No. 3-.] STUDIES ON THE HETERONEMERTINI. 425 



existence of such fibrils, though I have worked with a larger 

 number of preserving and staining reagents. He described 

 the " Faserstrang " of the lateral chord in Cerebratubts margi- 

 natus as continued directly into the fibrous core of the ventral 

 brain lobe; in Lineus I have found no such anterior continua- 

 tion. Lastly, Burger had not noticed the branching of the 

 nerve tube (his "Faserstrang"). 



Rohde, who previously maintained ('90a, '90b) that the 

 fibrillar structures of the dotted substance were the true 

 nervous elements, has later ('92) abandoned this view and, 

 following Leydig ('64), assumed the hyaloplasm to represent 

 the nervous elements. 



B. Cerebratulus. 



The dotted substance of C. lacteus was studied on sections 

 fixed with alcoholic solution of sublimate; and since I had no 

 material hardened in Hermann's fluid I could not determine 

 the course of the smaller nerve tubules, and only to a certain 

 extent that of the neurochords. 



The structure of the fibrous core of the lateral chords is 

 illustrated by the Figs. 30, 35 a-c, 40; the same structure is 

 found in the brain lobes, except that here the neurochords are 

 absent. As may be seen in Fig. 40, which represents a lateral 

 segment of a cross section out of about the middle point of the 

 fibrous core of the lateral chord, the dotted substance consists, 

 as in Lineus, of: (i) the staining fibres of the inner neuroglia 

 cells {Ngl.C); (2) axis cylinders {Ax. CI.) and their sheaths; (3) 

 irregular, unstaining spaces between the latter which, in refer- 

 ence to what we have learned in Linens, are probably filled with 

 body fluid. But the fibrous core of the lateral nerve chords in 

 Cerebratulus differs especially from that of Lineus in the pres- 

 ence of the colossal nerve tubules (neurochords of cells IV, 

 Ax.Cl. IV, Figs. 30, 35 b, 40). I have found these neuro- 

 chords to be pretty equally distributed throughout the fibrous 

 core, though they are more numerous centrally and laterally 

 than medially (Fig. 30). In the oesophageal region I traced 

 for a considerable distance a neurochord, which was situated at 

 the latero-ventral corner of the chord. The fibrous core pre- 



