. Cold-blooded vertebrates: part I. Fishes. Fishes; Amphibians; Reptiles. FISHES such great modifications; and although conditions through the ages have varied sufficiently to render extinct many species of fish, the number thus extinguished is small compared to the extinct species ot land animals. This theory of stabil- ity of marine conditions and slow evolution ol fishes gains support from the fact that some of the living fish forms, such as the bullhead sharks and the gar pikes, existed in their present form many ages ago, as their fossil remains prove. But what is a fish? It is customary
. Cold-blooded vertebrates: part I. Fishes. Fishes; Amphibians; Reptiles. FISHES such great modifications; and although conditions through the ages have varied sufficiently to render extinct many species of fish, the number thus extinguished is small compared to the extinct species ot land animals. This theory of stabil- ity of marine conditions and slow evolution ol fishes gains support from the fact that some of the living fish forms, such as the bullhead sharks and the gar pikes, existed in their present form many ages ago, as their fossil remains prove. But what is a fish? It is customary to call by this name all animals that live in water, especi- ally if they have a com- mercial value. The mere fact that an animal lives in the water, however, is insufficient to make it a fish, just as not all ani- mals that fly are birds. For present purposes a true fish may be defined as a cold-blooded animal with a backbone, with arms and legs represented by fins or rudiments ot fins, and living in the water in which it breathes by means of gills. Whales and porpoises and their allies often are spoken of as fish. They are excluded, however, because they possess lungs and do not breathe by means of gills. Our definition also excludes the so-called shellfish—oysters. Fig. I. African lungfish, Protopterus, aestivating in its cocoon of mucus. Note funnel by which air passes to mouth. After Newton Parker. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Hildebrand, Samuel F. (Samuel Frederick), 1883-1949; Gilmore, Charles Whitney, 1874-; Cochran, Doris M. (Doris Mable), 1898-1968. [New York, Smithsonian institution series, inc. ]
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