28 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
g. NERVOUS SYSTEM. 
The central nervous system, as in other leeches, consists in the middle part 
of the body of a ventral ganglionic chain of twenty-one distinct ganglia meta- 
merically arranged and joined by paired connectives. Forming an extension 
of this ganglionic chain at either end of the body, one finds a nervous mass 
representing several primitively distinct gangha more or less intimately fused 
together. In the central part of the body the ordinary position of the nerve 
ganglion is in the middle ring of its somite (Figure 4, somites XII.—XVIII.). 
Toward either end of the body, however, there is a slight, but increasing, 
centripetal displacement of the ganglia, just as is frequently the case in the 
central nervous system of Arthropoda. This displacement may amount to as 
much as two-thirds of a somite, or in extreme cases an entire somite. Thus 
we see in Figure 4 that the ganglion of somite vir. lies in the first ring of 
somite VIII, a displacement of two rings; in somites vi1L—x1. the displace- 
ment is only a single ring. About the same amount of displacement occurs in 
somites XIX.-XXII.; In somites XXIII. and XXIV. it amounts to about two 
rings ; and in somites XXV.-XXVII. it is still greater. The positions in which 
the nerve ganglia are shown in Figure 4 are average ones carefully computed 
from the observed positions in five different individuals. The ganglia are 
very constant in position, the extreme variations usually amounting to only a 
fraction of the width of a ring. 
The structure and morphological value of the ganglionic masses at the two 
ends of the body is a subject closely connected with the general question of 
the metamerism of the body. 
h. METAMERISM. 
(1) Number of Somites. 
A number of investigators have discussed the question of how many somites 
are found in the body of a leech, and have reached conclusions varying accord- 
ing as they placed emphasis on one or another of the following criteria: 
(1) The number of external rings ; (2) color markings of rings, or the recur- 
rence of peculiar papillz on certain rings of each somite; (3) metameric sense 
organs; (4) the number of ganglia in the central nervous system as determined 
(a) by a count of the nerve capsules, typically six to a ganglion, or (6) by 
ascertaining the number and peripheral distribution of the nerves arising from 
the ganglia. 
Whitman (’92), making use principally of the criteria named under 3 and 4, 
was the first to obtain an entirely satisfactory answer to the question. He has 
shown that in the central nervous system of ‘* Clepsine hollensis” (which is 
closely related to G. parasitica) there are present thirty-four ganglia, each giving 
off paired nerves. Six of these ganglia are found in the anterior ganglionic 
mass which encircles the pharyngeal sac; seven are found in the posterior 
