Article: Fine structure of the eye in two species of Phacops (Trilobita)

The anatomy of the schizochroal eyes of Phacops fecundus Barrande and P.latifrons (Bronn) was investigated by sectioning. Intralensar structures were best preserved in P. latifrons, where each lens has an outer crescentic region, perhaps bipartite, surrounding a smaller 'proximal nucleus'. In P. fecundus lens structure was less distinct, but a sublensar structure was seen projecting into the central lumen of the eye and forming a long slightly tapering cylinder continuous with the corneal-intrascleral membrane. This may be analogous with the crystalline cone in recent arthropods.In both species the sclera is thicker than the lenses, and each lens lies at the top of a cylindrical alveolus excavated in the sclera. The directions of these alveoli have been used in determining the probable original internal arrangement of the sublensar visual units. In horizontal section these are obliquely radial, in vertical section they are almost parallel and thus are not generally coaxial with the outwardly directed lens-axes. The functional significance of the latter phenomenon is obscure but may relate to day and night vision. The postulated internal arrangement necessitates shorter central visual units and longer outer ones. This would explain the observed size differentiation of the lenses on the visual surface.