. Animal life as affected by the natural conditions of existence. Animal ecology. KESPIEATION BY THE INTESTINE, 171 Less variety is found among the internal gills, whicli some- times are situated in the intestinal canal of water-animals. In the larvse of the Libellulidaj, for instance, leaf-shaped oi-gans are found inside the rectum, which apparently serve for respira- tion. I myself have described a system of foliated processes on the mucous membrane of the stomach of the Holothuridse (fig. 49) which have all the attributes of true gills—as an extensive surface, delicate membrane, and abundan
. Animal life as affected by the natural conditions of existence. Animal ecology. KESPIEATION BY THE INTESTINE, 171 Less variety is found among the internal gills, whicli some- times are situated in the intestinal canal of water-animals. In the larvse of the Libellulidaj, for instance, leaf-shaped oi-gans are found inside the rectum, which apparently serve for respira- tion. I myself have described a system of foliated processes on the mucous membrane of the stomach of the Holothuridse (fig. 49) which have all the attributes of true gills—as an extensive surface, delicate membrane, and abundant blood- vessels, with a constant renewal of the water that bathes the laminae. In most Annelida and many other Invertebrata, no. Fig, 49.—Part of the stomach of a Holothurian (Stichoptis variegaius) split open length- wise and laid flat, a, the dorsal furrow between the two series of gill-foliations; b, the broad tumid ventral surface which divides them ; c, the foliaceous gills. doubt a regular current of constantly renewed water passes through the intestine, which nevertheless bears no special gills; the more or less extensive folds of the mucous membrane here take the place of the absent organs. It may here be inci- dentally mentioned that even a fish (Gobitis fossilis, a species of Loach, fig. 50) breathes through the intestines ; but in this case the conditions are slightly different, inasmuch as it takes in air- bubbles at the surface of the water through its mouth, and swallows them, so that here the air comes into direct contact with the respiratory surface of the intestine.^" All these different organs of respiration ^' must act in. 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 Semper, C. (Carl), 1832-1893. New York, D. Appleton
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