2^8 THE ORIGIN OF VERTEBRATES 



I conclude, then, that the chelicera? must truly be included in 

 the pro-somatic group, but that they stand in a somewhat different 

 category to the rest of the prosomatic appendages, inasmuch as they 

 take up a very median anterior and somewhat dorsal position, and 

 their ganglia of origin are also exceptional in position. 



Next for consideration come the chilaria (7 in Fig. 103), which 

 Lankester did not consider to belong to appendages at all, but to 

 be a peculiar pair of sternites. Yet their very appearance, with 

 their spinous hairs corresponding to those of the other gnathites and 

 their separate nerve-supply, all point distinctly to their being a 

 modified pair of appendages, and, indeed, the matter has been placed 

 beyond doubt by the observations of Kishinouye, who has found 

 embryologically that they arise in the same way as the rest of the 

 prosomatic appendages, and belong to a distinct prosomatic segment, 

 viz. the seventh segment. In accordance with this, Brauer has found 

 that in the scorpion there is in the embryo a segment, whose ap- 

 pendages degenerate, which is situated between the segment bearing 

 the last pair of thoracic appendages and the genital operculum — a 

 segment, therefore, comparable in position to the chilarial segment of 

 Limulus. 



Coming now to the five locomotor appendages, we find that they 

 resemble each other to a considerable extent in most cases, with, 

 however, certain striking differences. Thus in Limulus they are 

 chelate, with their basal joints formed as gnathites, except in the 

 case of the fifth appendage, in which the extremity is modified for 

 the purpose of digging in the sand. In Pterygotus, Slimonia, Euryp- 

 terus, the first four of these appendages are very similar, and are 

 called by Huxley and Woodward endognaths; in all cases they 

 possess a basal part or sterno-coxal process, which acts as a gnathite 

 or foot-jaw, and a non-chelate tactile part, which possesses no pre- 

 hensile power, and in most cases could have had no appreciable 

 share in locomotion, called by Huxley and Woodward the palpus. 

 These small palps were probably retractile, and capable of being 

 withdrawn entirely under the hood. The fifth appendage is usually 

 different, being a large swimming organ in Pterygotus, Eurypterus, 

 and Slimonia (Figs. 8 and 104), and is known as the ectognath. 



Finally, in Drcpanopterus Bembycoides, as stated by Laurie, all 

 five locomotor appendages are built up after the same fashion, the 

 last one not being formed as a paddle-shaped organ or elongated as 



