42 THE LEECHES OF MINNESOTA 



and two being on the ventral side directly underneath the median 

 glia cells. The packets are usually ellipsoid in shape, but may be 

 variously distorted by constricting muscle and nerve fibres, so that 

 they are, in some cases, completely divided. 



The cell packets each contain a number of unipolar nerve cells 

 which send their fibres directly into the fibrous portion of the 

 ganglion, and one or more large glia cells ("Stirnzellen" Apathy, 

 '97). Each side packet contains one of these cells, sometimes cen- 

 trally located, but often lying near the outer margin of the packet. 

 This condition holds good throughout the nerve cord. The glia 

 cells are much larger than the ganglion cells, and as the nucleus 

 stains much more deeply with the ordinary chromatin stains, they 

 can easily be distinguished from the other cells of the packet. In 

 each ventral packet a pair of these glia cells are found lying side 

 by side near the distal end of the packet. This double condition 

 of the glia cells of the ventral packets is not unique for P. pediculata, 

 but is found in at least one other Placobdclla, viz. P. parasitica, 

 however, the glia cells in the side packets are also double. 



From each side of the fibrous portion of the ganglion, midway 

 between the two lateral packets, arise the three branches of the 

 somatic nerve; these pass backward and laterally, usually passing 

 under the posterior lateral packet, and then to the various annuli 

 of the somite in which the ganglion is situated. 



The connecting commissures consist of two large cylindrical 

 fibrous trunks, the lateral commissures, and a smaller central Nerve 

 of Faivre, (Plate E. Fav. n.) which takes its origin in the ganglion 

 between the two lateral commissures, or from one of these com- 

 missures near the ganglion. This central commissure may continue 

 its course independently between successive ganglia, occupying a 

 position between the lateral commissures; or it may, in places, 

 anastomose with one of the lateral commissures. A lateral commis- 

 sure, instead of always remaining a single trunk, frequently divides 

 into two branches which, after a short distance, again unite into 

 one cylindrical cord. In some cases all three commissures are fused, 

 for a short distance, into one solid cord. Lying along the central 

 axis of the commissure are two or more large spindle-shaped inter- 

 commissural cells. The normal number of these cells seems to be 

 two, one lying near each end of the commissure; but in many 

 cases one or both of these cells have divided, forming three, four, 

 and, in some cases, up to eight cells scattered along the whole 



