4 6 THE LEECHES OF MINNESOTA 



dently belongs to the central glia cell of this somite. It will also 

 be remembered that in the ventral ganglia each lateral packet con- 

 tains only one large glia cell, while the ventral packets each have 

 two. A further examination of the packets of somite II reveals 

 the fact that the four packets at the sides of the collar likewise have 

 only one glia cell, while the two dorsal packets each contain two. 

 This must be considered as still farther and more conclusive evi- 

 dence of the homology of the supraoesophageal ganglion and the 

 ganglia of the ventral chain, and it also makes the two central 

 packets at the dorsal side of the collar homologous with the ventral 

 packets of the ventral ganglia. No trace of the Nerve of Faivre, 

 which is present in the openings between all other ganglia, could be 

 found here. It seems to have completely disappeared. 



The Posterior Ganglionic Mass. 



(PI. E, Figs. 20, 21, and 22.) 



Throughout the posterior portion of the body of the leech, the 

 nerve cord runs nearly parallel to the ventral surface ; but when the 

 commissures enter the posterior sucker and become united in the 

 posterior ganglionic mass, they bend sharply dorsad making nearly 

 a right angle with the line of the ventral cord. The posterior mass 

 shows unmistakable evidence of being composed of seven somatic 

 ganglia which have been crowded together in a manner similar to 

 those of the anterior mass. Each of the original ganglia retains 

 its six cell packets, two central glia cells, and its pair of somatic 

 nerves. Each nerve, however, has only two trunks instead of three 

 as in the ganglia of the ventral chain. 



In the anterior ganglion of this mass, neuromere XXVIII, the 

 packets have retained more nearly their typical arrangement than 

 in any of the succeeding ganglia. The two ventral packets occupy 

 practically the same position as in the ganglia of the ventral chain, 

 but the side packets have been crowded together so that the anterior 

 one comes to lie nearly above the posterior and the somatic nerves 

 arise behind the posterior lateral packets. In all succeeding ganglia 

 the ventral packets lie side by side, making two rather irregular 

 rows, and the ventral packets of somite XXXIV are attached close 

 to the bases of the somatic nerves of that segment. In somites 

 XXIX, XXX, XXXI, XXXII and XXXIII, the posterior lateral 

 packets are attached by a narrow neck close above the bases of the 



