. Elements of comparative zoology. Zoology. MOLLUSCS. 199 In all gasteropods a lingual ribbon (p. 195) is present, and this works against a plate or 'jaw' on the upper side of the mouth. The alimentary canal is rarely straight. Usually there are convolutions, and the whole is so bent upon itself that the vent is carried far forward, and may be placed upon the 'neck' just above the mouth. Some- times it, or the liver connected with it, becomes greatly branched. Subclass I.—Streptonettra. In these the nervous system is twisted; there is but a single pair of tentacles upon the head; and the gills
. Elements of comparative zoology. Zoology. MOLLUSCS. 199 In all gasteropods a lingual ribbon (p. 195) is present, and this works against a plate or 'jaw' on the upper side of the mouth. The alimentary canal is rarely straight. Usually there are convolutions, and the whole is so bent upon itself that the vent is carried far forward, and may be placed upon the 'neck' just above the mouth. Some- times it, or the liver connected with it, becomes greatly branched. Subclass I.—Streptonettra. In these the nervous system is twisted; there is but a single pair of tentacles upon the head; and the gills are placed in front of the heart, a condition which leads many naturalists to call the group 'Prosobranchs.' Order I.—Diotocardia. In these forms the body retains its bilateral symmetry to a consider- able degree, and externally may ap- pear perfectly symmetrical. The name implies the existence of two auricles to the heart. In the limpets (fig. 37) the shell is a flattened cone; in the abalones it is somewhat ear-shaped and very weakly spiral, but in the top shells it is strongly spiral. The abalones alone have any economic value. Their shells, remarkable for having a series of holes in them, are composed of a greenish mother- of-pearl, which is extensively used in inlaid Fig. 37.—Limpet (Acmcea testudinalis). From Bin- ney's 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 Kingsley, J. S. (John Sterling), 1854-1929. New York, H. Holt and Company
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