650 'Journal of CoJiiparative Neurology and Psychology. 



forming a broad central loop between. The sheath is absent from 

 the ventral surfaces of the nerves, and an interchange of fibers 

 takes place. 



Fourteenth ventral commissure. — Before the loop of the thir- 

 teenth commissure has quite disappeared, figs. 12 to 13, other 

 fibers are seen ventral to it which extend across the space between 

 the oesophageal nerves. These fibers may be traced through the 

 ventral part of the oesophageal nerves, and toward the brain lobes, 

 and represent the slender anterior part of the fourteenth and last 

 ventral commissure, which extends altogether through ten sec- 

 tions. It gradually becomes broader until it measures 85/1 dorso- 

 ventrally, and in thickness is the second of the ventral commis- 

 sures. The oesophageal nerves are no longer distinguishable as 

 separate structures but have become a part of the commissure, 

 which is now very wide from side to side and extends from one 

 ventral brain lobe to the other. The very broad roots arise on 

 the dorsal surface of the commissure, and run upward to the 

 brain lobes, entering each fibrous core as a large bundle of fibers 

 several sections posterior to the termination of the central part of 

 the commissure, fig. i. The oesophageal nerves reappear, and 

 continue backward to the mouth, which begins fifteen sections 

 farther back. This commissure evidently corresponds with the 

 large, third and last, commissure between the oesophageal nerves 

 described by Hubrecht, Burger and Montgomery. 



4. NEUROCORD CELLS. 



a. Historical r^w^w.— BiJRGER ( 1 894) was the first to distinguish 

 the fourth type of ganglion cells, the giant cells which he terms 

 neurocord cells. He states (1899, p. 105), "Neurochordzellen 

 fand ich bei alien von mir untersuchten Cerebratulen, ferner bei 

 Langia formosa. Das Gehirn besitzt stets nur ein einziges Paar 

 von Neurochordzellen, welches an der medialen Flache der ven- 

 tralen Ganglien dort gelagert ist, wo die Schlundnerven ent- 

 springen. Zahlreiche Neurochordzellen befinden sich indessen im 

 Ganglienzellbelag der Seitenstamme * * * " The statements of 

 Burger (1895) in regard to the presence of neurocord cells in the 

 Heteronemerteans may be summarized as follows. In several 

 genera a single pair of neurocord cells is found on the medial sides 

 of the ventral brain lobes in the region of the origin of the oesopha- 



