30 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
somites XXVI. and XXVII., are shown, followéd 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 (XXvII.-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 denoting 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 (6., 
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 vi. 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 metameric nerves given off from it is 
shown in Figure 8; a view of its dorsal surface in Figure 12. Figure 10 
shows the arrangement 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, while below a pair of ventral 
capsules are arranged in the usual tandem order (6, 6, Figures 8, 10).2 
Ganglia 3-5 likewise present no’special difficulties, their lateral capsules 
being present in pairs with nerve roots attached (3,3- 4, 4; 5, 5, Figure 8). 
1] 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 (v1., Figure 8). as 
we should expect from the fact that somite vr. consists of three distinct rings 
(Figures 4,7). Most of the nerves anterior to this one, perhaps all, are either 
double or triple, but as I have been unable to determine accurately which con- 
dition exists in some of them, I represent the nerve as undivided in the case of the 
first five somites (Figure 8). Fora like reason I follow a similar course in repre- 
senting the segmental nerves of the posterior ganglionic mass (Figure 9). I think 
that all of these nerves are made up of at least two distinct bundles of fibres ; 
whether the small third nerve is also present as a distinct element in any or all of 
them, I am unable at present to say. 
