Page:The Eurypterida of New York Volume 1.pdf/44

 that the eye of Eurypterus had advanced one step beyond that of Pterygotus leading to the separation of the lens cone into an independent crystalline cone, and that this crystalline cone had failed to become hardened. In other arthropods, as in such insects as form Grenacher's "aconous type" of the compound eye, the whole ommatidial cell may remain as a clear transparent body [Watase, ibid].

The bearing of the position of the compound and simple eyes on the habits of life of the eurypterids will be noted in another chapter.

The median eyes or ocelli, which are also frequently termed the larval or simple eyes, consist of two single, transparent spots of the integument in round pits with ringlike walls situated on a small mound or tubercle. In Eurypterus and Eusarcus the transparent spots are distinctly thickened into lenslike bodies. The ocellar tubercle is always situated on the median line of the carapace and it wanders forward and backward with the lateral eyes so that as a rule it is on a cross line connecting the posterior extremities of the lateral eyes. Exceptions are made by the genera Eusarcus, Pterygotus and Slimonia where the lateral eyes have become marginal at the front of the carapace while the ocelli remain on the middle of the carapace, thus retaining the advantage of their position at the apex of the shield. In Limulus, on the contrary, the lateral eyes have remained in the posterior half and the ocelli have wandered to the front.

As to the function of the ocelli and compound eyes, observations on other classes of arthropods which possess both groups have led to the conclusion that the two sets are complementary to each other, the compound eyes being adapted to distant sight while the ocelli are myopic, as indicated by their highly convex lenses. This specialization has been necessitated by the slight adaptability of the stiff compound lenses to different distances. The most primitive ocelli lack lenses entirely and are mere pigment spots sensitive only to intensity of light.

Appendages of cephalothorax. The cephalothorax or prosoma bears six pairs of limbs which are homologous with those of Limulus. It is this close homology and the detailed comparison which is possible between