A pair of cephalic eyes located on the axes of the anteriormost filaments of the inner, left and right, demibranchs of the ctenidia and comprising simple, sensory pigment cups, occurs in many representatives of the Arcoida. and Pterioida. Such eyes, linked to the cerebral ganglia, also occur in some larvae and may represent an adaptation that is retained into adult life in only a few older bivalve lineages. Ectopic pallial eyes also occur in many representatives of the Arcoida and Pterioida on specialised sub-folds of the outer mantle folds and are typically most numerous posteriorly. These, too, are generally of simple construction and in the Arcoida comprise either a sensory cap or cup of pigmented and sensory cells. The cells of the sensory cap are formed into an "ommatidium-like" eyespot. Sensory reception is, moreover, ciliary-based in the former and microvillous in the latter so that the ommatidial structure is analogous to, not homologous with, eyes of superficially similar design in other phyla, notably the Arthropoda. More complex invaginated eyes occur in Enigmonia (Anomiidae) and Ctenoides (Limidae) on the outer mantle surface and the middle folds, respectively. The most complex, inverse pallial eyes occur on the middle mantle folds in representatives of the Pectinoidea. They have a lens, double ciliary distal and microvillous proximal retina, argentea and pigment cup. Somewhat simpler pallial eyes occur in the Cardioidea, including the Tridacnidae, on the inner mantle folds, but there is still a lens, sensory retinal cells, argentea and either a pigmented tentacle epithelium (Cardiidae) or zooxanthellae serving as a reflecting surface (Tridacnidae). Eyes virtually as complex as those of the Pectinoidea occur in the Laternulidae (Anomalodesmata: Pandoroidea) but on the inner mantle folds. A ciliated, accessory sense organ accompanies more complex bivalve pallial eyes and is especially well-developed in Laternula. Pallial eyes are thus of seemingly stochastic occurrence in the Bivalvia and their progressive sophistication has not, hitherto, been thought to follow a general plan. This is not so, however, and increasing sophistication follows a loose phylogenetic timetable; that is, older lineages have simpler eyes whereas the eyes of younger lineages are more complex. Moreover, each eye is structurally unique to each family and its representatives. The function of such complex eyes remains problematical. Virtually all bivalves, even those with no eyes, have a shadow "off" reflex which provokes adduction, siphonal retraction or digging, or a combination of all three reactions. Those with the most complex eyes do little more when stimulated by a shadow, Laternula truncata (Laternulidae), for example, simply flicks sand grains over the siphonal apertures to camouflage them. Members of the Pectinoidea with an "off" and an "on" response to light can detect adjacent movement and could, theoretically, respond to this by swimming. They rarely do, however, and adduction is the usual response and usually only after either mechanical or chemical stimulation. The pectinid eye can also form a simple image but this is probably not resolvable in the optic lobes of the visceral ganglia which receive such stimuli and initiate a response to them. Ectopic eyes, in particular, may have developed from the expression of the eyeless (ey) homeobox gene, as in other metazoans. But, more importantly, why have such complex eyes evolved when simpler ones in other bivalve phylogenies seem able to achieve the same defensive response, and an even wider array of equally successful lineages, notably the numerically dominant Heterodonta, have no eyes? The concave mirror, or argentea/tapetum, of the pectinid eyes with a retina positioned at the focal point allows movement but not objects to be detected on the distal retina. This structure makes such an eye more efficient at low light intensities, thereby improving the chances of predator avoidance over, effectively, a longer length of the day and/or at greater depths. Such variations in light intensity are detected on the proximal retina. In the Bivalvia, therefore, miniscule improvements in visual acuity, that is, spatial resolution but not spatial vision. have conferred a selective advantage over evolutionary time. Natural selection operates upon each eye structure to improve it at independent paces in the lineages that possess them so that there has not been, until now. a distinguishable picture of phylogenetic improvement with time as there is with the cephalic eyes of the Gastropoda. Rather, there is individual sophistication as and when the ey gene is expressed stochastically. Increasing sophistication of a few eyes (as in the Pectinoidea and Laternulidae) may also be energetically less expensive than thousands of them (as in the Arcoida), conferring yet another selective advantage.
