132 THE VOYAGE OF H.M.S. CHALLENGER. 



especially the brain-lobes.^ In a former publication (IX), where the medullary nerve 

 was for the first time noticed and described as the proboscidian-sheath-nerve, I traced its 

 origin to the dorsal commissure between the two lateral halves of the brain {loc. cit., 

 pi. i. fig. 1). Thanks to certain very favourable specimens in the Challenger collection, 

 I have now been able to add new data to this statement. Sections through the brain of 

 Cerehratulus macroren, Cerehratulus corrugatus and Cerehratulus angusticL'ps (PI. XII. 

 figs. 1, 7, 8; PI. XIII. fig. 1) show that the condition of things is indeed less simple than 

 this original statement would imply, — that the medullary nerve is not an eminently fibrous 

 cord springing at right angles from the eminently fibrous upper brain-commissure, but 

 that the nerve-tissue constituting the foremost and uppermost portions of the ujjper 

 brain-lobes spreads out over a far more considerable surface than the fil.)rous tract which 

 is known as the dorsal commissure. This expansion of nerve-tissue, in which the cellular 

 elements are no less conspicuous than the fibrous, is posteriorly directly continuous 

 with the plexus above described, laterally with the brain-lobes, anteriorly with the 

 cephalic nerves springing from these lobes. It attains its fullest dev-elopment just 

 before and behind the region where a transverse bundle of fibres uniting the fibrous core 

 of the lateral brain-lobes forms the well-known dorsal brain-commissure. This commis- 

 sure is a transverse fibrous tract forming part of a more extensive nerve-plate. To this 

 expansion of nerve-tissue the presence of nerve-cells gives a more primitive, at any rate 

 a less specialised, character. These nerve-cells and nerve-fibres are directly continuous 

 with those of the medullary nerve and (backwards) with those of the nerve-plexus, of 

 which this nerve is only the median longitudinal thickening. There is -even more reason 

 to look upon the fibres of this medullary nerve as a tract of the general fibrous stroma 

 not necessarily connected with the fibres of the brain-commissure. In other cases a more 

 direct continuity between the commissural and the medullary nerve-fibres was however 

 observed. 



In order clearly to understand the relative importance of the difi"erent parts of the 

 nervous system here noticed, the primitive Palseonemertea off"er th-e best starting-point. 



Thus in CarineUa we find the brain-lobes not yet separated into distinct upper and 

 lower lobes, nor do we find a posterior lobe (side-organ). The brain is a double lateral 

 and anterior thickening in the nerve-plexus, situated like it and like the lateral nerve- 

 stems outside the muscular body-wall in the deeper strata of the integument. The only 

 difi"erence between the medio-dorsal medullary nerve in this species and the lateral nerves 

 with their anterior enlargements (the brain-lobes) is its position and its greater tenuity 

 (PI. XVI. fig. 1), which, however, does not prevent its being very clearly observable in every 

 transverse section (PI. XL figs. 3, 4). Its connection with the brain-commissure was 

 already described (IX, p. 25), and figured by me {loc. cit., pi. iii. fig. 31). It must, 



' In the course of theae considerations a certain amount of repetition of facts already noticedin the paragraph ou 

 the nervous system cannot well be avoided. 



