90 MEMOIRS NATIONAL ACADEMY OF SCIENCES, VOL. X, NO. 4. 



We must admit that we have been very unsuccessful in the attempt to study the nervous 

 system of the Actinotrocha by means of methylene blue and ammonium molybdate. Gold 

 chloride has given no better results than staining- with iron hematoxylin. 



If the dorsal surface of the hood of a live Actinotrocha Species B. be examined, one will find 

 that there are a great many fibres winch run in more or less definite tracts (tig. 36). Many of 

 these fibres have nuclei along their course and are undoubtedly muscle fibres, while others run to 

 the edge of the hood and there seem to be continuations of certain cell-like bodies which Ikeda 

 was the first to describe (fig. 37). Although we have seen these bodies on all occasions in surface 

 views stained with methylene blue, yet in sections we have never been able to make them out, if 

 they are nerve cells. It must be mentioned, however, that in transverse sections through the 

 edge of the hood every 3 n section shows at least one nucleus closely applied to the ring of 

 nervous tissue running round the edge of the hood (tig. 43). These occup3 T the same position 

 with reference td the edge of the hood that the cell-like bodies do which are seen in surface views, 

 but we take them to be the nuclei of muscle cells, and frequently we have traced deeply stained 

 muscle fibres arising from them (fig. 43). Within the nervous tissue of the preoral ring we have 

 found no structures which we could consider to be the cell-like bodies mentioned by Ikeda (9). 



Ikeda has figured a great many fibres arising from the ganglion, but in the Actinotrocha 

 that we have examined we have not been able to see the connections; however, we do not wish 

 to deny that they exist. 



The three median nerves arising from the anterior side of the ganglion and running forward 

 to the edge of the hood, and two longitudinal tracts of nerve fibres arising from the posterior 

 side of the ganglion, can be easily made out, but the large majority of fibres which compose the 

 broad tract shown in tig. 36 are not connected with the nerve ganglion. There are some indi- 

 vidual differences in the arrangement of the above tracts, but in general they are about as shown 

 in fig. 36. 



On each side of the medio-dorsal line in the region of the youngest tentacles a tract of fibres 

 can be seen running longitudinally. In the region where the edge of the preoral hood is inserted 

 into the collar a small tract made up of a few fibres branches off from the dorsal longitudinal 

 tract and passes into the edge of the preoral lobe. Somewhat farther forward each dorsal longi- 

 tudinal trunk spreads out sometimes into three rather indefinite tracts, most of whose fibres seem 

 to reach the edge of the hood. Many of the fibres of the anterior branch appear to end in the 

 region at the sides of the ganglion, but no connection with the latter could be found. 



Immediately posterior to the ganglion a tract of fibres (fig. 36) is seen which runs for a short 

 distance transversely to the long axis of the Actinotrocha. On both sides the fibres of this tract 

 soon diverge from one another and in this way distribute themselves over the anterior part of 

 the hood, ending at the edge of the latter (tig. 36). 



Masterman (15) has figured (PI. XVIII, fig. 2) certain nerve tracts to the right and left of 

 the three nerves arising from the anterior end of the ganglion and finds that these "run 

 forward and outward and then bend backward and take a course to the posterior corner of 

 flic hood." 



A lateral view of the hood of Actinotrocha Species B. shows sometimes fibres gathered 

 together in trunks, but these never take the direction as shown by Masterman. They diverge 

 rather regularly and end all along the edge of the hood instead of at the posterior corners of the 

 same (fig. 37). They are in no way associated with the ganglion and do not have the appearance 

 of being even when the hood is turned upward out of its usual position. 



For several reasons we believe that the complicated tracts of fibres seen in a surface view of 

 a live Actinotrocha Species B. are not nerve fibres but muscle fibres. First, many of them show 

 along their course nuclei resembling nuclei of muscle cells. Second, cross sections through the 

 hood show that there is a rather heavy lining of muscle tibres which run in the same general 

 direction as do the fibres shown in the surface view. Third, there is no connection between 

 these fibres and the nerve ganglion. 



