The Descent of Man

the Molluscoida of some authors), for most of
these animals are permanently affixed to a support
or have their sexes united in the same individual.
In the Lamellibranchiata, or bivalve
shells, hermaphroditism is not rare. In the next
higher class of the Gasteropoda, or univalve
shells, the sexes are either united or separate.
But in the latter case the males never possess
special organs for finding, securing, or charming
the females, or for fighting with other
males. As I am informed by Mr. Gwyn Jeffreys,
the sole external difference between the sexes
consists in the shell sometimes differing a little
in form; for instance, the shell of the male periwinkle
(Littorina littorea) is narrower and has a
more elongated spire than that of the female.
But differences of this nature, it may be presumed,
are directly connected with the act of reproduction,
or with the development of the
ova.