have given in my Journal of Researches, 1845, p. 7.) Certain
Cephalopoda, however, are characterised 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 influential to a
certain extent; for although, as repeatedly stated by Mr. Gwyn
Jeffreys, the shells of some species living at a profound depth are
brightly coloured, yet we generally see the lower surfaces, as well as
the parts covered by the mantle, less highly-coloured than the upper
and exposed surfaces. (4. I have given ( Geological Observations on
Volcanic Islands, 1844, p. 53) a curious instance of the influence of
light on the colours of a frondescent incrustation, deposited by the
surf on the coast-rocks of Ascension and formed by the solution of
triturated sea-shells.) In some cases, as with shells living amongst
corals or brightly-tinted seaweeds, the bright colours may serve as a
protection. (5. Dr. Morse has lately discussed this subject in his
paper on the Adaptive Coloration of Mollusca, Proc. Boston Soc. of
Nat. Hist. vol. xiv. April 1871.) But that many of the nudibranch
Mollusca, or sea-slugs, are as beautifully coloured as any shells, may
be seen in Messrs. Alder and Hancock s magnificent work; and from
information kindly given me by Mr. Hancock, it seems extremely doubtful
whether these colours usually serve as a protection. With some species
this may be the case, as with one kind which lives on the green leaves
of algae, and is itself bright-green. But many brightly-coloured,
white, or otherwise conspicuous species, do not seek concealment;
whilst again some equally conspicuous species, as well as other
dull-coloured kinds live under stones and in dark recesses. So that
with these nudibranch molluscs, colour apparently does not stand in any
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