y chief examination has been directed, at this
stage of development, to the larvae of _Lepas australis_, which are of
unusual size, namely, from .065 to even almost .1 of an inch in length;
I examined, however, the larvae of several other species of Lepas, of
Ibla and of Balanus, with less care, but sufficiently to show that in
all essential points of organisation they were identical; this, indeed,
might have been inferred from the similarity of the larval prehensile
antennae, preserved in the bases of all mature Cirripedes, and which I
have carefully inspected in almost every genus. The larvae in this final
stage, in most of the genera, have increased many times in size since
their exclusion from the egg; for instance, in _Lepas australis_, from
.007 to .065, or even to .1 of an inch. They are now much compressed,
nearly of the shape of a cypris or mussel-shell, with the anterior end
the thickest, the sternal surface nearly or quite straight, and the
dorsal arched. Almost the whole of what is externally visible consists
of the carapace; for the thorax and limbs are hidden and enclosed by its
backward prolongation; and even at the anterior end of the animal, the
narrow sternal surface can be drawn up, so as to be likewise enclosed.
As in several Stomapod crustaceans, the part of the head bearing the
antennae and organs of sense, in front of the mouth, equals, or even
exceeds in length, and more than exceeds in bulk, the posterior part of
the body, consisting of the enclosed thorax and abdomen. I will now
briefly describe, in the following order, the carapace, the organs of
sense, mouth, thorax and limbs, abdomen, and internal viscera.

The form of the _Carapace_ has been sufficiently described; it consists
of thick chitine membrane, marked with lines, and sometimes with stars
and other patterns; it is obscurely divided into two halves by a line or
suture along part of the dorsal margin; these halves or two valves are
drawn together by an adductor muscle, in the same relative position as
in the mature Cirripede. The part overhanging and enclosing the thorax
is lined by an excessively delicate membrane, obviously homologous with
the lining of the sack in the mature animal, and is nothing but a
duplicature of the carapace, rendered very thin from being on the under
or protected side: a layer of true skin or corium, probably double,
separates these two folds.

_Acoustic Organs._--On the borders of the carapace, at the anter