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 mesosomatic. 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 Eurypteridæ, and the scorpions there are seven prosomatic segments which carry (1) the cheliceræ, (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 cheliceræ 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 {255}ventro-dorsal muscles. Among the muscles of the appendages the system of large tergo-coxal muscles is especially apparent. From these considerations it follows that the number of segments in this region in the vertebrate ought to be seven; that the musculature supplied by the trigeminal nerve ought to represent seven ventral or splanchnic segments, of which only the last two are likely to be conspicuous; and that the musculature supplied by the eye-muscle nerves ought to be dorso-ventral in direction, which it is, and represent seven dorsal or somatic segments.