Respiration by the Unmodified Skin and Food-Tract.—The general surface of the body has in all soft-skinned animals an aërating or respiratory action; and the simplest expression of organs is found in those animals where the body-surface is the only breathing-organ. Fig. 4.—Tubicola. a, Serpula contortuplicata, showing the branchiæ and operculum; b, Spirorbis communis. But a simpler expression of the function is shown in an amœba, for example, where by the amœboid movements every part of the structureless body eventually becomes surface, and is brought into immediate contact with the water. The amœba has no permanent skin, and no organs of any kind; consequently it breathes without special organs, and the other nutritive functions are equally destitute of them.
Many animals of higher groups breathe mainly or entirely by the skin. Among articulates, the leech and earth-worm are examples. In these a network of minute blood-vessels is spread beneath the delicate skin, thus bringing the blood into proximity with the water or air. The lowest crustaceans, and some sea-snails, and sea-spiders also, respire by the skin alone.
Most amphibians use the skin largely in respiration. In cold water, frogs will breathe entirely by the skin, and can not be killed by forced immersion, so long as they are provided with food. Even at ordinary
temperature, it is easier to drown some fishes by preventing them from reaching air than it is to suffocate frogs under the same conditions; yet the former have only gills, and the latter lungs alone. The hell-bender, found in our Western rivers—the largest American salaman-