In view of this inferred structural identity in the lateral eyes of Pterygotus and Limulus, the stage of development which the latter has attained becomes a matter of interest to us here.
Packard [Am. Nat. 1880, 14:212] describes the eye of Limulus as follows:
The structure of the eye is very unlike that of any other arthropod eye. The cornea is simply a smooth convex portion of the integument, which is much thinner than the adjoining part of the chitinous skin. There are no facets, the cornea externally being structureless, simply laminated like the rest of the integument. In the internal side of the cornea are a series of solid chitinous conical bodies, separated from one another by a slight interspace and in form resembling so many minié-rifle balls; the conical ends of these solid cones project free into the interior of the body, and are enveloped in a dense layer of black pigment. Within the base of these cones are secondary shallow cuplike bodies or shallow secondary cones. It is these primary cones which, seen through the smooth convex translucent cornea, give the appearance of a faceted surface to the external eye.
All the parts thus far described except the pigment layer, are molted with the rest of the crust, and the large slender cones can be easily seen by viewing a piece of the cast-off eye; the solid cones being seen projecting from the inner surface of the cast-off cornea.
The author adds: "So far as we can ascertain, no arthropod eye is so simple as that of Limulus."
Watase [Biol. Studies, Johns Hopkins Univ. 1880, 4:287] in consequence of his investigation of the compound eyes of arthropods considers the ommatidium of the lateral eye of Limulus as making the nearest approach to the primitive condition. "It is nothing more and nothing less," he states, "than a depression in the skin, with the thickened chitinous cuticle fitting in the open cavity and acting as a lens to condense the light." We have copied two of Watase's excellent figures to illustrate this structure [text figs. 8, 9].
In view of the close relationship clearly uniting Pterygotus with the other eurypterids, notably Eurypterus and Eusarcus, we have little reason to doubt that the lateral eyes in all were of like structure. Nevertheless,
