326 (JRESSWKI.L SIIEAIIKK. 



ai'e four well-marked ganglia corresponding to the four main 

 segments of the trunk. In addition to this, there are a few 

 cells that probably form a fifth, corresponding with the 

 somewhat reduced caudal segment. If we compare the 

 reconstruction iigure he gives of the nervous system with 

 that of cither the male or female Histriobdella given in the 

 present paper, it will be seen that, with the exception of this 

 greater separation of the cords, there is a remarkable resem- 

 blance in the general configuration of the nervous system of 

 the two forms. The brain and the oesophageal commissures 

 are much the same. In transverse sections the cords hold 

 similar positions in the ectoderm. The ventral sinus found in 

 the brain of Histriobdella, it would seem, is also present 

 in Dinophilus as a small closed cavity in the bi-ain substance 

 itself. 



In minor histological details they bear a striking resem- 

 blance to one another. The brain is clothed dorsal ly and 

 laterally with a mass of nerve-cells, having the peculiar 

 granular nuclei so characteristic of these cells in Histrio- 

 bdella. They are similarly differentiated from the supporting 

 tissue cells. The circum-oesophageal commissures are better 

 developed, however, in Dinophilus, and pass backwards 

 round the oesophagus just below the dorsal longitudinal 

 muscle-strands. The centre of the brain is composed of a 

 mass of clear fibrillar material that stains with difficulty. 

 As in Histriobdella, fibres are given off by the oesopha- 

 geal commissures at the point where these leave the brain. 

 They are much bigger in the case of Dinophilus, and are 

 more easily traced through consecutive sections. In His- 

 triobdella there are no pre-oral commissui-es, and the 

 ofano-lia are more circumscribed and definite than in Dino- 

 pliilus. 



As compared with the nervous system of Protodrilus 

 there is a greater difference than in the case of Dinophilus. 

 This is due to the lack of ganglia on the ventral cord. In 

 Protodrilus the ventral cord shows no ganaflionic divisions 

 corresponding to the external segmentation. This is very 



