Page:The Eurypterida of New York Volume 1.pdf/152

 some of which lie in the form of the carapace, the early embryonic differentiation of the pedipalps from the other appendages, the strong prelarval differentiation of preabdomen and postabdomen and the disappearance of the abdominal appendages in the scorpions and their persistence in the eurypterids.

It is yet to be determined whether these differences are of phylogenetic significance or only due to the pushing back, by acceleration, of later adaptations of the scorpions into the embryonic and larval stages. The latter is undoubtedly the case in the embryonic development of the pedipalps and of the narrow, distinctly defined, scorpionic tail. The early appearance and later atrophy of the abdominal appendages is, however, clearly a feature that points to a common ancestor for the scorpion and the eurypterids having such appendages, and we believe that the cephalothorax in the embryo of the scorpion, retains ancestral features from the facts that its length corresponds to about six abdominal segments and it equals the latter in width [text fig. 29]; that, however, the strong development of the procephalic region is tachygenetic.

A comparison of the larvae of all three, the eurypterids, Limulus and the scorpion, shows both the latter to have lost the primitive form of the abdomen by acceleration, that of Limulus being much broadened, that of the scorpion abruptly contracted to the tail or postabdomen while the eurypterids have best preserved the original gradual and uniform contraction. The carapaces of the eurypterids and the scorpion have most nearly retained the original proportions and form of the common ancestor. Of the cephalothoracic appendages the chelicerae are alike in all three groups and obviously ancestral in their form; the remaining legs have taken quite different courses of adaptation, the scorpions having developed the powerful chelate pedipalps, the eurypterids the swimming legs, while those of Limulus have remained relatively undifferentiated, and show no tachygenetic features in the embryos except the chelae. The embryo of the scorpion shows simple walking legs, like those of the eurypterids, and lacks the two movable claws. This simple form of the walking leg is also