In this last and
complete state, cirripedes may be considered as either more highly or more
lowly organised than they were in the larval condition. But in some genera
the larvae become developed into hermaphrodites having the ordinary
structure, or into what I have called complemental males; and in the latter
the development has assuredly been retrograde; for the male is a mere sack,
which lives for a short time and is destitute of mouth, stomach, and every
other organ of importance, excepting those for reproduction.
We are so much accustomed to see a difference in structure between the
embryo and the adult, that we are tempted to look at this difference as in
some necessary manner contingent on growth. But there is no reason why,
for instance, the wing of a bat, or the fin of a porpoise, should not have
been sketched out with all their parts in proper proportion, as soon as any
part became visible. In some whole groups of animals and in certain
members of other groups this is the case, and the embryo does not at any
period differ widely from the adult: thus Owen has remarked in regard to
cuttle-fish, "there is no metamorphosis; the cephalopodic character is
manifested long before the parts of the embryo are completed." Land-shells
and fresh-water crustaceans are born having their proper forms, while the
marine members of the same two great classes pass through considerable and
often great changes during their development.
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