there may here exist a difference from that of Pterygotus, for in the other eurypterids no facets have been observed. It is obvious that both Eurypterus and Eusarcus possessed an exteriorly smooth cornea just as Pterygotus; as to the interior of their compound eye, however, several possibilities present themselves: either the lenses were separate from the cornea and thereby lost in fossilization, not being united by the sclera as in the holochroal eyes of the trilobites; or the facets were so feebly
 |
 | |
| Figure 8 Limulus. Two ommatidia shown side by side, partly schematic. The thick unshaded body is the chitinous covering of the eye. L. lens cone, fitting into the depression of the skin. Rt. retinula. G. ganglion cell. (From Watase) | Figure 9 Diagram of the compound eye of Limulus, the black, heavy line representing the ectoderm and each depression in this layer corresponding to an ommatidium. (From Watase) |
developed as to escape observation. There are no records as far as we are aware, either among living forms, or among the fossil merostomes, to support the former hypothesis. Numerous recent crustaceans, among them the venerable Apus, either lack the facets entirely or have them so poorly developed that they are hardly noticeable,[1] and this fact seems well suited to shed light on the failure to detect the facets in Eurypterus and Eusarcus.
In some arthropods the crystalline cone assumes a transparent semiliquid state [see Watase, 1890, p. 147], and it may perhaps be assumed
- ↑ See Parker, G. H., The Compound Eyes in Crustaceans. Mus. Comp. Zool. Bul 1891. v. 21.
