Growth patterns in the eye of Petrolisthes elongatus (Crustacea; Decapoda; Anomura).

The eye of Petrolisthes elongatus is of the reflecting superposition type. The facets are square and the dioptric apparatus consists of a crystalline cone and a conestalk, each made up of four identical radially arranged units. The rhabdom has an elonagated (2-3 microns diameter) distal segment and a much thicker (6-16 microns diameter) proximal section. The distribution of screening pigments alters dramatically in a migration associated with changes in adaptational state. The following trends with increasing age, monitored as carapace length, were found: (1) increasing number of facets; (2) increasing size of eye and eye-stalk; (3) increasing length of eye-stalk in proportion to eye length; (4) increasing breadth of eye-stalk in proportion to eye breadth; (5) increasing breadth of eye-stalk and eye in proportion to length of eye-stalk and eye; (6) increase in volume of both retinal and dioptric structures; (7) slight increase in rhabdom width; (8) increase in crystalline cone length and breadth and (9) increase in focal length. The results of these trends were presumed to be, firstly, an increase in absolute sensitivity of single ommatidia; secondly, an increase in sensitivity to point sources of light and, thirdly, an increase in angular visual field of the whole eye.