. Cave vertebrates of America; a study in degenerative evolution. Cave animals; Heteropygii. EYES OF TYPHLOTRITON FROM MISSOURI. 37 In the larvae up to 90 mm. long the skin passes over the eye without forming a free orbital rim and the eye does not protrude beyond the general contour of the head. In the adult from 97 mm. on, the eye forms a beadlike projection. There are in the adult distinct lids. These are closed over the eye, covering it entirely, the slit being much too small for the eye. The lower lid is free from pigment, but Fig. g. (a) Diagrammatic Representation of Eye of Typfd
. Cave vertebrates of America; a study in degenerative evolution. Cave animals; Heteropygii. EYES OF TYPHLOTRITON FROM MISSOURI. 37 In the larvae up to 90 mm. long the skin passes over the eye without forming a free orbital rim and the eye does not protrude beyond the general contour of the head. In the adult from 97 mm. on, the eye forms a beadlike projection. There are in the adult distinct lids. These are closed over the eye, covering it entirely, the slit being much too small for the eye. The lower lid is free from pigment, but Fig. g. (a) Diagrammatic Representation of Eye of Typfdolriton drawn to scale. (6) Vertical Section through Cornea and Lids of Adult. the upper lid, which closes over the lower, is as thickly pigmented as any other part of the body. Stejneger says of the eyes that they are "small, only slightly raised, and covered by the continuous skin of the head, with only a shallow groove to indicate the open- ing between the lids, the underlying eyes visible as two ill-defined dusky ; In sections the lids are seen to overlap one another some distance, forming an obscure, free orbital rim. Figure 9 b is a median section of the lids and corneal epithelium of an eye mm. in diameter, taken from an adult specimen 106 mm. in length. In this section the upper lid overlaps the lower lid mm., or more than one-fifth the diameter of the eye. Passing from the median section toward the corners of the eye, the lower lid unites with the underlying tissue first. When observed from the top, the upper lid covers the eye entirely. The orbital slit is o. 17 mm. in length. The conjunctival pocket extends some distance forward and back- ward beyond the slit. The eye increases in size but little from the larval to the adult stage and its growth is not proportional to the growth in length of the ani- mal. (See comparative measurements of the eyes at the close of the chapter.) The following is a series of measurements (in millimeters) on th
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