XI] OF VARIOUS UNIVALVES 821 



that the ^ew movement along the axis has changed its direction. 

 For if I take a roll of tape and push the core out to one side or to 

 the other, or if I keep the centre of the roll fixed and push the rim 

 to the one side or to the other, I thereby convert the flat roll into a 

 hollow cone, or (in other words) a plane into a gauche spiral. Whether 

 we push one way or other, whether the spiral coil be plane or gauche, 

 positively or negatively deformed, it remains right-handed or left- 

 handed as the case may be ; but it does change its direction as soon 

 as we turn it upside down, or as soon as the animal does so in assuming 

 its natural attitude. The linear spirals within and without the cone 

 may change places but must remain congruent with one another; 

 for they are merely the two edges of the ribbon, and as such are 

 inseparable and identical twins. But of the shell itself we may 

 reasonably say that a^right-handed has given place to a left-handed 

 spiral. Of these, the one is a mirror-image of the other; and the 

 passing from one to the other through the plane of symmetry 

 (which has no "handedness*') is an operation which Listing called 

 perversion. The flat or discoid apple-snails are Uke our roll of tape, 

 which can be converted into a conical spire and perverted in one 

 direction or the other; and in this genus, by a rare exception, it 

 seems wellnigh as easy to depart one way as the other from the 

 plane of sjmametry. But why, in the general run of shells, all the 

 world over, in the past and in the present, one direction of twist is 

 so overwhelmingly commoner than the other, no man knows. 



The phenomenon of reversal, or "sinistrality," has an interest of 

 its own from j:he side of development and heredity. For careful 

 study of certain pond-snails has shewn that dextral and sinistral 

 varieties appear, not one by one, but by whole broods of the one 

 sort or^the other; a discovery which goes some way to account 

 for the predominant left-handedness of Fusus amhiguus in the 

 Red Crag. The right-handed, or ordinary form, is found to be 

 "dominant" to the other; but the Mendelian heredity is of a curious 

 and complicated kind. For the direction of the twist appears to 

 be predetermined in the germ even prior to its fertilisation; and 

 a left-handed pond-snail will produce a brood of left-handed young 

 even when fertilised by a normal, or right-handed, individual*. 



* See A, E. Boycott and others, Abnormal forms of Limnaea peregra . . .and their 

 inheritance, Phil. Trans. (B), ccxxix, p. 51, 1930; and other papers. 



