bisegmented structure in the metastoma, indicating that it originated from a paired organ. He considers it homologous to the chilaria of Limulus, a pair of movable sclerites set behind the coxal segments of the last pair of legs [text fig. 10] and remarks that the metastoma of the eurypterids certainly represents a much higher development of the organ than the chilaria of Limulus. Pocock [1901, p. 302] considers the metastoma as the homologue of the sternum of the scorpion but the observations of Kishinouye [1891] upon the embryo of Limulus longispina and those of Brauer [1895] on the embryo of the scorpion demonstrate that it represents the appendages of a distinct suppressed segment. For practical reasons we have not counted this abortive first segment of the preabdomen [see diagram p. 24].
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| Figure 13 Eurypterus fischeri Eichwald. Endostoma, seen from below (outside). (From Holm) | Figure 14 Eurypterus fischeri Eichwald. On the left the right coxa, seen from the interior and showing the doublure, the large cutting tooth and the smaller teeth; and its connection with the metastoma (on the right), which also shows its interior doublure. (From Holm) |
Gaskell, in his lately published The Origin of Vertebrates, in order to derive a vertebrate prosomatic or oral chamber fully separated from the gill chambers, has assumed that the metastoma and the operculum of Eurypterus became fused [op. cit. p. 242 and our text fig. 16]. It is safe for us to say that we have no evidence in the eurypterids of any tendency toward the fusion of these organs and that it seems to us such a procedure
