organs of sense, and to reach by their active powers
of swimming, a proper place on which to become attached and to undergo
their final metamorphosis. When this is completed they are fixed for
life: their legs are now converted into prehensile organs; they again
obtain a well-constructed mouth; but they have no antennae, and their
two eyes are now reconverted into a minute, single, simple eye-spot.
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. Spiders, again, barely undergo any metamorphosis. The
larvae of most insects pass through a worm-like stage, whether they are
active and adapted to diversified habits, or are inactive from being
placed in the midst of proper nutriment, or from being fed by their
parents; but in some few cases, as in that of Aphis, if we look to
the admirable drawings of the development of this insect, by Professor
Huxley, we see hardly any trace of the vermiform stage.

Sometimes it is only the earlier developmental stages which fail. Thus,
Fritz Muller has made the remarkable discovery that certain shrimp-like
crustacea