crustacean, the _Isaura cycladoides_, which has a persistent
bivalve shell.

The delicate tunic lining the sack, (a mere duplicature of that thick
one, forming the outside of the capitulum, and generally transformed
into valves,) and the integuments of the whole body, are regularly
moulted. With these integuments, the membrane lining the oesophagus, the
rectum, and the deep olfactory pouches, and the horny apodemes of the
maxillae, are all cast together. I have seen a specimen of Lepas, in
which, from some morbid adhesion, the old membrane lining one of the
olfactory pouches had not been moulted, but remained projecting from the
orifice as a brown shrivelled scroll. The new spines on the cirri (and
on the maxillae) are formed within the old ones; but as they have to be a
little longer than the latter, and as they cannot enter these up to
their very points, their basal portions are not thus included, but are
formed, running obliquely across the segments of the cirri; and what is
curious, these same basal portions are turned inside out, like the
fingers of a glove when hastily drawn off. After the exuviation of the
old spines, the new spines have their inverted basal portions drawn out
from within the segments, and turned outside in, so as to assume their
proper positions.

All Cirripedia grow rapidly: the yawl of H. M. S. _Beagle_ was lowered
into the water, at the Galapagos Archipelago, on the 15th of September,
and, after an interval of exactly thirty-three days, was hauled in: I
found on her bottom, a specimen of _Conchoderma virgata_ with the
capitulum and peduncle, each half an inch in length, and the former
7/20ths in width: this is half the size of the largest specimen I have
seen of this species: several other individuals, not half the size of
the above, contained numerous ova in their lamellae, ready to burst
forth. Supposing the larva of the largest specimen became attached the
first day the boat was put into the water, we have the metamorphosis, an
increase of length from about .05, the size of the larva, to an whole
inch, and the laying of probably several sets of eggs, all effected in
thirty-three days. From this rapid growth, repeated exuviations must be
requisite. Mr. W. Thompson, of Belfast, kept twenty specimens of
_Balanus balanoides_, a form of much slower growth, alive, and on the
twelfth day he found the twenty-first integument, showing that all had
moulted once, and one individual twice within