The microscopic anatomy of the eye of the Weddell seal was studied with various light and electronmicroscopic methods with aview to correlating morphological findings with the biology of this seal whichis adapted to the extremes of the Antarctic environment and to extreme diving excursions into the lightlessdepth of the sea. In the retina and area centralis was found but no fovea centralis. Thedensily packed photoreceptors consist exclusively of highly differenciated rods, which inprimates detect light at low intensity but have rather poor image discrimination.The ganglion cells are relatively scarce, suggesting a high degree of convergence of thelight-sensitive cells on the ganglion cells. The pigment epithelium is almost devoid ofpigment granules. The extensive tapetum lucidum is about 400-500 micrometer thickand is composed of about 30 layers of specialized cells. the cornea is 650 (center)to 800-900 (periphery) micrometer thick. Its structure and glucosaminoglycan histochemistrycorrespond to that of other mammals. The iridocorneal angle is unusually deep andpervaded by an elaborate trabecular meshwork, which together with a complex canalof Schlemm can be correlated with the ability to absorb large amounts of fluid. Theciliary muscle and its antagonist, the membrane of Bruch, are poorly developed., suggestingrelatively poor abilities of accomodation. The combination of a well-developed tapetumlucidum, and unpigmented pigment epithelium, well-developed rods, and a high number of rodconverging on only few ganglion cells is obviously an adaptation to an extreme lightsensitivity, enabling the animal to make use of the little light available in the deep sea.