180 . BULLETIN OF THE 



2. Large Ganglion Cells. 



The large ganglionic cells of the brain and ventral nerve cord were 

 called by Biirger "giant cells." So far as mere size is concerned, 

 the name is well chosen, but it has been pre-empted for the neuro- 

 chord cells of higher groups. To avoid the confusion which has en- 

 tered into other divisions of morphology owing to the use of a single 

 name for a multiplicity of organs, this designation should not be em- 

 ployed here unless there is some reason for regarding them as homolo- 

 gous with the cells of Annelids and Crustacea which first received the 

 name. In comparing these large cells with the neurochord cells of 

 Annelids, the first point of difference to be noted is the number of the 

 former. There are, as we have seen, at least five pairs of such cells in 

 the brain and others along the ventral nerve cord. In the Nemertines, 

 as in the Annelids, there is only a single pair of neurochord cells in the 

 brain ; and those in the ventral nerve cord are distributed in pairs and 

 at regular intervals, which does not seem to be the case in Nectonema. 



The second prominent point of dissimilarity has to do with the pro- 

 cesses. The fibres of these large cells vary somewhat in size, and do 

 not possess any very definite shape, being now nearly round, now angu- 

 lar, with a variety of form which may, however, be in part due to the 

 effect of reagents. In optical appearance and in reaction toward stain- 

 ing fluids they recall strongly the neurochords or giant fibres of higher 

 groups. They also extend for long distances, perhaps the entire length 

 of the worm, in an unbranched condition. But they differ from the giant 

 fibres in one striking respect, — they have no sheath ; in fact, it is very 

 difl&cult to say that they are even enclosed by a delicate membrane, so 

 fine is the boundai-y between them and the surrounding tissue. On the 

 other hand, the sheath of the giant fibres is the most striking pecu- 

 liarity which they possess, and often exceeds in prominence the fibre 

 itself. 



These seem to me sufficient reasons for regarding the cells in question 

 as not homologous with the giant cells of other groups. I have therefore 

 avoided using the expression " giant cells " to designate them, in order 

 not to suggest a false homology. It cannot be denied that these may 

 represent the primitive form of the giant cells, in which the fibres have 

 not yet acquired the highly differentiated sheath ; but until this be- 

 comes more probable by reason of evidence as yet lacking, it is better 

 to use the non-committal term, and to designate them as large gan- 

 glion cells. 



