EEPORT ON THE NEMERTEA. 83 



and 8 represent the whole of the brain tissue, figs. 5-7 and 9 the fibrous core, as it is 

 enclosed by the uerve-cells, the limit of this cellular investment being given in outline in 

 the latter figures. It must, however, from the first be remarked, that this outline should 

 be completed by the plexus, and by the median medullary nerve. They are not indicated 

 in these figures, although both of them are found along the whole length of the lateral 

 nerve-stems, and reach forward as far as the region of the dorsal brain commissure. 



It is seen at a glance that the fibrous core repeats the external folds and prominences 

 of the brain-masses, that, the lateral nerve-stem is continued into the lower lobe, and 

 that the upper lobe is distinguished by a prominent fold of its surface, a gyrus (fig. 1, 

 SL), into which a separate knob of the fibrous core is seen to pass, and by two other 

 fibrous projections — the one stretching towards the blunt end of the posterior lobe, the 

 other running forwards and accompanying the ciliated canal, which is also marked in 

 outline in fig. 6, and (in red) in fig. 5. The canal cc, in figs. 2, 3, 4, 8, is the exterior 

 portion of this duct. The difi'erent thickness of dorsal and ventral brain commissure may 

 be gathered from figs. 1, 3, 8, 9 ; from the latter two, the fact that the nerve-fibres are 

 very strongly preponderant in these commissures over the cells. Close behind the ventral 

 commissure the nerve for the oesophageal wall, vg, the so-called vagus nerve, is seen to 

 leave the common fibrous core of the brain, whereas the nerves for the proboscis (pn) 

 spring from the inner sur&ce of the ring, where the fibrous core turns up from the ventral 

 to the dorsal commissure (figs. 5, 9). The vagus nerve is soon after its origin connected 

 by transverse fibres with its opposite neighbour ; this vagus commissure is sometimes 

 repeated ; it will be again referred to in the general considerations on the nervous system. 

 The cephalic nerves that leave the brain and innervate the head are only very imperfectly 

 rendered in these figures ; their number is far greater than might be concluded from 

 figs. 5, 6, an. 



The aspect of several j^wrtions of the brain of Eupolia, in transverse section, is repre- 

 sented in PI. VI. 



It will there be noticed that fig. 4 represents an anterior section through the inferior 

 brain commissure and the point of innervation of the proboscis, fig. 5 one just behind 

 this, cutting the dorsal commissure and the vagus root at the same time. The exact 

 situation of these sections will be best understood by comparing them with PL V. fig. 9, 

 where the respective positions of the commissural ring, the proboscidian nerve, and the 

 vagus are clearly indicated. Fig. 7 is a transverse section lying further backwards, 

 almost in the level where the dotted line, SL, in PL V. fig. 4 terminates, whereas the 

 section fig. 8 lies again somewhat behind this, at a point where the "gyrus" of the superior 

 brain-lobe actually divides the central fibrous nerve-substance into an upper and a lower 

 portion. These sections, at the same time, show the difference in size between the 

 brain-cells and the glandular elements partaking in the constitution of the brain, along 

 the superficial part of what I have called the posterior brain-lobe (side-organ, auct.). 



