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. 



oc.Zenl 



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. 

 J 7 2 > x 73) 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, i.e., 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 



