Right- and Left-Händed Spirals in Gastropods

Abstract

A CORRESPONDENT has raised the question as to whether there is any explanation for the fact that spiral shells of present-day marine gastropods wind in the opposite direction to similar shells found in the Crag formation of East Anglia. The first point is that only certain species of fossil shells wind in this manner, and although it is usual for present-day gastropods to be dextral, that is to say, they wind in the ordinary clock-wise spiral, there are certain genera which always wind in the opposite direction and are sinistral also in species which are usually dextral, there are frequently sinistral varies ties. The well-known fossil shell Fusus contrarius from the Red Crag is so common that it has given rise to the idea that all fossil shells of this kind are sinistral, but there are a large number of fossil gastropods which are normally dextral and, to take but one example, the common ‘Buckie’, Buccinum undatum, closely related to Fusus, sometimes is sinistral although normally dextral. Other common recent gastropods which are known to have sinistral varieties are the edible snail, Helix pomatia, and the common fresh-water snail, LimnÅ"a peregra. A good deal of work has been done, and is still going on, on the breeding of the last-named mollusc in order to find out the reason for these sinistral varieties (Diver, Boycott and others), the conclusion being that the general organisation of the germ cell determines whether the animal is to be dextral or sinistral, the factors for right-handedness or left-handedness being in the chromosomes and inherited in special ways. A sinistral individual may have an entirely sinistral brood or it may have an entirely dextral brood or a mixture, the broods from these varying to a large degree. Besides truly sinistral shells which also have a sinistral animal, there are species with a sinistral shell and dextral animal. It is practically certain, however, that the fossil Fusus must have been wholly sinistral both in shell and animal.