No. 3.] STUDIES ON THE HETERONEMERTINI. 429 



cylinder, which is probably of a fluid (or at least a viscid) con- 

 sistency in life, and of (2) a fine spongioplasmic sheath envelop- 

 ing the latter. The hyaloplasm of the axis cylinder is in direct 

 connection with that of the cell, and its spongioplasmic sheath 

 with the peripheral spongioplasmic layer of the latter. " Primi- 

 tive nerve fibrils" are absent in the homogeneous axis cylinder, 

 and the nerve tube is not composed of a bundle of separate 

 "primitive nerve tubes" (Nansen), though in a few cases 

 irregular strands of spongioplasm may penetrate from the cell 

 into the most proximal portion of the axis cylinder. The nerve 

 tubule gives off fine, unbranching collaterals, and decreases in 

 diameter distally ; it does not terminate in a number of brush- 

 like processes. There is no structural difference between the 

 proximal and distal portions of the nerve tubule, i.e., no struc- 

 tural distinction into a proximal "ganglion-cell process" and 

 a distal " nerve fibril." 



7. The colossal nerve tubules of Cerebrattdus, the so- 

 called neurochords, differ from the other tubules (i) in their 

 superior size ; (2) in that they do not give off collaterals but 

 divide dichotomously, the two branches formed by such 

 division being of equal size ; (3) in their segmental con- 

 strictions. 



8. I found no substance resembling myelin in the nervous 

 system. 



9. The colossal ganglion cells (IV) of Cerebratulus (absent 

 in Lineus) are present in three pairs in the ventral brain lobes, 

 and are distributed irregularly along the lateral chords, but are 

 wholly absent in both ends of the latter (namely, in the oeso- 

 phageal region and in the caudicle). In the lateral chord they 

 increase in number posteriorly, and are dorsally more abundant 

 than ventrally ; those in the one chord do not regularly alter- 

 nate with, nor are they paired with, those in the other. The 

 only apparent regularity in the distribution is that areas where 

 they are comparatively numerous alternate with zones where 

 they are relatively scarce. 



10. In each lateral chord of Cerebratzdus, but apparently 

 not of Lineus, both dorsally and ventrally the radial clusters of 

 cells II show a bilateral arrangement. 



