. Animal life as affected by the natural conditions of existence. Animal ecology. 208 THE INFLUENCE OF INANIMATE SURROUNDINGS. Sedentary animals, not properly swimmers, are either fixed to their dwelUng-pIace or only temporarily attached. The way in which these animals either yield to or overcome the effects of currents has already been indicated; the hardness and shape of the shells, the adhering power of the foot, or of the skeleton— as in corals—^protect them against the steady lateral pressure of currents, but if they lose their hold or are broken off, they are soon destroyed. Only certain
. Animal life as affected by the natural conditions of existence. Animal ecology. 208 THE INFLUENCE OF INANIMATE SURROUNDINGS. Sedentary animals, not properly swimmers, are either fixed to their dwelUng-pIace or only temporarily attached. The way in which these animals either yield to or overcome the effects of currents has already been indicated; the hardness and shape of the shells, the adhering power of the foot, or of the skeleton— as in corals—^protect them against the steady lateral pressure of currents, but if they lose their hold or are broken off, they are soon destroyed. Only certain creatures, as many sponges and some polyps (Hydroida), although they are fixed, escape by other means the destructive effects of strong currents; their extreme tenacity of texture, elasticity, and pliancy, in which. Pig. 60.—Creeping MoUusca. rt, Naiica, "which can extend its foot very widely; h, Erycina, (?), which creeps by means of its foot, like a univalve, on the skin of a Synapta. they resemble many water plants, qualify them to live even in strongly agitated currents. Creeping creatures, which attach themselves only tempo- rarily to stones and plants, and can quit them at pleasure, have special organs of attachment. Thus, in most univalves and some bivalves, the part known as the foot serves this purpose ; the broad under-surface clings closely to the object they adhere tp, but at the same time can leave go of its hold. The force, however, with which the foot adheres is not merely not absolute, but is even relatively different in individuals of the same species at various ages. In the course of my experiments on the pond-snail [Lymncea stagnalis) I found that those just come. 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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