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 (z.). 
3. The nerve (¢ent.) 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 
endognaths, 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 Ammoccetes ? 
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 branchiz. 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 branchize 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 
VIIth nerve corresponding to the opercular appendages of the 
