40 THE LEECHES, OF MINNESOTA 



the sensory annulus the first annulus of the body somite so that his 

 neuromere did not correspond with the body metamere. 



Bristol, '98, worked out the metamerism similarly for Nephelis, 

 and, in addition, investigated the sympathetic system. He found the 

 sympathetic system connecting- with the central nervous system at 

 the collar near nerve roots I and II and forming a nerve circle in 

 front of the collar, with six capsules containing nerve cells, and a 

 plexus over the wall of the alimentary tract. In the work of Bristol 

 as well as the work of Whitman, the sensory annulus is taken as the 

 first ring of the somite. Castle, '00, and Moore, '00, working by 

 entirely independent methods, and without knowledge of each 

 other's work, came to the same conclusion with respect to the an- 

 nulation of the somite, viz. — that the sensory annulus is not the 

 first annulus of the somite, but the middle one in both the three ring 

 and five ring types. This brings the neuromere and the body somite 

 into harmony and seems to be supported by all conditions. Liva- 

 now, '03, has worked out the innervation of the body somite for 

 both the three and the five ringed types much more elaborately than 

 they have heretofore been studied, and endorses the view of Castle 

 and Moore. 



In this review of the work done upon the anatomy of the central 

 nervous system of the leech I have omitted such work as Apathy, 

 '97, and Havet, '00, and Retzius, '91, which are of a purely histo- 

 logical or cytological character. Nor have I attempted to review 

 the publications of all those who have done valuable work upon the 

 outer form of the nervous system. Much of this work has been of 

 a substantiating character and while these writers have added many 

 minor details which are of value, they have made no decided ad- 

 vance beyond their predecessors. Some of the works referred to I 

 have been unable to procure and so have been obliged to depend 

 upon the quotations of other authors who were more fortunate than 

 I and who have been able to review them first hand. 



Central Nervous System of Placobdella pediculata. 



(PI. E, Figs. 17 to 22.) 



The central nervous system may be divided into a cephalic por- 

 tion, a trunk portion, and a caudal portion. The anterior part con- 

 sists of the supra-oesophageal mass, the oesophageal collar, and the 

 sub-oesophageal mass. The trunk portion consists of a ventral 



