So again the two main divisions of
cirripedes, the pedunculated and sessile, though differing 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 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 must be 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 locomotive organs, a simple single eye, and a probosciformed mouth, with
which they feed largely, for they increase much in 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 out 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, simple eye-spot.
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