No. 3-] STUDIES ON THE HETERONEMERTINI. 413 



But does the nerve tubule consist of two separate parts, — 

 a proximal ganglion-cell process and a distal "Nervenfibrille " ? 

 Biirger's ('91b) figures drawn from preparations stained in life 

 with methylene blue certainly seem to support such a view. 

 But his descriptions do not mention any difference of structure 

 between the two parts, but simply a difference in dimension 

 and a slight distinction in staining power; and my own observa- 

 tions have taught me that there is no structural distinction 

 between the proximal and distal portions, but only a gradual 

 difference of diameter. Thus, if the nerve tubule be divisible 

 into a proximal ganglion-cell process and into a distal portion, 

 this distinction will probably be found, not to be one based 

 upon differences of structure in the nerve tubule itself, but 

 either upon differences of dimension or of differences in the 

 structure of its connective-tissue envelope (sheath of Schwann). 

 And with Apathy ('91), I think the supposed varicosities of the 

 nerve tubule might also be referred to the sheath of Schwann. 



B. Cell IV. 



The nerve tubules of the cells IV are the largest in the 

 nemerteans, and deserve the term " neurochords " applied by 

 their discoverer, Biirger ('90b), with reference to their simi- 

 larity to the neurochords (Eisig, '87; "Leydig'sche Fasern," 

 Friedlander, 89) of the annelids and crustaceans. 



This colossal nerve tubule (Figs. 27-29, 32, Ax. CI) is com- 

 posed of a thin spongioplasmic sheath, which is a continuation 

 of the peripheral alveolar layer of the cell ; and of an enclosed, 

 unstaining, homogeneous, hyaloplasmic axis cylinder, which is 

 in direct communication with the hyaloplasm of the distal por- 

 tion of the cell. The neurochord nerve tubule differs from the 

 tubules of the other ganglion cells mainly in its superior size. 

 I have never found any evidences of a finer structure in the 

 hyaloplasmic axis cylinder, but always found it to be homogene- 

 ous ; my results being in accord with those of Friedlander ('89) 

 on annelids and crustaceans, and with those of Apathy ('91) on 

 Hirudinea. But it is to be noted that on studying, for example, 

 a longitudinal section of such a neurochord by elevating or 



