ON THE DIPLOCHORDA. 297 



111 addition to these main uervous tracts the iuucr ends of 

 the whole epiblast form a fine fibrillar uervous network in every 

 direction. In the thin epiblast of the trunk I have not been 

 able to make these out, but in surface view this area shows, 

 as already described, fine nerves branching out in all directions 

 from the ventral and dorsal trunks. 



PI. 18, fig. 2, is a surface view from the side of the whole 

 larva, with the chief nerves indicated in black, whilst in fig. 1 

 are indicated the chief nerves of the hood in dorsal view at a 

 slightly earlier stage, and figured as they actually appear, i. e. 

 as fine unstained tracts. 



The appearance of the nerve-tracts, as seen in section, is 

 shown in PI. 19, figs. 6 to 17. 



In fig. 6 are to be noticed the three main dorsal nerves of 

 the hood {a. I. n. and a. m.n.), which at the level of fig. 11 

 commence to pass down the ventral floor of the neural tube, 

 and in fig. 13 the median nerve is seen to pass into the 

 ganglion. 



In fig. 15 the posterior end of the ganglion is reached, and 

 it is seen to diverge into two main dorsal trunks situated 

 (fig. 7) upon the dorsal surface of the collar area. These 

 may be traced through fig. 8 at the level of the mouth, and 

 fig. 9, which is almost at the posterior dorsal termination of 

 the collar area. Here they diverge, and are found at the base 

 of the tentacles, and they are seen to give ofi" branches to the 

 tentacles. 



In fig. 9, immediately below the mouth, may be seen the 

 ventral nervous area formed by the partial fusion of two tracts 

 situated lateral to the mouth in fig. 8. 



Fig. 10 shows the collar-ring (c. 72. r.) still more diverging 

 into two lateral branches, and the mid-dorsal region shows in- 

 dications of the fibres running down to the perianal band, 

 though the attenuation of the cells makes them difficult to 

 follow. On the right of this figure is seen (p. o. n.) the 

 termination of the pre-oral nerve-ring, and branches from it 

 may be noticed passing dorsally and ventrally. 



In fig. 36 the foregoing features of the collar-ring, &c.. 



