RELATION OF THE NEMERTEA TO THE VBRTEBRATA. 635 



branch from the facialis, establishing a connection between it 

 and the vagus. 



Ahlborn mentions the existence of a similar connecting 

 stem reaching further forward still, and connecting the trige- 

 minus and facialis. How these connections vary in the different 

 adult Vertebrata will not be discussed here. 



The different facts and speculations here brought forward in 

 connection with the cephalic ganglia and the nervus lateralis 

 vagi may suffice for the present. They may severally be 

 brought to bear upon the question of the eventual homology 

 of Vertebrate cephalic ganglia and nervus lateralis, on the one 

 hand, and Vermian paired brain-lobes and lateral nerve-stems 

 on the other. The parts here compared being indicated in 

 figs. 1 and 2 of PL XLII, with corresponding letters {Lg and 

 In), a glance at these figures may further convey a notion of 

 the purport of these speculations. 



There is one fact, however, which is not indicated in these 

 figures, which is nevertheless of very high importance for the 

 views here considered, and which I must therefore develop 

 more in detail. 



It is the connection between the successive spinal nerves 

 and the ramus lateralis vagi. 



The existence of similar connections between the (eminently 

 sensory and cutaneous) dorsal roots and the (similarly sensory 

 and cutaneous) lateral nerve is for the first time mentioned 

 by Ransom and d^Arcy Thompson for Petromyzonin the 

 following passage (loc. cit., p. 422) : 



" The dorsal rami of the posterior roots pass up (over the 

 lateralis nerve) to the skin of the back, but appear also to 

 send fibres into the lateralis. (For this statement we 

 at present rely only on sections, but we hope shortly to test it 

 by dissections of the large Petromyzon marinus.)" 



It hardly needs comment that if this observation should be 

 confirmed the fact would be of the utmost importance for the 

 hypothesis under discussion. We should then be permitted to 

 consider these metameric connections between the dorsal roots 

 and the nervus lateralis of Petromyzon, as the relics of an 



