The pallial eyes of Ctenoides floridanus (Bivalvia : Limoidea)

The structure of the pallial eye in the Limidae has never been elucidated p roperly, largely because they are difficult to see among the mass of surrou nding mantle tentacles and because they are few, small, and lose their pigm entation when preserved. Possibly two eye types are present, simple cup-sha ped receptors in species of Lima, like those seen in the Arcoida, and more complex invaginated ones in Ctenoides. The pallial eyes (similar to 18 on b oth lobes) of Ctenoides floridanus are formed by invagination of the middle mantle fold at the periostracal groove, so that all its contained structur es are derived from the outer and light is perceived through the inner epit helia of this fold. The eye comprises a simple multicellular lens and a pho toreceptive epithelium beneath it of lightly pigmented cells and alternatin g vacuolated, support cells. In some species of the Arcoidea, Limopsoidea and Pterioidea, pallial eyes o ccur on the outer mantle fold and thus beneath the periostracum land shell) . The pallial eyes of Ctenoides floridanus and other pterioideans, e.g. spe cies of the Pectinidae, occur on the middle fold and may thus have improved vision. In the Cardiodea, Tridacniidae and Laternulidae (Anomalodesmata) p allial eyes occur on the inner folds. There is thus a loose phylogenetic tr end, in which Ctenoides is a critical link, of increasing eye sophisticatio n correlated with the historical age of the clades possessing them.