. The biology of the amphibia. Amphibians. THE SENSE ORGANS AND THEIR FUNCTIONS 327 The eyes of modern Amphibia vary enormously in size. Arboreal and terrestrial forms tend to have larger ones than fossorial or aquatic types. The eyes are directed laterally in most forms, but in Centrolenella, Zachaenus, and a few other Salientia they are directed partly forward and possibly effect a binocular vision. Verrier (1927) estimated that even in the European tree frog 40 per cent of the field of vision was binocular. The eyeball is covered distally by a transparent cellular mem- brane, the cornea, th
. The biology of the amphibia. Amphibians. THE SENSE ORGANS AND THEIR FUNCTIONS 327 The eyes of modern Amphibia vary enormously in size. Arboreal and terrestrial forms tend to have larger ones than fossorial or aquatic types. The eyes are directed laterally in most forms, but in Centrolenella, Zachaenus, and a few other Salientia they are directed partly forward and possibly effect a binocular vision. Verrier (1927) estimated that even in the European tree frog 40 per cent of the field of vision was binocular. The eyeball is covered distally by a transparent cellular mem- brane, the cornea, the remainder, or concealed portions, being protected by a dense fibrous coat, the sclera. The latter is strengthened in the adults of some primitive urodeles, as well as in the larvae of most forms, by a ring or cup of cartilage (Stadt-. A B Fig. 118.—Sections representing three stages in the development of the eye of the frog, Rand esculenta. , anlage of lens; Br., brain; Ep., epidermis; L., lens; , optic cup. (After Giesbrecht.) muller, 1914; Okajima and Tsusaki, 1921). This cartilage, which is also found in frogs, was partly ossified at least, in some of the first tetrapods. It becomes enormously thick in Cryptobranchus, where Plate (1924) considers it a case of disharmonic growth conditioned by the degeneration of the eyes of this form. The cornea is arched in metamorphosed Amphibia and due to the inward migration of the lens (Fig. 118) is part of the refractive system. The cornea of larval Amphibia is frequently double, as in the case of certain bottom-living fish. The inner cornea arises from subcutaneous tissues and later fuses with the outer on metamorphosis (Giesbrecht, 1925). The eyelids first develop at metamorphosis. They are reduced in some aquatic Salientia and entirely absent in two genera of. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these ill
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