Page:Popular Science Monthly Volume 42.djvu/206

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THE POPULAR SCIENCE MONTHLY.

ing open, unprotected apertures (Helix clausa).[1] The former were found to be alive after several days, but the unprotected snails were eaten from their shells during the first night.

From tropical America we will now follow a great circle half round the globe, pausing in India or Ceylon. Here, too, the snails are exposed to the conditions of free competition for life in a tropical climate. What structures have been evolved in this totally dissimilar snail-fauna, corresponding to the evolution-products of American life under similar climatic conditions? As we would expect on a priori grounds, the protective structures, while strictly analogous, are in no way homologous, having arisen wholly independently in the two hemispheres. The Asiatic snails, instead of developing projecting teeth upon the edge of the aperture, have a system of calcareous blades or folds situated a distance within the shell, behind which the animal retreats when needful. The figures tell, better than any description, the extreme degree of complication which has been attained by the more highly organized forms. Beetles have occasionally been found sticking in the interstices of the folds, unable either to force their way into the interior or to extricate themselves and retreat.

The culminating point in the series of obstructive structures is perhaps reached by the narrow-throated snail of China (Stegodera angusticollis). In this bizarre form, the last of the spiralPSM V42 D206 Chinese narrow throated snail.jpgFig. 6.—Chinese Narrow-throated Snail. The narrow throat is shown by the dotted line. whorls is distorted and crowded against the preceding volution, producing an extremely narrow passage into the more spacious interior, as shown in the figure.

But, in spite of these various expedients for the protection of the snail, they have some enemies able to overcome or to evade all obstacles. It is sad to learn that in this case, too, civil wars are the bloodiest; the most deadly of the "malacophagi" are brother snails of the genera Selenites and Glandina. When the hungry Selenites discovers a temptingly juicy snail, a Helicina perhaps, the victim retreats into his shell, barring the entrance with his strong door or operculum. The Selenites thereupon sets to work cutting a hole through the large whorl of the Helicina, in order to gain entrance behind the barricade. The tongue-like odontophore with


  1. The Helix uvulifera is a Southern, the other a Northern species.