398 MONTGOMERY. [Vol. XIII. 



In Cerebratuhis the cyptoplasm of cells III is not, as a rule, 

 as vesicular as in Linetis, but both hyalo- and spongioplasmic 

 constituents are pretty evenly distributed. In some cases the 

 hyaloplasm nearly fills the whole cell body, though even in such 

 cells a peripheral, fine-grained, spongioplasmic layer envelops 

 the cell, and a similar layer surrounds the nucleus. Occasion- 

 ally, also, I found a large, flattened vacuole, resembling in shape 

 a concavo-convex lens, situated between the nucleus and the 

 proximal end of the cell ; the structure of the cytoplasm in the 

 rest of the cell being finely granular. 



The cells of the commissures of the oesophageal nerves are 

 a modification of the cells III of the brain, from which they 

 differ in their inferior size and denser cytoplasm (Fig. 20). 

 In the lateral chords, two or three cells III often surround a 

 cell IV, and such enveloping cells are often difficult to distin- 

 guish in structure from the enclosed IV (Fig. 29). 



Biirger ('90b) has already shown that the cells III occur both 

 in the dorsal and ventral lobes of the brain and in the lateral 

 chords. It may be noted that in Lineus they are compara- 

 tively more numerous on the periphery of the ventral lobe than 

 in Cerebratuhis ; while in the latter genus they are relatively 

 more abundant on the medial side of the dorsal lobe than in 

 Lineus. Only in L. sp. do I find a few cells III on the lateral 

 aspect of the lateral nerve chord ; this, as well as the median 

 side of the chords, being always devoid of ganglion cells in 

 Lineus gesserensis and Cerebratuhis . 



D. Cell IV {Cerebratuhis). 



These colossal cells, termed by their discoverer, Burger 

 ('94b), "neurochord cells," on account of their similarity to 

 the neurochord cells of other forms, have been found by him 

 to be limited to the genera Cerebratuhis, Langia, Drepanopho- 

 rus, and Prosadenoporus. They are absent in Lineus, but I 

 have found them in Cerebratuhis lacteus. Coe ('95a) did not 

 observe them in the latter species, which is a strange over- 

 sight, since they are the largest cells of this nemertean, with 

 the exception of the ova ; however, it is possible that this 

 author classed them together with cells III. 



