Page:Popular Science Monthly Volume 20.djvu/767

Rh become only mud or even entirely dry. It then travels over scorched and dusty ground in quest of water, and has been kept alive without water for six days. Tropical fresh-water fishes are commonly "able to survive droughts, living in semi-fluid mud or lying in a torpid state



below the hard-baked crust at the bottom of a tank, from which every drop of water has disappeared."

The lung of vertebrates is an offset, "diverticulum," of the food-canal, and in some form is possessed by all classes of back-boned animals. In the fishes it is represented by the "swim-bladder," which is mechanical in function, serving to vary the specific gravity of the body. Yet in some species it has also a respiratory function. It is quite wanting in those fishes which, like the skate, grovel on the sea-bottom, and it is relatively large in the flying-fishes. In most adult



fishes the air-bladder is entirely closed, having no communication with any other organ; and the inclosed gas is obtained from the blood. This is largely nitrogen in fresh-water fishes, and oxygen in