ring of finely granular texture. M. :je Quatrefages likewise 



* This species lias since been found in the Channel Islands. It is the Carebratuliis sjjectabill^ 

 of M. de Quatrefages. 



f Op. cit. pi. xviii. fig. 1. and pi. xix. fig. 1 ; also in his recent Hist Nat. des Anneles, pi. iv. 

 figs. 2 and 3. 



