18 THE LIFE OF THE MOLLUSCA 
round and round, each coil forming a “ whorl,” the 
last being the “ body whorl” (Plate V., Figs. 1 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 Ampullaridez 
there are animals with dextral organization occupying 
sinistral shells (Lanistes), while in the Planorbide 
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 
