Page:The Eurypterida of New York Volume 1.pdf/126

 The lack of differentiation of the preabdominal and postabdominal segments is likewise shown in the embryo and larva of Limulus, while in that of the scorpion the two regions are distinguishable almost as soon as segmentation sets in, obviously owing to the very pronounced differentiation between the broad preabdomen and the narrow taillike postabdomen in the adult. In the Cambric Strabops there is less differentiation observable between the two abdominal regions than in any later eurypterid and the more primitive of these later forms distinguish themselves by a more uniform and gradual contraction of the abdomen. It is a wholly proper assumption that the undifferentiated condition is the more primitive. The lack of differentiation of the abdomen in the larvae is hence to be interpreted as an inherited palingenetic feature.

The smaller number of segments in the nepionic stage is, for theoretical and observational reasons, to be considered as a purely larval feature incidental to the growth of the organism. The Cambric Strabops possesses the same number of segments as the Upper Siluric or Carbonic eurypterids and if any change in the number of segments in the development of the higher Arthropoda has taken place, it has generally been a reduction. The trilobites with the smaller number of segments in the immature stages and the greater number of segments in mature conditions of the earlier and more primitive species such as Paradoxides and Harpes, furnish an excellent analogy.

The smaller size of the telson is in accordance with the ontogeny of Limulus and is manifestly a palingenetic character indicating the primitive condition. This view is supported by the short, blunt telson of Strabops and the primitive later eurypterids, such as Hughmilleria.

In summing up the larval characters observed in these immature eurypterids we consider as coenogenetic or purely larval the relatively larger size of the carapace, of the compound eyes and of the swimming legs, and the smaller number of the abdominal segments; as palingenetic and phylogenetic, the approximation of the compound eyes to the margin, the prominence of the ocelli and their tumescences, the lack of