114 PROP. MARSHALL AND MR. SPENCER. 



would suggest that it is due mainly to an extension forwards, and 

 accumulation at the anterior end of the head, of the special tegumentary 

 sense organs — the mucous canals — ,this extension forwards involving 

 a corresponding extension of the nerves supplying these organs ; in 

 connection with this suggestion it is of interest to note that no one of 

 the nerves in front of the fifth sends any branches to these organs. 

 Whether there is any trace of a ramus dorsalis to the third is very 

 doubtful j at any rate the fourth nerve cannot be the ramus dorsalis 

 of the third, as its course is at right angles (fig. 11) to the rami dor sales 

 of the fifth and seventh nerves ; and, secondly, it is a motor and not a 

 sensory nerve. 



We now come to a far more intricate problem, viz., the import of 

 the connecting branches between the third, fifth, and seventh nerves, 

 with which it will be convenient to consider the nerve N. (figs. 10, 11, 

 12, and 15). 



These three nerves, N.c, 2V T .c/, and N. all appear very early ; we 

 have failed to determine the date of their first origin, but by stage k 

 they are fully established. The posterior one (N.c.), connecting the 

 fifth and seventh nerves together, is the most difficult to investigate, 

 owing to its appearing from the first as merely the deeper portion of 

 the buccal nerve (fig. 6); in longitudinal sections, however, it appears 

 very distinct (fig. 10). It is from the first much shorter than 

 either of the other two nerves we are considering, and in the later 

 stages (fig. 14) and the adult condition, owing to the close approxi- 

 mation of the fifth and seventh nerves, ceases to be visible as a dis- 

 tinct trunk. 



The second of the three nerves (iV.c, figs. 10 and 11) forms, as 

 already noticed, a direct connection between the Gasserian ganglion of 

 the fifth and the ciliary ganglion (e.g.) of the third nerve, and is much 

 more slender than N.c'. Concerning the nerve in question, it is of the 

 utmost importance to notice that not only is it fully established at the 

 stage at which our observations commence, but that it is from the very 

 first a connecting nerve, and that there is no reason whatever in the early 

 stages for considering it as belonging to the fifth rather than to the third 

 nerve. We have, therefore, in this paper given it a perfectly neutral 

 name. 



The last of these nerves, N., is still more remarkable \ like the others 

 it is present at k. Starting at this stage from the ciliary ganglion it 

 rims in an almost perfectly straight course to the anterior end of the head, 



