254 THE ORIGIN OF VERTEBRATES 
It is worthy of note that, in respect to their ccelomic cavities, as in 
the position and origin of their nerves in the central nervous system, 
the first pair of appendages, the chelicers, retain a unique position, 
differing from the rest of the prosomatic appendages. 
In the table I have shown how the vertebrate ccelomic cavities 
may be compared with those of Limulus. The next question to con- 
sider is the evidence obtained by morphologists and anatomists as to 
the number of segments supplied by the trigeminal nerve-group ; this 
question will be considered in the next chapter. 
SUMMARY. 
In Chapters IV. and V. I have dealt with the opisthotic segments of the 
vertebrate, including therein the segments supplied by the facial nerve, and 
shown that they correspond to the mesosomatic segments of the paleostracan ; 
consequently the facial (VIT.), glossopharyngeal (IX.), and vagus (X.) nerves 
originally supplied the branchial and opercular appendages. 
In this chapter the consideration of the pro-otic segments is commenced, 
that is, the segments supplied by the trigeminal (V.) and the eye-muscle nerves 
(III., IV., VI.). I have considered the VIth nerve with the rest of the eye- 
muscle nerves for convenience’ sake, though in reality it belongs to the same 
segment as the facial. Of these, that part of the trigeminal which innervates 
the muscles of mastication corresponds to the splanchnic segments, while the 
eye-muscle nerves belong to the corresponding somatic segments; but the 
pro-otic segments of the vertebrate ought to correspond to the prosomatic 
segments of the invertebrate, just as the opisthotic correspond to the meso- 
somatic. Therefore the motor part of the trigeminal ought to supply muscles 
which originally moved the prosomatic appendages, while the eye-muscles ought 
to have belonged to the somatic part of the same segments. ~ 
The first question considered is the number of segments which ought to be 
found in this region. In Limulus, the Eurypteride, and the scorpions there are 
seven prosomatic segments which carry (1) the chelicere, (2, 3, 4,5) the four 
first locomotor appendages—the endognaths, (6) the large special appendage— 
the ectognath—and (7) the appendages, which in Limulus are known as the 
chilaria, and are small and insignificant, but in Eurypterus and other forms 
grow forwards, fuse together, and form a single median lip to an accessory oral 
chamber, which lip is known as the metastoma. Of these appendages the 
cheliceree and endognaths tend to dwindle away and become mere tentacles, 
while the large swimming ectognath and metastoma remain strong and 
vigorous. 
In this, the prosomatic region, the somatic segmentation is not characterized 
by the presence of the longitudinal muscle segments, for they do not extend 
into this head-region, but only by the presence of the segmental somatic ventro- 
