236 PROCEEDINGS OF THE AMERICAN ACADEMY. 



least one crossing near the parapodia ; in no case have more than five 

 crossings been found in a single segment. Thus it seems that in regions 

 where two fibres exist side by side they show more evidence of metamer- 

 ism than does the rest of the nervous system. 



In the posterior segments of the worm it is diflicult to obtain single 

 sections that show any considerable length of Ley dig's fibres. The whole 

 nervous system follows the course of the epidermis, which in these seg- 

 ments is thrown into small folds or convolutions, whatever be the method 

 employed in killing. 



Attention may here be called to the fact, that in both these Maldanidoe 

 the central nervous system lies external to the ring muscles, which serve 

 as a mechanical support to it. In these cases, therefore, it is probable 

 that Leydig's fibres do not themselves fulfil the same function. 



f. Function of Leydig's Fibres. 



That Leydig's fibres are truly nervous in nature is supported, first, 

 by their direct connection with ganglion cells located in the midst of the 

 central nervous system ; secondly, by the finer structure of the fibre, both 

 sheath and contents; and, thirdly, by the position of the fibres in the 

 central nervous system. 



The cells with which these fibres are connected certainly lie within 

 the territory of the central nervous system, and are surrounded by other 

 nerve cells. Their processes are traceable among the fibres of the ventral 

 cord until they reach the Leydig's fibre. Such cells are found nowhere 

 else in the animal except in the region of the nervous system. They 

 show a marked resemblance in certain very important and peculiar details 

 of their internal structure to the ganglionic cells described by von Len- 

 hossek ('95" ) for the central nervous system of the frog. These points 

 of resemblance will be discussed later, when a detailed description of 

 the cells is given. 



It would seem as though the evidence that the cells in question are of 

 a nervous nature was almost beyond question ; but if this is so, then 

 Leydig's fibres, which clearly are in direct continuity with the processes 

 of the cells must likewise be of a nervous character. So far as I know, 

 those writers who have themselves demonstrated the continuity of cells 

 with Leydig's fibres have not denied the nervous nature of these struc- 

 tures. Only a very few of all the writers upon this subject, it is true, 

 have actually shown such a connection. These authors are Spengel ('81), 

 Rohde ('87), Haller ('89), Friedlaender ('89), and Cerfontaine ('92), 

 all of whom are supporters of the nervous nature of Leydig's fibres. 



