No. 3-] STUDIES ON THE HETERONEMERTINI. 423 



A large number, though by no means all and perhaps not 

 even one-half, of the nerve tubules of the lateral chord are 

 enclosed in this nerve tube. For a glance at Fig. 36 demon- 

 strates that a considerable number of cross-sectioned axis cylin- 

 ders are distributed pretty evenly throughout the fibrous core, 

 without being grouped together as nerve tubes. In fact, there 

 appear to be nearly as many nerve tubules scattered throughout 

 the fibrous core of the lateral chord, and not enclosed in its 

 central nerve tube, as we find in the fibrous core of the dorsal 

 and ventral brain lobes, where no nerve tubes occur (cf. Fig. 36 

 of a lateral chord with Fig. 37 of the dorsal brain lobe). 



For each nerve tubule of cell III there is usually a special 

 opening in the inner neurilemma, while, as already noted, for 

 each radial cluster of cells II (Fig. 25) there is but a single 

 opening through which all their nerve tubules penetrate to the 

 fibrous core. Fig. 36 shows that the bundle of nerve tubules 

 from such a cluster of cells II penetrates nearly to the center 

 of the fibrous core, where the individual nerve tubules then 

 diverge and bend towards the posterior end of the chord. Fig. 

 37, representing a transverse section of the core of the dorsal 

 brain lobe, exhibits the same phenomenon in regard to clusters 

 of cells I. Probably new accessions of nerve tubules are being 

 continuously added to the central nerve tube of the lateral 

 chord from such metameric bundles of nerve tubules of the 

 radial cell clusters of II. 



Attention may be drawn to Fig. 41, from a preparation 

 hardened in sublimate, afterwards stained with methylene blue 

 followed by brasilin, of Tetrastemma vermiculum. In the 

 longitudinally sectioned fibrous core {F. C.) of the lateral chord, 

 I find a rod-like structure which had been stained a deep green. 

 This body probably represents just such a nerve tube as I have 

 described for Lineus. 



I must not be misconstrued into supposing that all the 

 unstaining portions of the dotted substance represent axis 

 cylinders. On the contrary, I have found irregular, even more 

 or less tubular, unstaining spaces in the fibrous core which can 

 represent nothing more nor less than spaces filled with body 

 fluid, just as the unstaining spaces enclosed by the muscle 



