290 THE ORIGIN OF VERTEBRATES 



to my theory, to the epimeral nerves of Limulus ; the other coloured 

 red, which supplies muscles belonging to the visceral or splanchnic 

 muscle-group, and contains also the nerves to the tentacles. 



This latter group, which is formed by two distinct well-defined 

 nerves, viz, the mandibular and the internal branch of the maxillary, 

 corresponds, according to my theory, to the amalgamated nerves of 

 the prosomatic appendages, and is clearly divisible into three distinct 

 nerves — 



1. The lower lip-nerve or the metastomal nerve {met.). 



2. The tongue-nerve (t.). 



3. The nerve (tent.) to the upper lip and tentacles. 



Of these three pairs of nerves it is suggested that the first pair 

 were derived from the nerves to the metastomal appendage. The 

 second pair of nerves ought, on this theory, originally to have sup- 

 plied the pair of appendages immediately in front of the metastoma 

 — that is, the pair of ectognaths, and therefore the ventral pair of 

 tentacles, known as the tongue, would represent the last remnant of 

 these ectognaths. Similarly, the other tentacles would represent the 

 enclognaths, and therefore the third pair of nerves would represent 

 the fused nerves to these concentrated endognaths, which, in the 

 Eurypterids, stand aloof from the ectognaths. 



Let us consider these three propositions separately. In the first 

 place, have we any right to attribute segmental value to the man- 

 dibular nerve ? What evidence is there of segments in this region 

 in Ammocoetes ? 



The Segment of the Lower Lip, or Metastomal Segment. 



We have seen that in the branchial or mesosomatic region the 

 segments corresponding to the mesosomatic appendages were mapped 

 out by means of their supporting or skeletal structures, their seg- 

 mental muscles, and their nervous arrangements, as well as by the 

 arrangement of the branchiae. Similarly, the segments in front of 

 the branchial region, corresponding to the prosomatic appendages, 

 ought to be definable by the same means, although, owing to the 

 absence of branchiae and the greater concentration in this region, 

 the separate segments would probably not be so conspicuous. 



The last segment considered was the segment belonging to the 

 Tilth nerve corresponding to the opercular appendages of the 



