ly half that size. In Ibla this orifice is
seated lower down (Pl. IV, fig. 8 _a'_, _e_), between the bases of the
first and second cirri, which are here far apart: in _Alepas cornuta_ it
is placed rather nearer to the adductor scutorum muscle, namely, beneath
the mandibles. The orifice leads into a rather deep and wide meatus; the
external integument is turned in for a short distance, widening a
little, and then ends abruptly. The meatus, enlarging upwards, is lined
by thick pulpy corium, and is closed at the upper end; from its summit
is suspended a flattened sack of singular and different shapes in the
different genera. This, the so-called acoustic sack of _Conchoderma
virgata_, is figured Pl. IX, fig. 6. The deep and wide notch faces
towards the posterior end of the animal; the inferior lobe, thus almost
cut off, is flattened in a different plane from the upper part; the lobe
is lodged in a little pouch of corresponding form, leading from the open
meatus in which the upper part is included. In _Conchoderma aurita_, the
top of the acoustic sack is narrower and more constricted, the whole
more rounded, and the lobe more turned down. In _Lepas fascicularis_ the
notch is not so deep or wide, and the lobe larger. In _Ibla Cumingii_
the sack is of the shape of a vase, with one corner folded over. In
_Scalpellum vulgare_ it is small, oval, with the lower end much pushed
in, and furnished with a little crest. Lastly, in _Pollicipes mitella_
it is simply oval. In all cases the sack is empty, or contains only a
little pulpy matter: it consists of brownish, thick, and remarkably
elastic tissue, formed, apparently, of transverse little pillars,
becoming fibrous on the outside, and with their inner ends appearing
like hyaline points. The mouth of the acoustic sack (removed in the
drawing) is closed by a tender diaphragm, through which I saw what I
believe was a moderately-sized nerve enter; I have not yet succeeded in
tracing this nerve. The first pair of cirri seem, to a certain extent,
to serve as antennae, and therefore the position of an acoustic organ at
their bases, is analogous to what takes place in crustacea; but there
are not here any otolites, or the siliceous particles and hairs, as
described by Dr. Farre, in that class. Nevertheless, the sack is so
highly elastic, and its suspension in a meatus freely open to the water,
seems so well adapted for an acoustic organ, that I have provisionally
thus called it. In the larva,