322 CIJESSWELJ; SHEAREii. • 



cells, tlieii* nuclei are rich in chromatin. This peculiarity 

 renders them distinguishable from the supporting cells of the 

 surrounding tissues. Some of the ganglion cells are clearly 

 multipolar, but axons and dendrites are not recognisable. At 

 the base of the tentacles the cells are bipolar, one process 

 going into the tentacle while the other enters the neuropile. 

 They form a dense mass of cells on the anterior dorsal surface 

 of the brain-core. They are, however, quite distinct from it, 

 only sending a few fine threads into its substance. In the 

 median plane a small space, a prolongation of the general 

 blastocoelic space, extends up under the brain, and separates 

 them from the core, dividing them into two lateral masses. 

 The central core of the brain is composed of a dense mass 

 of interwoven nerve-fibres. It is distinguishable by its 

 yellow colonr and its non-nucleated character. It is remark- 

 able that both in relation with the bl•^un and the ventral cord 

 the nerve-cells seem quite apart, and outside the fibrillar part 

 of the nervous system. Their relationship seems closer with 

 the ectoderniic tissues of the head and the mesodermic 

 and ectoderniic tissues in the trunk than with the fibrillar 

 material of the nervous system in these regions. 



The fibres of the ventral portion of the neuropile seem to 

 run from side to side, while those of the superficial layers run 

 more longitudinally. In sagittal sections it is lenticular in 

 outline, and in the median plane is divided by a transverse 

 fissui-e into an anterior and posterior part. Haswell also 

 shows these divisions in the brain of Stratiodrilus (fig. 8). 

 This division is only limited to the median plane ; laterally 

 the neuropile swells out into two large lobes on either side. 

 Thus it Consists, as in Dinophilus, in a median and two 

 lateral lobes, the median being in turn divided into an 

 anterior and posterior portion. In the figures of the brain 

 accompanying this paper these divisions do not show, as the 

 brain surface is taken from the o-anglion cells and not from 

 the central core. Behind the brain, and dorsal to the 

 muscular apparatus of the jaws, there is a second accumula- 

 tion of nerve-cells. These may possibly have to do with the 



