[83] The name Balanus was used, almost as at present, by Lister and
Hill, before the introduction of the binomial system. Since that
period the first two authors, as far as I know, who used this name,
were Da Costa, in his 'Hist. Nat. Test. Brit.,' in 1778; and Bock,
in the 'Naturforscher,' for the same year; Bock, however, applied
it to a Chelonobia.
CONOPEA (pars generis). _Say._ Journal Nat. Sc. Philadelphia, vol.
ii, part ii, 1822.
MESSULA (do.). _Leach._ Zoological Journal, vol. ii, 1825.
CHIRONA (do.). _J. E. Gray._ Philosoph. Transacts., 1835, p. 37.
_Compartments six; basis calcareous or membranous; opercular valves
sub-triangular._
_Distribution._--Mundane: in the warmer seas.
_General appearance._--The shape of the shell in the different
species varies from depressed conical to cylindrical; the latter
form being generally assumed when specimens are crowded together; but
some species, as _B. balanoides_, _crenatus_, and _lævis_, seem more
subject than others to be thus affected. The colour is either white,
generally tinted by the yellowish or brownish epidermis, or any colour
intermediate between bright pink and rich blue, purple being the
prevailing tint. The persistence of the so-called epidermis is very
different in different species, being even sometimes highly variable in
the same species. The surface is either smooth or more commonly folded
longitudinally, or sharply ribbed. The orifice differs in form from
diamond-shape to trigonal; the carinal end, owing to the shape of the
carina, being always sharper or narrower than the rostral end. The size
of the orifice, in proportion to the shell, varies accordingly as the
latter is more or less conical or cylindrical. The orifice is either
entire or more or less deeply toothed, in proportion to the degree of
obliquity of the summits of the radii and alæ. The radii almost always
have smoother surfaces than the parietes. In some few species the
radii are not developed, the sutures being marked only by fissure-like
lines; in others they are very narrow, and in this case their upper
margins are generally rounded and smooth, instead of being straight
and jagged. The carino-lateral compartments are usually much narrower
than the lateral compartments, occasionally in an extreme degree, as in
_B. allium_. The shell is generally strong, sometimes to a wonderful
degree; but the strength and thickness vary in the individuals of some
of the species. By the action of hot caustic potash, the compartments
in several species, such as _B. Hameri_ and _crenatus_, separate on a
touch; in others, they adhere so strongly as to prove that the sutures
must be calcified together. In this genus we have the largest known
sessile cirripede, viz., the _B. psittacus_, and on the other hand many
small species; but it is very difficult, except in well-known species,
to ascertain the average or even the maximum dimensions.
_Scutum._--This valve is almost triangular, with the basi-tergal corner
more or less rounded off. The prominent lines of growth are sometimes
crossed by longitudinal striæ. Internally, the articular ridge projects
to a very different degree in the different species; its lower end is
sometimes (as in _B. lævis_, Pl. 4, fig. 2 _c_) produced downwards as a
small, sharp, free style; there is always an articular furrow receiving
the inflected margin of the tergum. There is always an impression left
by the attachment of the adductor scutorum muscle; and often its lower
side is bounded more or less closely by a sharp adductor ridge, running
some way down the valve; this ridge is occasionally almost confluent,
in its upper part, with the articular ridge, and in this case sometimes
it forms, together with the inflected tergal margin, a large tubular
cavity, running up, as in _B. psittacus_ (Pl. 2, fig. 3 _c_), almost
to the apex of the valve. Almost invariably there is a slight pit or
depression for the lateral depressor muscle; sometimes within the
depression there is a little ridge, as in _B. perforatus_ and _nubilus_
(Pl. 4, fig. 3 _a_, and Pl. 6, fig. 2 _a_); and in the case of _B.
vestitus_, _flosculus_, and _imperator_ (Pl. 8, figs. 3 _a_, 4 _a_),
there are regular crests for this same purpose. The rostral depressor
muscle is usually attached in a small pit formed by the folding over of
the lower part of the occludent margin: in _B. imperator_ (Pl. 8, fig.
4 _a_) there are regular crests for its attachment, and traces of them
may be discovered in _B. vestitus_.
_Tergum._--This valve is more nearly triangular than any other shape,
with the spur more or less prominent. The apex generally projects a
little above the level of the scutum; in some species it consists of
a triangular and solid, in others (Pl. 2, fig. 3 _b_) of an almost
cylindrical, extremely sharp, inwardly curved, and very prominent beak.
This beak is generally purple; it is sometimes hollow, and occupied by
a thread of corium. Its formation, and the apparent sliding up of the
whole tergum, so as to project above the scutum, has been described
under the family. From an account given to me by a person who kept
_B. porcatus_ alive, the beaks appear to be used, when the operculum
is touched, as an organ of defence,--the animal striking with them.
The tergal margin is more or less inflected; and the carinal margin
is convex in different degrees, and, in some species, is added to
by upturned zones of growth. The basal margin either forms a nearly
straight line on opposite sides of the spur, or more commonly slopes
towards it in various manners. The spur, or basal projection, is
rarely placed in the middle of the basal margin, generally near,
sometimes close to the basi-scutal angle; it varies much in length
and breadth, and is sometimes even half the width of the valve. The
surface of the valve is almost always more or less depressed, sometimes
so much as to form a deepish furrow, the "longitudinal furrow," which
extends from the apex to the extremity of the spur. When the furrow is
deep, its sides, as the specimen grows old, almost always become folded
inwards, so as to touch, and then the furrow becomes converted into a
closed fissure: in this latter case the folded sides generally form a
central crest on the spur. Internally, in the middle of the upper part
of the valve, the articular ridge is more or less prominent, forming
the carinal margin of the articular furrow, in which the articular
ridge of the scutum is lodged; occasionally, however, this articular
ridge can hardly be said to exist. In most species the tergal depressor
muscle is attached to sharp crests on the basi-carinal corner of the
valve, but these are almost obliterated in other species.
_Compartments._--The external appearance of the shell has already been
described. In the most typical species, the parietes consist of an
outer and inner lamina, separated by strong longitudinal septa; these
septa are denticulated on both sides at their bases, but only close
to the inner lamina; in fact the inner lamina is apparently formed by
the union, thickening, and production, of some of the denticuli. As
it is not the innermost of the denticuli on the basal edges of the
longitudinal septa, which thus become united into a solid layer, the
longitudinal septa form slightly projecting, longitudinal ribs on the
inner lamina. These internal ribs are longitudinally striated; in
old specimens they often become obliterated, especially in the upper
part of the shell. The parietal tubes or pores (occupied by threads
of corium) are generally square and large; but in _B. Ajax_ they
are very small, and in _B. glandula_ often extremely minute. In the
upper part of the shell, and sometimes low down, they are generally
crossed by thin, transverse, calcareous septa: in some species, as
in _B. perforatus_, and in some varieties of _B. amphitrite_, the
upper ends of the tubes are filled up solidly with shell. In some
varieties of _B. crenatus_ and of _amphitrite_, the longitudinal septa,
near the outer lamina, divide, thus giving rise to a very imperfect
row of outer short tubes. In _B. vinaceus_ (Pl. 2, fig. 7 _d_) the
inner lamina is cancellated instead of being solid, which is caused
by the basal denticuli of the longitudinal septa being simply united
together by their ends and crossed by transverse septa, instead of
being consolidated into a mass. In several species, as in _B. Hameri_,
the walls consist only of the outer lamina with longitudinal ribs, no
inner lamina having been formed; the ribs here evidently answer to the
longitudinal septa in the foregoing species. In _B. flosculus_ and
_imperator_ the walls are solid, their basal margins being formed of
irregular, elongated points, and little ridges (Pl. 8, fig. 4 _c_),
which apparently prefigure the more regular longitudinal ribs or septa.
In _B. balanoides_ the walls are generally either nearly smooth and
solid, or irregularly cancellated; in _B. cariosus_ (Pl. 7, fig. 3 _b_)
two or three rows of short irregular tubes are formed by unequally
branching septa, almost as in the genus _Tetraclita_.
The _Radii_, in all the species, are constructed essentially on the
same plan as the parietes; thus, in the typical forms, there is an
outer and inner lamina, with septa, which, near the inner lamina, are
furnished with denticuli on both sides; hence the radii are permeated
by pores or tubes, like the parietes; but this holds good only in the
first section of the genus, for, in the other species, the tubes are
filled up quite solidly. The denticuli on the septa often occur only on
one side, or disappear altogether; and, lastly, the septa themselves
often appear merely like little teeth, or disappear altogether as in
_B. Hameri_, or occur only near the bases of the radii, as in _B.
amaryllis_. A slight furrow in the compartment, against which each
radius abuts, is generally marked by the septa and their denticuli. In
regard to the _alæ_, their lateral or sutural edges are either thin
and smooth, or, more commonly, finely crenated or ribbed. The little
transverse crenations are homologous with the septa in the radii and
parietes. The edges of the alæ are usually received in a furrow. The
diametric growth of the shell is effected by the growth of the radii
and alæ, and chiefly by that of the former. The sutural and lateral
edges of both radii and alæ are added to, either quite up to their
summits, or only low down, and during the continued growth of the
shell, lower and lower down; in accordance with this difference in
growth, the summits of the radii and alæ become either very oblique,
or they extend parallel to the basis, that is, from tip to tip of the
adjoining compartments. When the radii and alæ are added to, as is most
usual, above the level of the opercular membrane, and therefore above
the sack, ribbons of corium run up the sutures from the sack, higher
or lower, according to the height to which, in the different species,
the edges of the radii and alæ continue to be added to. The obliquity
of the summits of the radii and alæ varies, in some cases, in the same
species. It often happens that when the summits of the radii are very
oblique, the summits of the alæ are but little so; and the converse;
both, however, are often either equally oblique, or both have square
summits. The _sheath_ extends either one third or more than half down
the shell; its basal margin often (Pl. 25, fig. 1, K′) freely depends
or overhangs the inner lamina of the walls.
_Basis._--In typical species the basis is calcareous, and consists of
an upper and lower lamina, separated by radiating septa, forming pores.
In the same manner as the septa of the parietes sometimes, though
rarely, become irregularly divided near the outer lamina, forming
outer pores, so it is, but in a much more marked degree, with the
basis. The basis in such cases becomes extremely thick, and consists
of an upper, thin lamina, with the regular radiating septa and pores,
and of an underlying, thick, cancellated mass, which seems wholly to
result from the dividing and sub-dividing of the septa. The basal
radiating pores, like the parietal pores, are closed at intervals by
calcareous transverse septa. The basal points of the parietal septa
enter the orifices of the basal pores, and the threads of corium pass
into the latter, between the denticuli of the parietal septa. In some
species, as in _B. crenatus_ and _Hameri_, the basis is perfectly
solid, the upper lamina being absent, just as in some species, the
internal lamina of the parietes is absent. In _B. flosculus_ the basis
is calcareous, but consists of so excessively thin a film as hardly
to be distinguished: it presents, moreover, as also is the case with
_B. imperator_, a beaded structure. Again, in some few species, as
in _B. balanoides_, the basis is simply membranous. When the basis
is thin, it is always flat, and is closely moulded to the irregular
surface of attachment; and in this case, when specimens are crowded
together, their elongation is effected exclusively by the growth of the
walls; but, when the basis is thick, it sometimes becomes, in crowded
groups, deeply, but irregularly, cup-formed, or cylindrical, as in _B.
psittacus_ and _perforatus_. In _B. allium_, however, which inhabits
massive corals, the basis is as regularly concave or cup-formed as
in the genus Pyrgoma: in _B. calceolus_ and its allies, and in some
varieties of the fossil _B. inclusus_, the basis is boat-formed, with
its lower surface deeply grooved longitudinally from clasping the
stem of the Gorgonia or other zoophyte, to which it was attached.
In certain varieties of _B. lævis_ it is very remarkable that the
deeply cup-formed basis becomes, owing apparently to the whole shell
having grown too deep for the animal, half-filled up with irregular,
calcareous, transverse plates (Pl. 4, fig. 2 _a_), resting one upon the
other by irregular points or pillars. The cementing apparatus has been
sufficiently described under the Family.
_Mouth._--The _labrum_ is always notched; sometimes it has no teeth,
but generally there are three on each side; in _B. balanoides_ there
are five or six on each side; and in _B. improvisus_ and _eburneus_
there is a whole row of teeth (Pl. 26, fig. 2, _e′_), graduated in
size, on each side of the notch. The _palpi_ are large, with their
apices nearly touching, and furnished with long spines. The _mandibles_
have, as it appears, normally, five teeth, but the two lower teeth
are always small and often rudimentary, and almost confluent with
the inferior, sometimes spinose angle. The _maxillæ_ have either a
simple edge, or a notch under the pair of large upper spines, or the
lower part forms (Pl. 26, fig. 7) a step-formed projection: there are
generally two lower spines, placed singly or not in pairs, larger than
the others, with the exception of the uppermost pair. The _outer
maxillæ_ are, on their inner faces, obscurely divided into two lobes.
_Cirri._--The rami of the first pair are unequal, the shorter one
sometimes not being more than half the length of the other ramus:
the segments of the shorter ramus are broad, and are, together with
the lower segments of the longer ramus, thickly clothed with spines;
in some species, as in _B. perforatus_, the anterior surfaces of the
segments, more especially of the shorter ramus, and of both rami of the
second pair are produced (Pl. 29, fig. 4), so as sometimes to form very
remarkable projections. The segments of the second and third pairs are
always thickly clothed with spines, as also are their pedicels. The
third pair is rather longer than the second; but in _B. vestitus_ and
_imperator_ it is much longer, and is otherwise somewhat different.
The dorsal and basal margin of the pedicel of the third pair, in some
of the species, as in _B. tintinnabulum_, is produced backwards on the
thorax, and forms a membranous plate fringed with fine spines. The
three posterior and longer pairs of cirri have from three to rarely
eight or ten pairs of long spines on each segment, with generally one
or two minute spines in the middle between each pair: their pedicels
have a regular double row of spines.
The _penis_ is long and hairy: in most of the species there is, at its
dorsal basis, a small, sharp, flattened, imperforate projection; first
observed by Poli: but this is sometimes absent, as in _B. crenatus_,
though present in the closely allied _B. balanoides_; and its presence
is variable in _B. tintinnabulum_. All the species have large plicated
_branchiæ_. The base of the _sack_ in several species is furnished
with inwardly projecting filamentary appendages. In _B. perforatus_,
_crenatus_, and _improvisus_, and I believe in other species, the upper
part of the _stomach_ is furnished with a circle of branching cæca.
_On the variation of the species_; _their arrangement and affinities_;
_value of the characters used_; _changes during growth_.--Owing to the
great variation in external characters, to which almost all the species
are subject, and likewise to the genus being a very natural one, that
is, to the species following each other in close and natural order, it
is not easy to exaggerate the difficulty of identifying the species,
except by a deliberate examination of the internal and external
structure of each individual specimen. Every one who has collected
sessile cirripedes must have perceived to what an extent their shape
depends on their position and grouping. The surface of attachment has
a great effect on that of the shell; for as the walls are added to at
their bases, every portion has at one time been in close contact with
the supporting surface; thus I have seen a strongly-ribbed species
(_B. porcatus_) and a nearly smooth species (_B. crenatus_) closely
resembling each other, and both having a peculiar appearance, owing
to their having been attached to a pecten. Dr. Gray has pointed out
to me specimens of _B. patellaris_, curiously pitted like the wood
to which they had adhered; and numberless other instances might be
added. Quite independently of the effect produced by the surface of
attachment, the degree to which the longitudinal folds and ribs are
developed on the parietes, is variable in most of the species, as in
_B. tintinnabulum_, _vestitus_, and even in _B. porcatus_; the presence
or entire absence of these ribs often surprisingly alters the whole
aspect of the shell. The persistence of the so-called epidermis is in
some degree variable; and in _B. lævis_ we have groups of specimens
absolutely naked, and others uniformly clothed with a brown membrane.
Again, some species in certain localities are all subject to the
disintegration of the entire outer lamina of the walls; and in such
cases (as with _B. perforatus_) there is not the smallest resemblance
between the corroded and perfect specimens. The size of the orifice,
and consequently of the operculum, compared with the shell itself,
varies accordingly as the shell is more or less conical or cylindrical;
in the latter case, the summits of the radii are generally more oblique
and the aperture consequently more deeply toothed than in the more
conical varieties. Size is a serviceable character in some cases, but
very many specimens are required to ascertain the average or maximum
size of each species, for there is no method of distinguishing a
half-grown from a full-grown specimen; and I believe, as long as the
individual lives, so long does it go on moulting and growing. _Colour_
is of very considerable service; though the precise tint varies greatly
in almost every species; and what is a far more serious evil, the
majority of the species have their white or nearly white varieties, the
latter sometimes as numerous as the coloured ones: in _B. perforatus_,
_lævis_, _flosculus_, _amphitrite_, and in several other species, the
common white varieties are eminently deceptive.
Besides the slight variation in the obliquity of the summits of the
radii and alæ, dependent on the more or less cylindrical form of
the shell, in some species, as in _B. tintinnabulum_, _amphitrite_,
_improvisus_, _trigonus_, and _porcatus_, their obliquity also varies
occasionally from unknown causes, and thus greatly affects the general
appearance of the shell. In some few species, as in _B. perforatus_,
the radii are often either not at all developed, or are of very
variable width; in others, when the shell has become cylindrical, or
when very old, the radii cease to grow, and from the disintegration of
the whole upper part of the shell, with the continued growth of the
lower part, the radii at last come to exist as mere fissures: I have
seen instances of this in _B. psittacus_, _nigrescens_, and _porcatus_.
Nevertheless, the obliquity of the upper margin, and the breadth of the
radii are useful characters; and still more useful is the fact whether
the upper margins are smooth and arched, or straight and jagged. The
fact of the terga being more or less beaked is useful: as is, likewise,
the presence of striæ, or furrows, or rows of pits, radiating from the
apices of the scuta; but to ascertain the presence of these marks,
it is almost invariably necessary to take out the scuta, clean, and
examine them with a lens; these ridges and furrows, moreover, in some
species, as is strikingly the case with _B. tintinnabulum_, and in
less degree with _B. trigonus_, _lævis_, and _concavus_, appear and
disappear, and vary without any apparent cause.
Now if we reflect that form, size, state and nature of the surface,
presence of epidermis, relative size of the orifice, presence of
longitudinal ribs, tint, and often the existence of any colour, are
all highly variable in most of the species; and that the obliquity
of the summits of the radii, and the presence of longitudinal striæ
on the scuta, are variable in several species; we shall perceive how
difficult it must ever be to distinguish the species from external
characters. As some evidence of this, I may mention that, after having
described nearly 40 species, and when my eye was naturally able to
appreciate small differences, I began carefully to examine varieties of
_B. tintinnabulum_, _amphitrite_, _improvisus_, _porcatus_, _vestitus_,
&c., without even a _suspicion_ that they belonged to these species, at
that time thoroughly well known to me; yet in the cases here referred
to, there could be no doubt, when a perfect series was examined,
that the specimens were only varieties. From this cause the labour
of naming a collection is great. Let no one attempt to identify the
species of this genus, without being prepared to disarticulate, clean,
and carefully examine with a microscope the basis and parietes, and
both the under and upper surfaces of the opercular valves; for I feel
convinced, that he will otherwise throw away much labour. Moreover, in
many cases, it is almost necessary, on account of the variability of
the characters, to possess several specimens. From these facts, I have
not hesitated to form my sections on characters which require close
examination, though I would gladly have seized on external characters,
could I have found such even moderately constant.
The least varying, and therefore most important characters, must
be taken from the internal structure of the parietes, radii, and
basis: not that these characters are absolutely invariable; thus the
porosity of the parietes is slightly variable in _B. glandula_, and
highly variable in the fossil _B. unguiformis_; it is also highly
variable in _B. balanoides_, but under a systematic point of view
this is unimportant, as the section including this latter species is
well defined by the membranous basis. The porosity of the basis is in
some degree variable in _B. nubilus_, _improvisus_, and _patellaris_;
and in _B. flosculus_ we see signs of a passage from a calcareous to
a membranous basis. Characters derived from the general shape, and
from the ridges and depressions on the under side of the scuta and
terga, especially of the scuta, are highly serviceable. The terga,
indeed, in many species, as in _B. amphitrite_, vary considerably,
and are affected by the general shape of the shell. Unfortunately the
differences are not very great between the scuta of the different
species. The cause of the opercular valves offering more useful
characters, as far as outline is concerned, than do the walls of
the shell, is no doubt due to their being almost independent of any
influence from the nature of the surface of attachment. Even the ridges
and depressions on the under side of the scuta, which are in direct
connection with the muscles and soft parts of the animal, vary to a
certain extent: thus the length and prominence of the adductor ridge
is decidedly variable in _B. concavus_ and _tintinnabulum_, and in a
less degree in _B. lævis_; the size and form of the little cavity for
the lateral depressor muscle varies in many species; so does the exact
shape and degree of prominence of the articular ridge. There is one
character in the terga, which at first would be thought very useful,
namely, whether an open longitudinal furrow, or a closed fissure
runs down the valve from the apex to the spur; but it is found that
the furrow almost always gradually closes up during growth; and as a
consequence of this, the width of the spur compared to that of the
whole valve, as well as its distance from the basi-scutal angle, and
the form of its basal extremity, all vary in some degree. The length
of the spur sometimes varies considerably, as in _B. concavus_ and
_amphitrite_. The parts of the mouth are only occasionally serviceable;
for the teeth on the labrum, and the state of the lower teeth on the
mandibles, and the presence of a step-formed projection at the lower
angle of the maxillæ, are all often variable. The relative lengths of
the two rami of the first pair of cirri, the degree of protuberance
of the segments, and the number of pairs of spines on the segments of
the posterior pairs of cirri, are sometimes useful; but the relative
lengths of the cirri, and more especially the numbers of pairs of
spines on the posterior cirri, are apt to vary. Finally, I must
express my deliberate opinion, that every part and organ, internal and
external, in Cirripedes, is liable to some amount of variation in some
of the species.
I must now point out the principal changes which supervene during
growth, and which cannot properly be called variations. In the first
place, I think, it is scarcely possible to recognise a species when
under the 1/10th of an inch in diameter. In some cases, as in _var.
d'Orbignii_ of _B. tintinnabulum_, the shell is invariably coloured
when old, but quite white when very young. Generally the tints become
very much darker with age. Some species, which usually or invariably
have, when mature, longitudinally folded walls, as _B. flosculus_
and _balanoides_, are perfectly smooth in youth. The walls in _B.
eburneus_, when young, have white, hyaline, longitudinal lines, and
are naked; whereas, with advancing age, these lines disappear, and the
subsequently formed shell becomes covered with membrane. The summits
of the radii are apt to be oblique in the young of _B. Capensis_,
_psittacus_, and _tintinnabulum_, whereas they are generally quite
square in old specimens. In _B. eburneus_, _cariosus_, and in a lesser
degree in _B. psittacus_, the scuta become longitudinally striated
only with age. On the other hand, in very young specimens of _B.
tintinnabulum_, the scuta sometimes are deeply impressed by little
pits placed in rows. I have already alluded to the longitudinal
furrow on the tergum so entirely changing its character, owing to the
edges becoming, during growth, folded inwards; this likewise causes a
decrease in the proportional breadth of the spur. In old specimens of
_B. flosculus, var. sordidus_, the whole tergum is much more elongated
than in young specimens. The basal margin of the sheath is hollow
beneath in the young of _B. cariosus_ and of some other species, but
in the old it is continuous with the inner surface of the walls. The
inner lamina of the parietes generally loses, to a certain extent, its
longitudinally ribbed character in old age. The basis is solid, instead
of being porose, in very young specimens of _B. improvisus_. In all the
species, the carino-lateral compartments, in early age, are very narrow
in proportion to the width of the lateral compartments; and in all, at
this early period, the operculum is large in proportion to the whole
shell. The number of spines on the edge of the maxillæ, the number of
segments in all the cirri, and the number of spines on each segment,
are few in early youth, and go on increasing with each successive
exuviation: the pedicels of the cirri are long in proportion to the
rami at this same early age.[84]
[84] In some specimens of _Balanus perforatus_ I made the following
enumeration of the number of segments in the cirri:--
+--------------+--------------+--------------+------------------+
| |Basal diameter|Basal diameter| Medium sized |
| | of shell | of shell | specimen about |
| | 1/20th of | 1/5th of |3/4ths of an inch |
| | an inch. | an inch. |in basal diameter.|
+--------------+--------------+--------------+------------------+
|First cirrus }| ? | 11 | 17 |
|shorter ramus}| | | |
| | | | |
|Second cirrus | 4 or 5 | 9 | 13 |
| | | | |
|Sixth cirrus | 9 or 10 | 19 |31, in another 36 |
+--------------+--------------+--------------+------------------+
In the specimen 1/5th of an inch in basal diameter, each segment
of the posterior cirri carried five pairs of spines; whereas,
in full-grown specimens, there are six or seven pairs. In the
1/20th of an inch specimen, on the inner maxillæ, there were
no spines between the upper large and the lower large pair of
spines; whereas, in the 1/5th of an inch specimen, there were five
intermediate spines, and in larger specimens nine or ten spines.
Notwithstanding the difficulties now enumerated, I hope that, owing to
having examined a vast number of specimens of the most varying species,
I have not fallen into very many errors. I have endeavoured to err
on the side of making too few instead of too many species. In those
cases, however, in which I have seen only a few specimens, I have been
sometimes compelled to decide without sufficient evidence.
I would gladly have divided this genus, already including 45 species,
into smaller genera; but so far from being enabled to do so, I
have been compelled to form my Sections (immediately to be given)
on characters not absolutely invariable, and far from obvious. I
was particularly anxious to separate the elongated species with a
boat-formed basis, which are attached to Gorgoniæ, and which form the
genus Conopea of Say, but I was unable to effect their separation even
as a sub-genus; for _B. navicula_ and _cymbiformis_ graduate in the
most insensible manner through _B. galeatus_ (the type of Say's genus)
and _B. calceolus_ into _B. stultus_, and this into _B. Ajax_; yet
this latter species has even been described as a mere variety of the
typical _B. tintinnabulum_! Indeed, so insensible is this graduation,
that the first and second sections of the genus are hardly distinct. I
fully admit, that if _B. stultus_ and _Ajax_ had never existed, _B.
calceolus_ and its three allies might have formed as natural a little
group, though difficult to be characterised, as does the sub-genus
Acasta; or perhaps this group and Acasta might have been combined
together. These same species, viz., _B. calceolus_ and its allies,
are intimately allied to _B. terebratus_ and _inclusus_, which are
contained in the last section (F) of the genus; and this shows that Dr.
Gray's proposed genus Chirona, including the species with non-porose
parietes in sections (E) and (F), could hardly have been instituted,
even if the porosity of the parietes had not been variable in _B.
unguiformis_, _balanoides_, and _glandula_. My fourth section (D),
founded on the basis not being porose, is perhaps the weakest of the
divisions, as may be seen in the list of exceptions appended to the
sectional headings.
The arrangement of the species is, I think, as natural as a linear
one could be made: every one knows how irregularly and in how many
directions the lines of affinity in every natural genus branch out.
Some few species stand rather isolated, as _B. declivis_; and _B.
allium_, _cepa_, and _quadrivittatus_ in a little group by themselves.
I have already shown how the species in the first and second sections
(A and B) blend into each other; and that the blending species are
likewise allied to some in the last section (F); furthermore, I shall
have occasion to show that these same species can hardly be separated
naturally from the sub-genus Acasta. The first section, moreover,
passes into the third (C) by _B. tulipiformis_; and the third into
the fourth (D) by _B. improvisus_, _nubilus_, _corrugatus_, and
_patellaris_: the fifth and sixth (E and F) sections are closely
connected together by _B. cariosus_ and _flosculus_; and these two
sections blend into the fourth (D) through _B. unguiformis_ and
_balanoides_, and lastly, into the third (C) section by _B. dolosus_
and _improvisus_.
The genus, as we have just seen, is hardly separated from the sub-genus
Acasta; by _B. allium_ it tends to pass through the sub-genus Creusia
into Pyrgoma; and by _B. imperator_ and _flosculus_ it graduates into
Elminius and Tetraclita.
_Geographical Distribution._--This, which is much the largest genus of
sessile cirripedes, has its species scattered over the whole world,
from the arctic regions, in lat. 74° 48′, where we have _B. crenatus_
and _porcatus_, throughout the tropical seas, to Cape Horn, where
_B. flosculus_ adheres to the coast-rocks. Many of the species have
individually very wide ranges; thus _B. tintinnabulum_ and _amphitrite_
are found throughout the warmer seas; but the wide distribution of
these species may be partly due to their frequent adhesion to ships'
bottoms: _B. crenatus_ ranges from the frozen seas, in lat. 74° 48′
north, to the West Indies and Cape of Good Hope--a wonderful endurance
of the most opposite climates. _Balanus improvisus_, again, extends
from Europe to Nova Scotia, thence southward to Patagonia, and up
the western coast of S. America, someway north of the Equator. Most
of the species have considerable ranges; thus of the six species
found on the eastern shores of northern America, five of them occur
in Great Britain. Of the thirty-six species of which the habitats
are known, exactly one third, or twelve, inhabit both the torrid and
temperate zones, these being divided by the isocryme of 68°; nine
are found exclusively in the torrid, and fifteen exclusively in the
temperate zones. Within the warmer latitudes, and especially in the
southern hemisphere, Tetraclita and Elminius to a certain extent
supplant Balanus. In depth, the species range from the upper limits
of the tidal zone to even fifty fathoms. _Balanus improvisus_ and
_eburneus_ are able to survive in brackish water. The different species
are attached to various surfaces--rocks, shells, timber, floating
objects, sea-weed, lamelliform corals, Milleporæ, Gorgoniæ, and even to
sponges. Mr. G. B. Sowerby has remarked[85] that in the species from
the southern hemisphere it is the basis, and in the species from the
northern hemisphere it is the parietes, which are elongated, when the
individuals, from being crowded together, become cylindrical; but this
is erroneous; _B. perforatus_, in the northern hemisphere, sometimes
has an elongated basis; but no doubt the basis of our commonest
species, as _B. balanoides_, _crenatus_, and _porcatus_, from being
either membranous or thin, does not become cup-shaped; whereas this
structure is conspicuous in _B. psittacus_ and _lævis_, the two
commonest species in southern South America.
[85] Darwin's 'Geology of S. America,' p, 264.
_Fossil Species._--Having already given, under the Family, some account
of the geological history of sessile cirripedes, short as it is, I here
only allude to the subject in order to state my conviction that species
cannot be satisfactorily distinguished in a fossil state, and rarely
in a recent state, without an examination of the opercular valves.
Nothing, indeed, could have been easier than to have affixed names
to many groups of specimens, having different aspects, but to feel
sure that these were really distinct species requires better evidence
than can be afforded by the shell, without the operculum. No doubt,
in some of the smaller sections of the genus--for instance, in that
characterised by a membranous basis--it would have been possible to
have distinguished some or several fossil species; but such have not as
yet been found. When the specimens are much fossilised, it is, indeed,
difficult to make out the primary points of structure--namely, whether
the parietes, radii, and basis are porose: to do this it is sometimes
necessary to rub down, polish, and carefully examine, a transverse
section of a piece of the shell.
_Sections of the Genus._
[A]
_Parietes, and basis, and radii permeated by pores._
[B]
_Parietes and basis sometimes permeated by pores, sometimes not; radii
not permeated by pores; shell elongated in its rostro-carinal axis;
basis boat-shaped, attached to Gorgoniæ and Milleporæ._
[C]
_Parietes and basis permeated by pores; radii not permeated by pores._
[D]
_Parietes permeated by pores. Basis and radii not permeated by pores._
[E]
_Basis membranous._
[F]
_Parietes and radii not permeated by pores. Basis sometimes permeated
by pores, sometimes not, sometimes excessively thin and hardly
distinguishable._[86]
[86] The following species might, owing to variation, or to the
obscurity of their structure, without care, be classed in the wrong
sections:--in A, _Balanus Ajax_, from living attached to Milleporæ,
is sometimes elongated in its rostro-carinal axis, and from having
its radii only finely porose, it might be classed in B: on the
other hand, _Bal. stultus_ is sometimes but little elongated, and
the basis hardly boat-formed, and hence might be classed in A. The
distinction between all the species in (B) and the sub-genus Acasta
is artificial.
In sections C and D, I have seen one specimen of _B. spongicola_
with a solid basis, and very young specimens of _B. improvisus_
are thus characterised, and therefore these species are liable to
be placed in the wrong section, D: _Bal. nubilus_, also, has part
of its base non-porose, and therefore likewise might be placed
in D: on the other hand, the circumference of the basis in _B.
patellaris_ is often porose, and hence this species, which belongs
to D, might be placed in C.
In _Bal. glandula_, in D, the parietes of the compartments, without
several were examined, might be thought to be solid, and therefore
this species might be wrongly placed in F; on the other hand, the
fossil _B. unguiformis_, in F, often has porose parietes, and such
specimens would naturally be placed in D. Lastly, without care, _B.
flosculus_ might be thought to have a membranous basis, and so get
placed in E.
The genus Pachylasma, without an examination of the animal's body,
might easily get misplaced in the wrong genus, amongst the species
in the last section (F) of Balanus, yet there can be no doubt that
Pachylasma belongs to the Chthamalinæ.
_Section_ A.