Structure of shell 34
" of the individual compartments 43
" of the radii 45
" of the alæ 47
" of the sheath 48
" of the basis 49
" of the opercular valves (scuta and terga) 51
Growth of whole shell and microscopical structure 54
Muscles of sack 61
Branchiæ 63
Thorax and body 65
Muscular system 68
Movements and muscles of the cirri 71
Mouth 74
Cirri 81
Caudal appendages 85
Alimentary canal 85
Circulatory system 87
Nervous system 88
Eyes and vision 93
Acoustic organs 95
Olfactory sacks 97
Male organs of generation 97
Female organs of generation 100
Metamorphoses and homologies 102
Larva, first stage 103
Larva, second stage 109
Larva, last or pupal stage 110
Act of metamorphosis 126
On the homologies of the carapace 131
Cementing apparatus 133
Affinities, classification, variation 152
Rate of growth, exuviation, &c. 156
Geographical range and habitats 159
Geological history 172
Almost every one who has walked over a rocky shore knows that a
barnacle or acorn-shell is an irregular cone, formed generally of six
compartments, with an orifice at the top, closed by a neatly-fitted,
moveable lid, or operculum.[19] Within this shell the animal's body is
lodged; and through a slit in the lid, it has the power of protruding
six pairs of articulated cirri or legs, and of securing by their
means any prey brought by the waters within their reach. The basis is
firmly cemented to the surface of attachment. The whole shell, basis,
and operculum consists, as we have already seen, of the first three
segments of the head, modified into a singularly constructed carapace,
which encloses the mouth and rest of the body. The anterior extremity
of the shell is situated in the centre of the basis, where indeed,
by due care, the antennæ of the pupa may be always detected; and the
posterior extremity is directed vertically upwards.
[19] The best published description of the structure of the
shell of a sessile Cirripede, is given by Dr. Coldstream, in the
'Encyclopædia of Anatomy and Physiology,' article 'Cirrhopoda.'
_Structure of Shell._
When the shell of a sessile Cirripede or barnacle, for instance,
of a Balanus, is first examined, the structure appears extremely
complicated; but this can hardly be considered as really the case. The
structure will, I think, be best understood by recalling to mind that
of Pollicipes,--the oldest known genus, from which, in one sense, all
ordinary Cirripedes, both sessile and pedunculated, seem to radiate.
I must premise, and the fact in itself deserves early notice, that
the homologies of the several parts in the pedunculated and sessile
Cirripedes admits of no doubt,--that is, if amongst the pedunculated,
the genus Pollicipes, or certain species of Scalpellum, be taken as a
standard of comparison.[20] The peduncle corresponds with the basis,
as may be clearly seen, if a Pollicipes with a short peduncle, and a
Balanus, with a deep cup-formed or cylindrical basis be compared, for
the contained parts are similar, and both grow at their upper edges
upwards and outwards. Secondly, the valves round the lower part of
the capitulum of a Pollicipes, though generally much more numerous,
and forming more than one whorl or circle, and not so closely packed
together, answer to the compartments forming the shell of a sessile
Cirripede; this is shown by their lateral and downward growth, by their
upper ends generally projecting freely above the cavity in which the
animal's body is lodged; and in the case _Pollicipes mitella_, by an
actual resemblance in outline, some being triangular, some broad at
the upper end, and some sub-rhomboidal, and, lastly, in the manner in
which they slightly overlap and indent each other: moreover further
resemblances in the relative position and even in the size of the
several valves, will hereafter be pointed out between certain sessile
genera amongst the Chthamalinæ and certain genera of the Lepadidæ.
Thirdly, the scuta and terga in Pollicipes, so strikingly resemble
in manner of growth in position relatively to the animal's body--in
shape--and even in being articulated together, the valves which form
the operculum or lid of sessile Cirripedes, that their identity is at
once obvious.
[20] Dr. J. E. Gray long ago observed these homologies. If Lepas be
taken, the comparison is not quite so simple, owing to the growth
of all the valves in that genus being upwards; but in several
species of Scalpellum we may see the intermediate steps between
the normal downward growth of the valves in Pollicipes, and the
abnormal upward growth in Lepas.
It may be well here further to premise, that apparently none of the
sutures in the shells of Cirripedes correspond with the articulations
between the three archetype cephalic segments, of which the whole shell
is formed; or with the eight elemental pieces, of which each separate
segment in the archetype crustacean is known to consist. But, as I
believe, the several valves in the shell of a Cirripede are homologous,
or at least analogous, with the sclerodermic plates,[21] of which
the carapace of the Podophthalmia is formed; with this difference,
that in the latter they become, after their first formation, united
together into a single piece, and are thus moulted as a whole; whereas
in Cirripedes, the valves or sclerodermic plates are not moulted, but
continue to be added to throughout life.
[21] Milne Edwards, 'Annales des Sciences Naturelles,' tom. xviii,
(1852), p. 236.
In Pollicipes, there is no difficulty in understanding the growth of
the lower valves of the capitulum, especially if a species be taken
in which these valves stand a little way apart: at each period of
growth, they are added to at their basal edges and a little way up
both sides; at the same time, a new membrane connecting them together
is formed, the old membrane disintegrating, or being left hanging in
tatters to the last zone of growth. Now if we look at the shell of
a sessile Cirripede, there is no essential difference in the growth
of the compartments or valves; all grow downwards and laterally; but
they overlap each other much more laterally than in Pollicipes, and
the connecting membrane is in most parts reduced to a mere film jammed
in between the valves; but, in the case of the opercular membrane, it
still remains wide, and is periodically moulted.
In the annexed woodcut (fig. 1), of the rostrum of _Balanus Hameri_,
the downward growth and the lateral growth on both sides is plain.
The modified sides (_rr_) for convenience sake, have been called the
_radii_; they invariably overlap the adjoining compartments. The middle
part (_p_), has been called the wall, or parietal portion: in the
specimen figured, the walls and radii are distinctly separated, but in
some cases, especially amongst the Chthamalinæ, the lines of growth are
absolutely continuous from one to the other. In fig. 2 of a Lateral
compartment of the same Balanus, we have the same essential structure;
but the left side (_a_) is more protuberant, and is hollowed out in its
lower half; it is, also, more distinctly separated from the parietal
portion: this side has for convenience been called the _ala_; it is
invariably overlapped by the adjoining compartment: in some few cases,
as in Pachylasma, the ala is not hollowed out in its lower part, and
from being added to in a straight line along its whole edge, with the
lines of growth continuous with those on the wall, it differs hardly at
all in appearance from a radius. Lastly, in fig. 3 of the carina, or
compartment facing the rostrum, we have alæ (_aa_) on both sides; these
being, as in all cases, overlapped by the adjoining compartments.
[Illustration: Fig. 1.
Fig. 2.
Fig. 3.
_p_, _p_, Parietes; _r_, _r_, Radii; _a_, _a_, Alæ.
Fig. 1, Rostrum with two radii, serving in the Chthamalinæ for
rostro-lateral compartments.
Fig. 2, always serving for lateral and carino-lateral compartments.