OF PERIPATUS CAPEXSIS. 889 



Here they at once approach and shortly meet in the~rrredian 

 dorsal line (fig. 16). They can only be traced for a very short 

 distance beyond their meeting point. These nerves are, without 

 doubt, the homologues of the sympathetic system of Chaetopods, 

 occupying as they do the exact position which Semper has 

 shewn to be characteristic of the sympathetic nerves in that 

 group, and arising from an almost identical part of the brain 1 . 



Histology of the Nervous System. 



Ventral Cords. The histology of the ventral cords and 

 oesophageal commissures is very simple and uniform. They 

 consist of a cord almost wholly formed of nerve-fibres, placed 

 dorsally, and a ventral layer of ganglion cells (figs. 16 and 20). 



The fibrous portion of the cord has the usual structure, being 

 formed mainly of longitudinal fibres, each probably being a 

 bundle of fibres of various sizes, enveloped in a sponge-work 

 of connective tissue. The larger bundles of fibres are placed 

 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 somewhat crescent-shaped in 

 section, and, as shewn 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 which 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 ganglionic 

 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. 



1 Vide Spengel, " Oligognatluis Ronelliac." Naph-s Mittht-ihiti^-tt, IJd. in. pi. iv. 



fur. 5:. 



B. 57 



