This truly comparative text takes a fundamental, biophysical approach toward animal physiology. Students majoring in zoology, biology, or premedicine will study animals ranging from simple invertebrates and protozoans to complex multicellular invertebrates and vertebrates. Emphasis on evolution shows the progressive changes, modifications, and developments of physiological systems from simple to complex animals. Comparisons show the similarities and differences in how animals function, but stress fundamentally similar adaptations in very different animals.