44 



THE LEECHES OF MINNESOTA 



are arranged side by side at the head of this row. The side packets 

 are not so regularly arranged. The fourteen side packets belonging 

 to somites VI, V, IV, and the posterior half of III, are arranged 

 in two irregular rows at either side of the fibrous mass. The 

 anterior packets of somites IV and V are shoved closer together 

 and dorsad, while the posterior packet of somite IV is crowded 

 ventrad. The lateral packets of somite II and the anterior lateral 

 packet of somite III lie along the posterior side of the oesophageal 

 collar, which bends sharply dorsad from the suboesophageal portion, 

 and extends in a wide loop around the digestive tract. An interest- 

 ing variation 'is to be noticed in the left posterior lateral packet of 

 somite II which has been completely divided, retaining only a com- 

 mon point of attachment. Partial or complete division of cell 

 packets is rather common in different parts of the nervous system 

 and is evidently the result of the mechanical pressure of organs 

 which may be in contact with them. In some cases it is probably 

 brought about by constricting muscle fibres, while in others it is 

 clearly the result of a nerve having been crow r ded against the 

 packet until it has pinched it completely in two. 



The six packets of cells of somite I are situated on the anterior 

 side of the oesophageal collar, the ventral packets having been 

 pushed to the extreme dorsal side of the loop, while the lateral 

 packets lie along the sides of the collar below the outer margins of 

 these dorsal packets. The posterior pair of lateral packets of somite 

 I are about as far ventral with respect to the oesophagus as the 

 anterior pair of somite II are dorsal. There is, then, in this species, 

 no distinct supraoesophageal ganglion, but a suboesophageal mass 

 and an oesophageal collar, around which are distributed ganglionic 

 packets belonging to the first three ganglia. The equivalent, how- 

 ever, of the supra-oesophageal ganglion is to be found here in the 

 part anterior to the collar. The oesophageal commissures which go 

 to form the collar are not the homologues of the lateral commissures 

 of the ventral chain, but are made up of the ganglionic fibre masses 

 of several ganglia. This state of affairs is exactly what we should 

 expect to find if several ventral ganglia had been crowded together 

 until the ventral commissures were practically eliminated, and then 

 the oesophagus had been forced between the anterior two, pushing 

 the median ventral bodies of these two ganglia with their crossing 

 fibre masses, to the extreme dorsal and ventral sides. The position 

 of the ganglionic parts would seem to indicate that there had been 



