ANATOMY AND DEVELOPMENT OF PERIPATUS CAPENSIS. 233 



near the inner borders of the cords. In this part of the cord 

 there are placed a very small number of ganglion cells. 



The layer of ganglion cells is someAvhat crescent-shaped in 

 section, and, as shown in figs. 16 and 20, envelopes the whole 

 ventral aspect of the fibrous parts of the cord, and even creeps 

 up slightly on to the dorsal side. It is thicker on the inner 

 than on the outer side, and increases considerably in bulk at 

 each ganglionic enlargement. The cells of Avhich it is com- 

 posed are for the most part of a nearly uniform size, but at the 

 border of the fibrous matter a fair sprinkling of larger cells is 

 found. 



The tracheal vessels supplying the nervous system are placed 

 amongst the larger cells, at the boundary between the gan- 

 glionic and fibrous regions of the cords. 



With reference to the peripheral nerve-stems there is not 

 much to be said. They have for the most part a similar struc- 

 ture to the fibrous parts of the main cord, but are provided with 

 a somewhat larger number of cells. 



Sheath of the Ventral Cords. — The ventral "cords are enve- 

 loped by a double sheath, the two layers of Avhich are often in 

 contact, while in other cases they may be somewhat widely 

 separated from each other. The inner layer is extremely thin 

 and always very closely envelopes the nerve-cords. The outer 

 layer is thick and fibrous, and contains a fair sprinkling of 

 nuclei. 



Supra- oesophageal Ganglia. — In the present state of our 

 knowledge a very detailed description of the histology of the 

 supra-oesophageal ganglia would be quite superfluous, and 

 I shall confine myself to a description of the more obvious 

 features in the arrangement of the ganglionic and fibrous por- 

 tions (PI. XVII, fig. 19 A— g). 



The ganglion cells are in the first place confined, for the 

 most part, to the surface. Along the under side of each gan- 

 glion there is a very thick layer of cells, continuous behind, 

 with the layer of ganglion cells which is placed on the under 

 surface of the oesophageal commissures. These cells have, 

 moreover, an arrangement very similar to that in the ventral 



