VESTIGIAL ORGANS
In many cave animals the eyes are vestigial. Thus in a number of cave fishes the eyeballs are small, spherical bodies hidden under the skin and are of no use whatever. The eyes of certain subterranean crayfishes, insects, and salamanders are similarly useless and hence vestigial.
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Snakes are commonly regarded as legless reptiles. But in the python (Fig. 2) a small claw can be see on each side of the vent, and these claws are supported by bones within the body in such a way that they are clearly vestiges of hind legs. Snakes are also peculiar in the structure of their lungs. Most air-inhabiting vertebrates have two lungs, one right and the other left. Many snakes have only the right lung, the left being represented only by a small protuberance. Vestigial organs are also well exemplified in birds. The ovaries and oviducts of most animals are evenly placed and equally developed on the two sides of the body, but in the birds these parts are functional only on the left side (Fig. 3). The ovary and the oviduct of the right side are abortive and quite useless, and hence vestigial.
The wings of certain birds are also vestigial. Wings used as organs of flight are among the most striking possessions of birds. Yet in the running birds, such as the ostrich and the cassowary, the wings are entirely useless for flight, for these birds are unable to rise off the ground. Relatively
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