No. I.] THE METAMERISM OF NEPHELIS. 37 



nervous system ; and that portion intimately connected with 

 the control of the organs of internal life which I shall call the 

 sympathetic system. These two parts differ widely in certain 

 characteristics of structure as well as of function. The central 

 nervous system is strongly metameric throughout its length. 

 Its cells are relatively larger and are referable to the unipolar 

 and bipolar types for the most part. The fibers of these cells 

 always tend to run in bundles and never to form plexuses. The 

 sympathetic, on the other hand, is free from any discoverable 

 trace of metamerism ; its cells are small and frequently multi- 

 polar, and the fibers always tend to form plexuses (PI. VIII, 

 Fig. 19). 



The Central Nervous System. 



The entire ganglionic chain in Nephelis, as in other leeches, 

 is contained in the ventral blood sinus, which, according to 

 Bourne and others, is one of the vestiges of the original coelomic 

 cavity. This sinus runs directly under the alimentary canal 

 and is readily distinguished by its dark pigmentation and the 

 swellings within which lie the ganglia. 



The anterior end of the chain, called the sub-oesophageal 

 ganglia and the brain, consists of a mass of neuromeres more 

 or less completely fused together, and forming a collar about 

 the oesophagus. The posterior end, called the "anal ganglia," 

 consists likewise of a number of neuromeres more or less com- 

 pletely fused. Between these terminal portions lie eighteen 

 neuromeres joined each to the next by two connectives. Between 

 these, and dorsal to the axis of the chain, lies a small bundle of 

 fibers known as the median nerve, or Faivre's nerve. These 

 connectives are longest in the mid-body region and decrease in 

 length towards either end of the body, becoming almost nil in 

 the most fused parts at both extremities. Within each connective 

 lies a "colossal axial" cell, the nucleus lying about midway be- 

 tween the neuromeres, as has been described for other leeches. 



At the points of junction between the connectives and the 

 neuromeres the fibers of the connectives do not separate into 

 small bundles as they do in Hirudo and Macrobdella, but each 

 continues into the body of the neuromere as a single bundle. 



