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  • Title: Cone photoreceptors lacking oil droplets in the retina of the echidna, Tachyglossus aculeatus (Monotremata).
    Author: Young HM, Pettigrew JD.
    Journal: Vis Neurosci; 1991 May; 6(5):409-20. PubMed ID: 2069895.
    Abstract:
    The echidna, Tachyglossus aculeatus, a monotreme mammal, is thought to possess an all-rod retina (O'Day, 1952). This study provides anatomical evidence for the presence of cone-like photoreceptors in the retina of the echidna. The cones, which constitute 10-15% of the photoreceptors, have all of the ultrastructural characteristics previously shown in the cones of placental mammals, and, like cones of other animals (Blanks & Johnson, 1984), they bind peanut agglutinin. Unlike the cones of another monotreme, the platypus, the cones of the echidna retina do not possess oil droplets. Twin cones, pairs of cones in which there is no obvious difference in the size, shape, or ultrastructural features of the members of a pair, are common. The density of cones varies from 9000 cells/mm2 in the superior periphery to 22,000 cells/mm2 in the central retina. Nearest-neighbor analysis suggests that the cone mosaic in the echidna retina results from the presence of single and twin cones in a relatively regular array.
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