622 A. A. W. HUBREOHT. 



Cerebratulus angusticeps ('Zool. Challenger, Exp./ 

 Part 54, 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 

 upper brain-lobes spreads out over a far more considerable 

 surface than the fibrous 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 deve- 

 lopment 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 

 commissure 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 different parts of the nervous system here noticed, the 

 primitive Palseonemertea offer the best starting-point. 



Thus in Carinella 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 



