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The ciliary muscle exhibits considerable variation in its development among the vertebrates. The muscle is considered to be well developed in humans, primates, ungulates, and aquatic carnivores, and the extent of development is thought to correspond to the amplitude of accommodation. The recent finding of a well-developed ciliary muscle in the hooded seal (Cystophora cristata) has initiated interest in an accommodative mechanism for vision in air and in water. This study is a comparative investigation of the ciliary muscle in a number of aquatic mammals. The methods include gross inspection of the ciliary region and histological examination of the ciliary body and iris by means of light microscopy. The results indicate that in completely aquatic mammals the ciliary muscle is rudimentary, and in fact, appears to be lacking in one species (beluga whale). The ciliary muscle appears to be better developed in the …