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 segmental muscles, and their nervous arrangements, as well as by the arrangement of the branchiæ. 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 branchiæ 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 {291}Eurypterid. The segment immediately in front of this is the next for consideration, viz. that corresponding to the chilarial appendages or metastoma; and as the basal part of this pair of appendages was fused with the basal part of the operculum, the one cannot be discussed without the other; therefore, the segment to which the lower lip belongs must be considered in connection with and not apart from the thyro-hyoid segments already dealt with. In Chapter V., p. 188, I stated that the supporting bars of the foremost mesosomatic segments, the thyro-hyoid segments, differed from the cartilaginous bars of the branchial segments, in that they were composed of muco-cartilage. Also in addition to the muco-cartilaginous skeletal bars, a ventral plate of muco-cartilage exists in Ammocoetes which covers over the thyroid gland. Similarly in the prosomatic segments the skeletal bars are composed of muco-cartilage and the ventral plate of muco-cartilage continues forward as the plate of the lower lip. It is of special interest, in connection with the segments indicated by such supporting structures, to find that this special tissue is entirely confined to the head-region, and disappears absolutely at transformation, thus indicating the ancestral nature of the segments marked out by its presence. This muco-cartilaginous skeleton is the key to the whole position, and requires, therefore, to be understood. It is of great importance, not only because it demonstrates the position of the segments in Ammocoetes which characterized its invertebrate ancestor, but also because it possesses a structure remarkably similar to that found in the head-plates of the most ancient fishes. For the present I will confine myself to the consideration of this muco-cartilaginous skeleton as evidence of the relationship of Ammocoetes to the Eurypterids, and in the next chapter will show how absolutely the same skeleton corresponds to that of the Cephalaspidæ, so that Ammocoetes is really a slightly modified Cephalaspid, the larval form of which was Eurypterid in character. {292}[Illustration: FIG. 115.--DORSAL HALF OF HEAD-REGION OF AMMOCOETES.