The transformations of these animals, as investigated by Mr. Darwin,
are of great interest. The Cirripede, whatever its genus, and whatever
its peculiarities of adult existence, begins its life in a form exactly
like that of a young Entomostracous Crustacean, with a broad carapace,
a single eye, two pairs of antennæ, three pairs of jointed, branched,
and well-bristled legs, and a forked tail. It casts off its skin twice,
undergoing, especially at the second moult, a considerable change
of figure. At the third moult it has assumed almost the form of a
_Cypris_, or _Cythere_, being enclosed in a bivalve shell, in which the
front of the head with the antennæ is greatly developed, equalling in
bulk all the rest of the body. The single eye has become two, which are
very large, and attached to the outer arms of two bent processes like
the letters U U, which are seen within the thorax.
In this stage the little animal searches about for some spot suitable
for permanent residence; a ship’s bottom, a piece of floating timber,
the back of a whale or turtle, or the solid rock. When its selection
is made, the two antennæ, which project from the shell, pour out a
glutinous gum or cement, which hardens in water, and firmly attaches
them. Henceforth the animal is a fixture, glued by the front of its
head to its support. Another moult now takes place; the bivalve shell
is thrown off, with the great eyes, and their U-like processes, and
the little Cirripede is seen in its true form. It is now in effect
a Stomapod Crustacean, attached by its antennæ, the head greatly
lengthened (in _Lepas_, etc.), the carapace composed of several pieces
(valves), the legs modified into cirri, and made to execute their
grasping movements backwards instead of forwards, and the whole abdomen
obliterated, or reduced to an inconspicuous rudiment.