CASTLE: NORTH AMERICAN RHYNCHOBDELLIDA. 29 
ganglionic mass which lies in the posterior sucker and supplies it with nerves ; 
these, added to the twenty-one distinct ganglia found in the central part of the 
body, bring the total up to thirty-four. An examination of the sense organs 
connected with these ganglia, and situated typically on the middle ring (first, 
Whitman) of each somite, yields corroborative evidence that the number of 
somites represented in the body is thirty-four. 
Bristol (99) subsequently made a similar study of the metamerism of 
Nephelis lateralis, his conclusions being for the Gnathobdellide entirely in 
harmony with those ef Whitman for the Rhynchobdellide. 
Oka (94), however, has cast doubt upon the general applicability of Whit- 
man’s determination, based as it was on the metamerism of a single species of 
Glossiphonia, by stating that in the several European species which he has 
studied (G. stagnalis, G. complanata, G. concolor, G. heteroclita, G. papillosa, 
G. marginata, and G. tessellata) he finds evidence of only five (not of six) fused 
ganglia in the brain. Moreover, in recent systematic papers, such as those of 
Blanchard (’94) and Moore (’99), we find the body of the leech still analyzed 
and described as consisting of twenty-six preanal somites, instead of twenty- 
seven, the number found in that portion of the body by Whitman (’92) 
and Bristol (99), and still earlier, though on less satisfactory evidence, by 
Apathy (88). 
Accordingly, I have thought it worth while to examine into this matter 
rather carefully in the case of the species studied by me. 
I may say at once that my results, in the case of all six species studied, are 
in complete accord with those of Whitman (’92), so far as the number of meta- 
meres is concerned. In determining the kmits of the somite, I have arrived at 
conclusions differing from those of my predecessors, as will presently appear 
(p. 31 ff.). 
a. Structure of a Typical Ganglion. — A typical ganglion from the middle 
of the body has its ganglion cells arranged in six groups enclosed in capsules of 
connective tissue. Four of these capsules are lateral in position, two on each 
side of the ganglion ; the other two occupy a mid ventral position, one in the 
anterior, the other in the posterior part of the ganglion. (See the ganglion of 
somite xxvI. in Figure 9, Plate 3.) Three nerves are given off close together 
from either side of the ganglion, and are distributed to the three successive rings 
of one and the same somite, as I have elsewhere (Castle, 1900) pointed out. 
If, then, we can determine exactly how many such ganglia are present in the 
eentral nervous system of a leech, we shall be in a position to say how many 
somites enter into the composition of its body. 
In the middle part of the body, as already stated, twenty-one distinct ganglia 
of the sort just described can easily be recognized. To determine how many 
are present toward either end of the body, where more or less fusion of ganglia 
has taken place, is a matter of more difficulty. 
8. Fused Ganglia. — Figure 9 (Plate 3) shows a dorsal view of the poste- 
rior part of the central nervous system of G. stagnalis, obtained by reconstruc- 
tion from a series of frontal sections. The last two distinct ganglia, those of 
