THE LEECHES OF MINNESOTA 45 



a concentration of ganglia around a small mouth followed by a 

 stretching of parts as the collar slipped back over the larger 

 pharynx, as supposed by Whitman, '92. 



In ganglion VI we have an arrangement similar to the typical 

 ventral ganglion in all respects except that the packets are more 

 closely pressed together and the somatic nerves arise nearly op- 

 posite the anterior bodies. Ganglion V is similar to ganglion VI 

 except that the central trunk of the somatic nerves (V2) takes a 

 dorsal course, passing across the inner anterior margin of the 

 anterior lateral packet of ganglion V, then passing between the 

 lateral packets of ganglion II and over the lateral dorsal margin or 

 the oesophageal collar. Ganglion IV is similar to ganglion VI ex- 

 cept that the somatic nerve of each side is composed of two trunks 

 instead of three. Ganglion III differs from ganglion IV in that 

 the anterior lateral packet is attached, to the posterior side of the 

 oesophageal collar directly above the posterior packet. The nerve 

 of this somite takes its origin just in front of the anterior lateral 

 packet. 



Ganglion II departs widely from the typical somatic ganglion, 

 the ventral packets being side^by side at the anterior ventral side 

 of the oesophageal collar and the lateral packets being far dorsal on 

 the posterior side of the collar. The nerves of this somite have each 

 been reduced to one trunk which takes its origin from the anterior 

 side of the oesophageal collar just outside and above the ventral 

 packet of that side. From the inside of the collar at the base of 

 this nerve, arises the stomatogastric nerve (Plate E, st. n.) which 

 arches toward the median line and then turns caudad along the 

 oesophagus. This nerve arises just where we should expect the 

 anterior "motor" trunk of the somatic nerve, but the part supplied 

 is very different from that which we should expect the anterior 

 branch of the somatic nerve to supply. The similarity of this nerve 

 to the vagus nerve of vertebrates, both in origin and termination, 

 is rather striking. 



Ganglion I is very similar to ganglion II, but the ventral 

 packets have been pushed to the dorsal side of the collar and the 

 lateral packets have migrated to the anterior side. The nerve of 

 somite I has only one trunk and arises from the inside of the collar 

 a short distance above nerve II. Just outside the origin of this 

 nerve lies the posterior lateral packet. In the collar, just above the 

 base of each nerve of somite I, there is a large nucleus which evi- 



