REPORT ON THE NEMERTEA. 77 



principal ones, and it is this fact that more or less obscures the metamery here alluded to 

 (PI. XIII. figs. 2, 4). This metamery in the nervous plexus is of the same character as 

 the metamery that is noticed in the intestinal arrangement, in the nephridia, in the 

 generative organs, and in the blood vascular system of the Nemertea ; it may indeed be 

 called incipient. To its significance, for the important question of the origin of segmen- 

 tation, we shall have occasion to return by and by. 



Amongst the forms in which I found the metamery to be very distinct, Cerebratulus 

 angusticeps stands foremost (PL XIV. fig. 1). And I must here call attention to the fact 

 that the transverse stems here described are not only dorsally, but also ventrally, most 

 regular and conspicuous, uniting the longitudinal nerve-stems below the intestine by a 

 regular series of transverse commissures in the plexus, which is the primary connecting 

 medium. It is important to note that there is no ventro-median longitudinal stem in 

 Nemertea opposite the dorso-median one ; and not less important, that the same favourable 

 species just named enables me to establish with certainty that the ventral transverse stems 

 reach much further forwards than might originally be expected. The mouth alone inter- 

 feres with their course; they are, however, found immediately before as well as behind it, 

 and whilst in front of the mouth the lateral stems very soon merge into the lower brain- 

 lobes, it is clearly seen that the transverse commissures are still recognisable, i.e., that the 

 lower brain-lobes are united by thin ventral commissures, separated by a very short 

 distance, till close up to the massive ventral commissure that has been hitherto regarded 

 as the only ventral connection between the brain-lobes. The thin commissures just 

 described are, however, not directly connected with the fibrous core of the brain-lobes, 

 which is, on the contrary, directly continued into the massive inferior commissure, but 

 they seem to derive their fibres from the outer cellular coating of these lobes. They pass 

 underneath the two vagus stems, where these spring from the lower brain-lobes, and 

 where these are in their turn, in front of the mouth, united by transverse commissures, 

 as was noticed above (p. 38, 45 ; cf. PI. XIV. fig. 5). 



The histological description of the plexus may be very short, and has already been 

 touched upon in the beginning of this section. Fibrous and cellular nerve-tissue are 

 very regularly intermixed, the direction of the fibres follows that of the tracts in which 

 they are found, and the fibres are, on the whole, closer together than they are often found 

 in other Platyelminthes, where the designation of the nerve-stems — before they were 

 recognised as such — as " spongiose strands" (spongiose Balkenstriinge) was current, and not 

 inadequate. The nerve-fibres, however, are not so closely bound together, that the 

 bundles are not very frequently found to be pierced by radial contractile fibres, as was 

 noticed above, and is rendered evident by comparison of PI. XIII. figs. 3, 4, rf. That 

 this intermixture is indeed a primitive character may safely be concluded, if we observe 

 that Lang in his monograph on the Polyclada (XVIII) specially mentions similar 

 features in the nervous arrangement of that group of Turbellaria, and also if we remem- 



