The Descent of Man

acterised by one extraordinary sexual character,
namely that the male element collects within
one of the arms or tentacles, which is then
cast off, and clinging by its sucking-discs to the
female, lives for a time an independent life. So
completely does the cast-off arm resemble a
separate animal, that it was described by Cuvier
as a parasitic worm under the name of
Hectocotyle. But this marvellous structure may
be classed as a primary rather than as a secondary
sexual character.
Although with the Mollusca sexual selection
does not seem to have come into play; yet many
univalve and bivalve shells, such as volutes,
cones, scallops, etc., are beautifully coloured
and shaped. The colours do not appear in most
cases to be of any use as a protection; they are
probably the direct result, as in the lowest classes,
of the nature of the tissues; the patterns and
the sculpture of the shell depending on its manner
of growth. The amount of light seems to be