No. 3-] STUDIES ON THE HETERONEMERTINI. 415 



has found the absence of collaterals characteristic for the neuro- 

 chords. Leydig ('85) has figured a dichotomic division of the 

 neurochord in Dytiscus. Another peculiarity of these neuro- 

 chords I find in their constrictions, which may be compared to 

 the " anneaux constricteurs " ("Ranvier'sche Einschniirungen") 

 of the vertebrate axis cylinder. To wit, just anterior to a point 

 of division, such a constriction of the axis cylinder may always 

 be found (Fig. 42 a, b) ; and in one case (Fig. 42 d) I found a 

 constriction far removed from any point of dichotomy. Such 

 segmental constrictions of the neurochords have been observed 

 by Friedlander ('89) in the polychaete MastobrancJms ; but in 

 this annelid the constrictions are of greater extent than in Cere- 

 braUdus. It will be recalled that three pairs of cells IV occur 

 in the ventral lobes of the brain, and none at all in the lateral 

 chords in the oesophageal region ; each lateral chord, therefore, 

 derives three neurochords from the respective side of the brain. 

 But these most anterior neurochords do not pass undivided 

 through the lateral chord in its oesophageal region, but divide 

 here dichotomously ; so that in studying a series of transverse 

 sections from this region, they are found to increase in number 

 and decrease in size posteriorly. Since, however, on a cross 

 section of the lateral chord in the posterior portion of this region 

 it is not infrequent to find a neurochord fully as large in diame- 

 ter as one in the more anterior portion, we must suppose, either 

 (i) that a neurochord or one of its branches may vary in diame- 

 ter in different portions of its course, or (2) that a number of 

 neurochords may fuse together, posteriorly, to produce a larger, 

 compound neurochord (or neurochord " bundle "). Though the 

 first alternate would appear to be the more probable, I was 

 unable to determine this question, even on longitudinal 

 sections. 



Burger first ('90b) maintained that the neurochords of Cere- 

 bratulus and Langia divide dichotomously, after observing their 

 numerical increase on successive cross sections. Later ('91b ) 

 he abandoned this view, and finding on mtra vitam methylene 

 blue preparations no branching of the neurochords, concluded 

 that the latter pass to the posterior end of the lateral chords 

 without division. My observations, which are epitomized in 



