THE PROSOMATIC SEGMENTS OF AMMOCCETES 29 1 



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 dis- 

 cussed 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 

 Ammoccetes which covers over the thyroid gland. 



Similarly in the prosomatic segments the skeletal bars are com- 

 posed 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 support- 

 ing structures, to find that this special tissue is entirely confined to 

 the head-region, and disappears absolutely at transformation, thus indi- 

 cating 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 

 Ammoccetes 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 Ammoccetes to the Eurypterids, 

 and in the next chapter will show how absolutely the same skeleton 

 corresponds to that of the Cephalaspidre, so that Ammoccetes is 

 really a slightly modified Cephalaspid, the larval form of which was 

 Eurypterid in character. 



In Chapter IV., Eigs. 63, 64, I have given a representation of the 

 ventral and dorsal views of an Ammoccetes cut in half horizontally. 

 Such a section shows with great clearness the series of branchial 

 appendages with their segmental muscles and cartilaginous bars 

 which form the branchial segments innervated by the IXth and Xth 

 nerves, according to my view of the branchial unit. As is seen (Fig. 

 64 or 115), the skeletal bar of the hyoid or opercular appendage, 



