254 THE O RIG IX 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 chelicerre, retain a unique position, 

 differing from the rest of the prosomatic appendages. 



In the table I have shown how the vertebrate crelomic 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 palaaostracan ; 

 consequently the facial (VII.). 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 seg-nients supplied by the trigeminal (V.) and the eye-muscle nerves 

 (III.. IV., VI.). I have considered the Vlth 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 seg-ments, while the 

 eye-muscle nerves belong* to the corresponding- somatic seg-ments ; but the 

 pro-otic segments of the vertebrate ought to correspond to the prosomatic 

 segments of the invertebrate, just as the oinsthotic correspond to the meso- 

 somatic. Therefore the motor part of the trigeminal ought to supply muscles 

 which orig-inally 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 Eurypterida?. and the scorpions there are 

 seven prosomatic segments which carry (1) the chelicerae, (2, 3. 4, 5) the four 

 first locomotor appendages — the endognaths. (6) the large special appendage — 

 the ectognath — and (7) the appendag-es, which in Limulus are known as the 

 chilaria, and are small and insig-nificant, 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 

 chnlicerae 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 seg-mental somatic ventro- 



