44 BRISTOL. [Vol. XV. 



the corresponding area in Nephelis, the ventral side of the ist 

 annulus together with the outer lateral sensillae. The posterior 

 nerve in Nephelis, as in Clepsine, is the principal sensory 

 nerve, innervates dorsal sensillae in all five rings, and ventral 

 organs in annuli 2 and 3 (Clepsine 2). 



Remembering, then, that annuli 4 and 5 represent annulus 

 3 in Clepsine, that 2 and 3 represent annulus 2 in Clepsine, 

 together with the homology of the nerves, the anterior nerve 

 of Nephelis representing the anterior and middle nerves of 

 Clepsine, we may use Professor Whitman's words (9, p. 388) to 

 describe the distribution of the nerves of Clepsine for Nephelis 

 as well: "They innervate three successive rings, the ist and 

 2d of their own segment and the 3d of the preceding segment. 

 The distribution is thus triannulate and dimeric." 



The Lnnervation of the Terminal Somites. 



We are now prepared to understand the modifications of the 

 plan of innervation found in a body somite as found in the 

 somites innervated by the more or less completely fused 

 terminal neuromeres in the head region, somites I to V, and 

 those in the anal region, somites XXV to XXXIV. These I 

 shall call terminal somites for convenience. Of these two 

 groups, those of the anal region present less departure from 

 the normal and hence will be described first. 



The Anal Region. 



As I have stated briefly elsewhere, the posterior portion of 

 the nerve chain is sometimes called the anal ganglia. As 

 Whitman and others have shown in other leeches, so in Nephe- 

 lis the nerve chain in this region consists of neuromeres more 

 or less fused together but retaining their fundamental charac- 

 teristics to such an extent that they can be easily identified. 

 PL VI, Figs. 14 and 1 5, show the dorsal and ventral views. The 

 number of neuromeres is ten, — the three anterior being less 

 modified by the fusion than the remaining seven. Koehler (14) 



" R. Koehler : Recherches sur la structure du systeme nerveux de la Nephelis. 

 8°. Nancy. 18S2. 



