parents. In most cases, however, the larvae,
though active, still obey, more or less closely, the law of common
embryonic resemblance. Cirripedes afford a good instance of this: even the
illustrious Cuvier did not perceive that a barnacle was, as it certainly
is, a crustacean; but a glance at the larva shows this to be the case in an
unmistakeable manner. So again the two main divisions of cirripedes, the
pedunculated and sessile, which differ widely in external appearance, have
larvae in all their stages barely distinguishable.

The embryo in the course of development generally rises in organisation: I
use this expression, though I am aware that it is hardly possible to define
clearly what is meant by the organisation being higher or lower. But no one
probably will dispute that the butterfly is higher than the caterpillar. In
some cases, however, the mature animal is generally considered as lower in
the scale than the larva, as with certain parasitic crustaceans. To refer
once again to cirripedes: the larvae in the first stage have three pairs of
legs, a very simple single eye, and a probosciformed mouth, with which they
feed largely, for they increase much in {441} size. In the second stage,
answering to the chrysalis stage of butterflies, they have six pairs of
beautifully constructed natatory legs, a pair of magnificent compound eyes,
and extremely complex antennae; but they have a closed and imperfect mouth,
and cannot feed: their function at this stage is, to search by their
well-developed 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, and very 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 either 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, or other organ of importance, excepting for reproduction.

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