No. 3.] STUDIES ON THE HETERONEMERTINI. 409 



more than a very short distance, nor have I found them 

 branched; my observations corroborate those of Burger ('91b) 

 on methylene blue preparations, who discovered the collaterals 

 (his " ungleichwerthige Nebenfortsatze "), and found them 

 always unbranched. After the nerve tubule has given off a 

 number of collaterals, it is greatly decreased in diameter, 

 becoming almost invisible. In this distal portion of its extent, 

 the finer structure can necessarily be no longer determined; but 

 there is no adequate reason for supposing that the structure of 

 the nerve tubule here differs from that in its proximal portion, 

 as maintained by Burger ('91b). The nerve tubule, after enter- 

 ing the fibrous core, certainly does not divide into a brush of 

 fine branches, as figured by Rohde ('90a) for polychaetes. 



It remains to state that even after the use of fluids contain- 

 ing OSO4 I have been unable to find any evidences of a myelin 

 sheath, such as has been described by Retzius ('89b) and 

 Friedlander ('89) for other invertebrates. And as little as 

 H. Schultze ('79) and Apathy ('91), have I found varicosities of 

 the axis cylinder. 



In Cerebratulus lacteiis I could not make as thorough a study 

 of the nerve tubules as in the preceding species, since I had 

 but one specimen, and that fixed with alcoholic sublimate. 

 But it may be stated that these preparations of Cerebratulus 

 showed the same structure, as those of Lineus fixed by the 

 same method. The spongioplasmic, peripheral layer of the 

 ganglion cell is produced to form the sheath of the nerve 

 tubule, and its hyaloplasm is in direct connection with that of 

 the axis cylinder (Figs. 20, 21, 23, 29, ///). As in Lmeus, 

 spongioplasmic strands sometimes penetrate from the cytoplasm 

 of the cell into the proximal portion of the axis cylinder (Fig. 

 23) ; but though found in a comparatively larger number of cells 

 than in that genus, they are nevertheless restricted to relatively 

 few cells. Such strands are of varying thickness, and have a 

 greater diameter proximally than distally; further, they vary in 

 number from one to about six, and, though more or less parallel 

 in position, are not placed at equal distances apart. Accord- 

 ingly, there is apparently no regular distribution of the spongio- 

 and hyaloplasmic substances in the proximal portion of such an 



