18 THE LIFE OF THE MOLLUSCA 



round and round, each coil forming a " whorl," the 

 last being the " body whorl " (Plate V., Figs, i to 10). 

 This coil may be a flat one, but most Gastropods 

 have the visceral dome and shell twisted dextrally — 

 that is to say, when the shell is placed with the 

 mouth uppermost, and the apex directed away from 

 the observer, the mouth lies to the right hand of the 

 axis of the shell. Some are wound in the opposite 

 direction and are sinistral. Reversed varieties of 

 animals and shells normally dextral, or normally 

 sinistral, are often met with, as well as species in 

 which the coiling of the animal and shell is indiffer- 

 ently dextral or sinistral in the individuals composing 

 it. There are, moreover, Gastropods whose shells 

 coiled in one direction enclose animals whose organ- 

 ization is that usually associated with shells having 

 the opposite twist. Thus among the Ampullaridae 

 there are animals with dextral organization occupying 

 sinistral shells (Lanistes), while in the Planorbidae 

 the common Flat-coiled Water Snail (Planorbis) is an 

 example of a normally sinistral animal in an appar- 

 ently dextral shell. This is brought about by what 

 is known as " hyperstrophy," and may be illustrated 

 by taking a perfectly flat, coiled shell and placing it 

 with the mouth downwards and directed away from 

 the observer (Plate IV., Fig. 3, A) ; then in dextrally 

 organized molluscs that assume a spiral growth, the 

 spire would normally be exserted towards the right 

 (Plate IV., Fig. 3, B, C), but may exceptionally be 



