Page:The Eurypterida of New York Volume 1.pdf/53

 segments of the legs. There is other evidence of their prehensile function. In Limulus the first pair of walking legs of the male becomes transformed at maturity into a hooklike grasping organ and in, the second pair of legs also develops in the mature male a hooked clasping organ.

The first pair of postoral appendages probably also served in most genera as a tactile organ. This is very clearly indicated in Slimonia where it is directly developed into an antenniform appendage. Its small size in Eusarcus and Stylonurus which contrasts with that of the following legs, is also evidence that it could have aided little, if at all, in walking or swimming.

The fifth pair of postoral appendages have been termed the ectognaths, ectognathites or swimming legs, because in most genera (Eurypterus, Dolichopterus, Eusarcus, Hughmilleria, Slimonia, Pterygotus) the terminal segments are flattened into a paddle-shaped organ that is currently considered as having functioned in swimming. Hall figured the swimming legs of the crab  [1861, pl. 84A, fig. 6, 7] to point out the remarkable analogy in its structure with that of the last leg of Eurypterus; and Holm has carefully worked out the characters which so excellently adapted this organ for a swimming function [1899, p. 27]. The most important of these are the sharp, knifelike edge of the anterior margins of the fourth to sixth segments, the thin blade of the seventh and eighth segments and their articulation, by which they were enabled to form a continuous oar blade at the time of the backward stroke, while in the forward stroke, the eighth segment could be turned backward on the seventh like the blade of a shears to diminish the resistance of the water. The oar blade form of the extended seventh and eighth segments is well shown on plate 4, figure 3; the reflexing of the eighth segment is