30 bulletin: museum of comparative zoology. 



somites xxvi. and xxvii., are shown, followed by the nerve mass of the poste- 

 rior sucker, made up of seven fused ganglia. In it seven pairs of lateral 

 capsules appear on either side, a segmental nerve root being closely connected 

 with each pair (xxviii.-xxxiv.). The more posterior of the lateral capsules 

 has in the case of each pair been displaced outward and downward (ventrad) 

 and been reduced in size. The position of the seven pairs of ventral capsules 

 is indicated by dotted outlines, the numeral denotiiig the somite to which each 

 capsule belongs. In the first and last of the fused ganglia of this region, the 

 ventral capsules occupy their typical tandem position (as in ganglion 26) ; in 

 the case of the intervening ganglia (29-33), we find a more or less complete 

 displacement of the ventral capsules to a side-by-side position. A similar dis- 

 placement occurs in ganglion 27, which lies close back against the septum which 

 divides the lacunar space of the posterior sucker from that in which the more 

 anterior portions of the central nervous system lie. The same mechanical 

 cause, crowding in an antero-posterior direction, explains both phenomena of 

 displacement. 



The evidence presented in Figure 9 leaves no room for doubt that seven 

 primitive ganglia are found in the nerve mass of the posterior sucker in this 

 species. Determination of the number of ganglia represented in the brain mass 

 is not quite so easy, but the evidence is likewise convincing. The brain (ft.. 

 Figures 4, 7) forms a ring of nervous substance situated commonly in the last 

 ring of somite vi. and the first two rings of somite vii. It surrounds the 

 thin-walled pharyngeal sac (sac. phy., Figure 1), there being in leeches no 

 recognizable separation into supra- and sub-cesophageal ganglia. 



A lateral view of the brain and the metaraeric nerves given off from it is 

 shown in Figure 8 ; a view of its dorsal surface in Figure 12. Figure 10 

 shows the arrfingement of the capsules on its ventral surface. An examination 

 of Figures 8 and 10 shows that the capsules (6, 6) of the last brain ganglion 

 have quite their typical arrangement. A triple segmental nerve (vi., Figure 8) 

 emerges from under a pair of lateral capsules, Avhile below a pair of ventral 

 capsules are arranged in the usual tandem order (6, 6, Figures 8, 10). ^ 



Ganglia 3-5 likewise present no special difficulties, their lateral capsules 

 being present in pairs with nerve roots attached (3, 3 It, J^: 5, 5, Figure 8). 



1 I have been unable to determine to what extent in the reduced somites at 

 the two ends of the body the original triple nature of the segmental nerves per- 

 sists. The nerve of the last brain ganglion is certainly triple (vi., Figure 8). as 

 we should expect from the fact that somite vi. consists of three distinct rings 

 (Figures 4, 7). Most of the nerves anterior to this one, perhaps all, arc either 

 double or triple, but as I have been unable to determine accurately which con- 

 dition exists in some of tliera, T represent the nerve as undivided in the case of the 

 first five somites (Figure 8). For a like reason I follow a similar course in reiire- 

 senting the segmental nerves of the posterior ganglionic mass (Figure 9). I think 

 that all of these nerves arc made up of at least two distinct bundles of fibres ; 

 whether the small tliinl nerve is also present as a distinct ulement in any or all of 

 tiiem, I am unable at present to say. 



