ABSTRACT

The ocular sinuses of seven genera of ostracodes (Trachyleberididae) were studied through the use of internal molds and thin and polished sections. This is the first published account of the three-dimensional aspects of the ocular sinus in the following genera: Actinocythereis, Henryhowella, Malzella, Orionina, Puriana, and Radimella. Echinocythereis, previously studied, was included for comparison.

Morphology of the sinuses varies from the low mound-like form in some specimens of Henryhowella, to the long structure in Actinocythereis. With the exception of Henryhowella), all other genera have ocular sinuses with many features in common. These properties include a stalk-like appearance marked by a distinct constriction someplace along the length of the stalk. Most of the sinus is circular in cross-section except the distal portion which is expanded and irregular in shape, presumably to accommodate the portion of the eye with the lens cell, retinal cells, and rhabdoms. The surface of the distal portion is dominated by a concavity that is the complement of the convexity of the adjoining eyespot. Adjacent to the convexity is an anterior lobe or rim and usually a higher posterior rim. Channels with diameters of 2 to 6 micrometers probably are passageways for axons as mentioned by Andersson and Nilsson (1981).

Ontogenetic development includes an early low, cone-shaped sinus with a central concavity. Later juveniles in most taxa have a stalk as do adults.

Despite overall similarities of the ocular sinuses, detailed examination indicates that genera and species can each be distinguished using this feature. Variations in size, shape, proportions, and secondary topography are diagnostic; thus, another character is available for taxonomic purposes.