. A manual of zoology. xi PHYLUM MOLLUSCA 285 ture from symmetry is very marked. The left side of the body has become very much more strongly developed than the right, and this side of the body is drawn out into a spirally twisted prominence — the visceral spiral—enclos- ing the liver and other organs. The anal aperture, instead of being median and posterior, is situated on the right side, and in front of it on the same side is the reproductive aperture. lent pulm. Fig. 171. — Helix nemoralis. an, anus : gen. ap, genital aperture ; oc. tent, pos- terior eye-bearing tentacles ; pulm, o
. A manual of zoology. xi PHYLUM MOLLUSCA 285 ture from symmetry is very marked. The left side of the body has become very much more strongly developed than the right, and this side of the body is drawn out into a spirally twisted prominence — the visceral spiral—enclos- ing the liver and other organs. The anal aperture, instead of being median and posterior, is situated on the right side, and in front of it on the same side is the reproductive aperture. lent pulm. Fig. 171. — Helix nemoralis. an, anus : gen. ap, genital aperture ; oc. tent, pos- terior eye-bearing tentacles ; pulm, opening of pulmonary sac ; tent, anterior tentacles. (After Pelseneer.) The shell is of simple conical form in the limpets. In most of the Gastropoda it is in the shape of a spiral (Figs. J72> x73) with the turns usually in close contact with one another, the inner walls of the turns coalescing to form an axial, hollow, or solid column — the columella. By far the greater number of such spiral shells are dextral, , if we begin at the apex of the spiral to reach the opening of the shell we have to pass from left to right with the columella always on our right-hand side; in a few cases, however, the spiral is sinistral, taking the opposite direction from that of the ordinary dextral shell. The form of the shell varies with the degree of obliquity with which the whorls are set. 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 Parker, T. Jeffery (Thomas Jeffery), 1850-1897; Haswell, William A. (William Aitcheson), 1854-1925. New York, The Macmillan Company; London, Macmillan & Co. ,Ltd.
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