. Cave vertebrates of America; a study in degenerative evolution. Cave animals; Heteropygii. C THE EYES OF ZYGONECTES NOTATUS. Of the eyes of a number of species of normal fishes, namely those of Cyma- togaster aggregates, Carassius auratus, Amciurus sp., Coregonus sp., and Zygonectes riotatus examined, I shall briefly describe the eyes of but one. Zygonectes notatus (Rafinesque) was selected for comparison, since it is a member of the Cyprinodontidae, a family closely related to the Amblyopsidae. I am not aware that this species has any advantage over other species of the family. It has large
. Cave vertebrates of America; a study in degenerative evolution. Cave animals; Heteropygii. C THE EYES OF ZYGONECTES NOTATUS. Of the eyes of a number of species of normal fishes, namely those of Cyma- togaster aggregates, Carassius auratus, Amciurus sp., Coregonus sp., and Zygonectes riotatus examined, I shall briefly describe the eyes of but one. Zygonectes notatus (Rafinesque) was selected for comparison, since it is a member of the Cyprinodontidae, a family closely related to the Amblyopsidae. I am not aware that this species has any advantage over other species of the family. It has large, well-developed eyes, that we may assume to be fully and normally devel- oped. The material examined was alcoholic. It had been preserved by simply placing in alcohol without any intention of future histological examination, but the structures were all well preserved for making out the horizontal relations of the single and twin cones. The protoplasmic and nervous processes of the cells were of course not brought out as with Golgi's method. A specimen 38 mm. long had the eye mm. in length, 2 mm. in vertical diameter, mm. from axis of optic nerve to front of iris, mm. from axis of optic nerve to front of cornea; lens mm. in diameter; pigment layer measures 56 /i.; outer nuclear layer, 36 /x; outer reticu- lar, 4/i.; tangential cell layer, 9 /a; inner nuclear, 40 /*; inner reticular, 52 /x; ganglionic layer, 12 /a; optic-fiber layer, 28 ft; total thickness of retina, 237 ti. The regularity of arrangement of single and twin cones is very striking. The basal part of the single cones con- tains refractive granules increasing in size outward where the series ends in a lenticular vacuolated body separating the granular from the distal part of the rod. The twin cones are all without granulation. This marked difference between the two enables one to distinguish between them at a glance in tangential sections. The twin cones are arranged in series in such a manner tha
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