II. The inheritance of sinistrality in Limnæa peregra ( Mollusca, Pulmonata )

Abstract

Limncea peregra is one of the commonest British fresh-water molluscs. Like other aquatic Pulmonates, it is hermaphrodite and breeds either by self- or cross-fertilisation. Normally the shell and body are coiled in a right-handed spiral (“dextral”) . Very rarely a reversed (“sinistral”) form occurs in which the whole symmetry is completely inverted : the shell and body are coiled in a left-handed spiral, the heart and kidney are reversed ; the rectum, penis and vagina open on the left-hand side of the neck instead of the right; the osphradium is on the left and the consequent asymmetry of the nervous ganglia is reversed. A sinistral snail is a complete mirror image of a dextral. The difference involves the whole development of each individual : it is apparent in the first division of the egg and obvious in the second. To be twisted either to the right or to the left seem to be the only available morphological possibilities. A shell which is not dextral is not necessarily sinistral, nor vice versa ; the shells might be coiled on the flat, something like a Planorbis . We have had a few of these monsters (fig. 8, Plate 10), of which the animals have been sinistral : if fertile they have normal dextral or sinistral young and appear to play no particular part in the genetic scheme. But a spiral mode of cleavage in the egg and a spiral twist in the soft parts of the adult seem to be essential qualities of gastropod mollusca, and an animal which is neither dextral nor sinistral is presumably impossible : the twist being obligatory there is no third alternative.