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 ganglia 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.-xviir.). 

 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 vii. lies in the first ring of 

 somite viii., a displacement of two rings ; in somites viii.-xi. 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.-xxvri. 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 fcAhnving criteria : 

 (1) The number of external rings ; (2) color markings of rings, or the recur- 

 rence of peculiar papillae 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 (l>) by 

 ascertaining tlie number and peripheral distribution of the nerves arising from 

 the ganglia. 



Whitman ('92), making use principally of the criteria named under Sand 4, 

 wa.s the first to ol)tain an entirely satisfactory answer to the question. He has 

 shown that in the central nervous system of " Clepsine hollensis" (whic^h is 

 closely related to O. parasitica) there are present thirty-four ganglia, each giving 

 off paired nerves. Six of these ganglia are foniul in the anterior ganglionic 

 m;iss which encircles the pharyngeal sac ; seven are found in the posterior 



