The fine structure of the retinal photoreceptors has been studied by light and electron microscopy in the short-tailed stingray (Dasyatis brevicaudata). The duplex retina of this elasmobranch contains rods and cones in a ratio of about 10:1. No multiple receptors were noted nor was a repeating or mosaic arrangement of the cones obvious. Only light-adapted specimens were studied but retinomotor movements of the photoreceptors were felt to be minimal or absent. The rods are large cylindrical cells with inner and outer segments of much the same diameter. Cones are short stout cells with a conical outer segment and a wider inner segment. Rod outer segment discs show peripheral incisures while cones do not. The inner segment of rods and cones are rich in organelles indicating much synthetic activity. The nuclei of rods and cones appear quite similar but cone nuclei are invariably at least partially protruded through the external limiting membrane which is formed by a series of zonulae adherentes between photoreceptor cells and M?ller cells. The synaptic region of both rods and cones display both invaginated (ribbon) synapses and superficial (conventional) synapses.
Adaptation, Physiological Animals