. Animal life as affected by the natural conditions of existence. Animal ecology. DEaENEKATION OF ORGANS. 81 only by the anus ; but when the young larvEe of the Crustaceans have once found their way in—which is not difficult by reason of the strong and rhythmical indraught of water through the cloaca —they never seem to quit the situation of their own choice; at the same time they greatly irritate the organ, and as they grow they stop up the tubular vessel more and more till at last serious degeneration of the organ is induced (fig. 21, 6). The main trunk is greatly distended, while the latera
. Animal life as affected by the natural conditions of existence. Animal ecology. DEaENEKATION OF ORGANS. 81 only by the anus ; but when the young larvEe of the Crustaceans have once found their way in—which is not difficult by reason of the strong and rhythmical indraught of water through the cloaca —they never seem to quit the situation of their own choice; at the same time they greatly irritate the organ, and as they grow they stop up the tubular vessel more and more till at last serious degeneration of the organ is induced (fig. 21, 6). The main trunk is greatly distended, while the lateral branches, which usually form a highly ramified structure, dwindle alto- gether, and are visible only as thin filaments, sometimes feebly branched. The young larvsB now produced are excluded, and become wandering bodies, in obedience to the law which. Fia. 22.—Zoea stage of the larva of Pinnotheres HolothuMw. governs all ; this they do under the form of the larva, or Zoea (fig. 22), which is common to all crabs, and they have the well-developed eyes of the typical character. Even when they enter the animal, they still preserve these eyes; but as they grow they gradually iDecome blind or half-blind, the brow grows forward over the eyes, and finally covers them so completely that, in the oldest individuals, not the slightest trace of them, or of the pigment, is to be seen through the thick skin; while at the same time the eyes seem to undergo a more or less extensive retrogressive metamorphosis. ' The instances here adduced show very clearly that the absence of light sometimes occasions degeneration from disuse,. 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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