CHTHAMALUS. _Ranzani._ Memoire di Storia Naturale, 1820.
EURAPHIA. _Conrad._ Journal Acad. Nat. Sc. Philadelphia, vol. 7, 1834.
_Compartments six: basis membranous, but sometimes in appearance
calcareous, owing to the inflected parietes._
Distribution mundane; attached generally to littoral rocks and shells.
This, the typical genus, is the largest and widest distributed group of
the sub-family Chthamalinæ. The founder of the genus apparently did not
perceive its essential character; Savigny, however, as is evident from
the excellent figure in the great work on Ægypt, perfectly understood
the difference between Chthamalus and Balanus. I was first indebted
to Dr. J. E. Gray for explaining to me this difference; but the only
published account which I have met with is in a paper by the Rev. R. T.
Lowe,[130] in which he states, on the authority of Mr. Clark of Bath,
that in Chthamalus the anterior compartment or rostrum has alæ like
the posterior compartment or carina, the anterior or rostro-lateral
compartments being destitute of alæ. These characters being exactly
reversed in Balanus, as I have already explained (p. 176) under the
sub-family of Balaninæ.
[130] 'Zoological Journal,' vol. 3, p. 76, 1828.
The shell, owing apparently to its containing much animal matter, is
particularly subject to disintegration; and when thus much affected it
is quite impossible to distinguish the species by external characters.
It is, in fact, best to cast on one side external appearance, though
when the shells happen to be well preserved, each species has its own
peculiar aspect. We have in this genus smooth and plicated, cylindrical
and depressed varieties of most of the species. The development of the
radii is very apt to vary, and even the compartments often become so
completely united and calcified together that the sutures are almost
or quite obliterated. A more serious difficulty in discriminating
the species, arises from the fact of the opercular valves, not only
varying extremely in external appearance in consequence of the greater
or less disintegration of their apices, and consequent exposure of
their articular ridges and furrows (compare fig. 1 _a_, 1 _b_, 1 _c_,
in Pl. 18), but from their truly varying in outline with the varying
shape of the shell: this latter circumstance is probably due to the
opercular membrane which unites the valves to the shell being very
narrow, and in consequence, differences in the shape of the shell
affect the opercular valves, in a manner and to a degree to which the
Balaninæ are not subject (compare fig. 1 _e_, 1 _f_, 1 _h_, in Pl. 18).
The scuta, on the other hand, differ to an unusually slight degree in
the different species. In the common _Chthamalus stellatus_, which
abounds on the southern British shores, the whole external aspect
of the shell is often so completely masked, owing to its varying
shape, its obliterated sutures, its deeply disintegrated and punctured
surface, and by the corroded condition of its opercular valves, that
I have found this species, in the collections of naturalists who have
attended to cirripedes, arranged actually on the same tablet, mingled
with specimens of _Balanus balanoides_. I have myself several times
found it less troublesome to discriminate these two genera by the
included animal's body than by the shell,[131] though the latter, when
well preserved and developed, possesses such obviously well-marked
characters: the same thing has occurred to me with some of the other
species.
[131] It will be found ultimately to save time and trouble, to soak
for half an hour in hot caustic potash, a specimen out of each
group of shells to be examined, and then well wash and brush the
separated valves and compartments; this process has been followed
by me with all the species here to be described.
_General Appearance._--The shell is generally depressed, but sometimes,
when growing in groups, cylindrical. The surface is either smooth or
longitudinally folded; and, as already stated, very apt to be deeply
disintegrated. The radii, when developed, are narrow, with their
summits oblique and rounded; but they are often quite absent, and
sometimes even the sutures are almost obliterated. The rostrum and
carina are of the same shape and size, and the two lateral compartments
on each side are of nearly equal breadths. The orifice is generally
sub-rhomboidal, being widest towards the carinal, instead of towards
the rostral end, as is usual in Balanus: but in _Chthamalus fissus_
the orifice is narrow and elongated. The opercular valves have their
apices generally disintegrated and worn away, and are then seen to be
deeply locked together. The colour of the shell is dirty white or dull
purplish-red or brown; but in _C. intertextus_ rich violet-purple. The
species are small, not often exceeding half an inch in basal diameter,
with the exception of _C. Hembeli_, of which I have seen a specimen two
and three quarters of an inch in diameter.
_Scuta._--These present no particular character, excepting in most
cases the large development of the articular ridge, and sometimes the
presence of a furrow above the articular ridge, of which only traces
can rarely be detected in Balanus; hence in these cases, as in _C.
Hembeli_ (Pl. 18, fig. 5 _a_, 5 _c_) and _scabrosus_ (Pl. 19, fig. 2
_a_, 2 _c_), the line of articulation between the scutum and tergum
is more complicated than is usual. There is generally a slight pit,
sometimes even furnished with small crests, for the lateral scutal
depressor muscle: in _C. scabrosus_ (fig. 2 _d_, _q_) a part of this
muscle is attached to a small pit at the basi-scutal corner of the
tergum,--a fact of which I have observed no instance in any other
genus. In _C. intertextus_ the terga and scuta are calcified together,
without even a suture being visible on their internal faces. In _C.
Hembeli_ the valve is externally marked by a few longitudinal furrows.
The _Terga_, like the scuta, have a prominent articular ridge and deep
furrow. In many specimens of _C. stellatus_ and in _C. scabrosus_
the valve is narrow: in _C. fissus_ it is triangular and nearly
equilateral. The crests for the depressores muscles are well developed:
in _C. scabrosus_ (Pl. 19, fig. 2 _d_, _p_) these crests are united
into a plate, which, together with the outer lamina of the valve, forms
a deep narrow pit: in _C. Hembeli_ the crests are furnished with small
sub-crests. The opercular membrane is narrow; it is sometimes furnished
with a few minute spines.
_Structure of Parietes._--The parietes are solid, and composed of
successive layers of shell; the inner surface varies in condition
in the same species, being either smooth or marked with branching
impressed lines, or mamillated, or often irregularly punctured for the
entrance of tubuli. Owing to these tubuli, the walls, when externally
disintegrated, often become punctured. In certain depressed varieties
of both _C. stellatus_ and _scabrosus_ the walls are supported by
irregular depending columns, placed along either one side or both sides
of the sutures. In _C. intertextus_ we have the remarkable character of
the wall of the shell growing (I presume after a certain age) almost
rectangularly inwards, thus forming a rather wide, flat, calcareous rim
round the central basal membrane. In _C. Hembeli_ the internal basal
edges of the parietes, in one moderately young specimen, were rugged
with irregular points, but presented no other remarkable appearance;
but in five old and very large specimens, the whole basis was
calcareous, being absolutely continuous with the inner lamina of the
parietes, showing that the latter had grown flatly inwards all round,
and had then become confluent in the middle, so that there was no
longer any basal membrane; excepting, no doubt, that which had existed
in the younger stage, and which would be preserved in a functionless
condition between the surface of attachment and the inflected parietes.
When an opercular valve or compartment is dissolved in acid, layers of
tissue are left, and these are seen to be penetrated by tubuli, which
enter at the punctures before mentioned on the inside of the shell:
these tubuli often stand in groups of three or four together; they
are about 1/10000th of an inch in diameter. Besides these irregularly
scattered tubuli, there are in the opercular valves of _C. antennatus_,
innumerable smaller parallel tubuli, running to the external investing
membrane.
_Structure of the Radii and Alæ._--The radii, when developed, are
always rather narrow. Their recipient furrows are generally nearly as
broad as the radii themselves. Their edges are either quite smooth, as
in _C. antennatus_; or very finely crenated; or, as in _C. dentatus_
and _Hembeli_ (Pl. 18, fig. 3 _a_, 5 _a_), so strongly crenated as to
make the suture, both externally and internally, toothed: in these
two species, the radii are ribbed in transverse lines parallel to
the basis, each rib corresponding with one of the projecting and
interlocking teeth on the sutures. In _C. intertextus_, and much less
plainly in most specimens of _C. scabrosus_ (Pl. 19, fig. 1 _a_, 2
_a_), we have a structure in appearance very different, for the radii
here consist of several very oblique plates, (_i. e._ nearly parallel
to the parietes) on both sides of the sutures, which are interfolded or
locked together: I believe that this structure is a mere modification
of that in _C. dentatus_ and _Hembeli_, the transverse ridges on the
radii of those species being here developed into oblique plates. We
shall hereafter meet with a similar structure in the genus Verruca;
to which genus, until meeting with these two species of Chthamalus, I
had thought that the interfolding sutures had been confined. The alæ
have their edges generally finely crenated: during diametric growth
(when such takes place), they are rarely added to above the level
of the opercular membrane, and hence their summits are oblique: in
_C. intertextus_, however, the alæ are laterally added to above the
opercular membrane, and their recipient furrows are likewise added to,
of which fact I have seen no other distinct instance in any genus;
hence on both sides of the sutures, in the sheath of this species, the
lines of growth are upturned. In some much disintegrated specimens,
both of _C. stellatus_ (var. _depressus_) and of _C. antennatus_, the
radii have been corroded away, and the diametric growth is effected
exclusively by the growth of the alæ, which are moreover much exposed,
and rendered conspicuous. The sheath descends a moderate distance down
the shell. When a shell is boiled in potash, the sutures (excepting
when abnormally calcified together, as very often happens with some
species) always fall apart, showing that the union is simply by
animalised matter.
_Basis._--The basis is always membranous; but we have seen, in _C.
intertextus_, that the walls form a flat ledge all round the base,
and that in _old_ specimens of _C. Hembeli_, they grow so far inwards
and become so completely confluent, that they might most easily be
mistaken for a true calcareous basis. I may add, that in one elongated
specimen of _C. stellatus_ from La Plata, the walls had likewise
grown rectangularly inwards, forming a flat base, and had then turned
upwards in the middle, forming a medial crest, with the edges not quite
calcified together. The true basal membrane is very obscurely divided
into concentric slips. I observed in several species, attached to the
lower surface, an excessively fine network, quadrangular or hexagonal,
of yellow vessels, which seemed insensibly to pass into the sheets,
discs, and globules of cement, by which the membranous basis adheres to
the supporting surface. I saw, in _C. antennatus_, numerous irregular,
bifurcating, and inosculating cement-ducts, of unequal diameter, often
crossing each other, and sending off branches ending in points: the
older ducts, instead of being solidly filled up with cement, were only
divided by septa. I did not succeed, in any species, in discovering the
cement-glands.
_Mouth._--The _labrum_ is slightly bullate, with the middle portion
depressed, but not forming a notch; in some species it is hairy, and
in some pectinated with short spines. The _palpi_ are of moderate size.
The mandibles have from three to five main teeth, the number sometimes
varying even in the same species: the lower teeth are either plainly
double laterally, or very obscurely double, or to all appearance
quite single: a rather large lower portion of the mandible is finely
pectinated. The maxillæ are always notched under the two or three large
upper spines: the notch bears some fine spines: beneath the notch there
are some large spines, and at the inferior angle some smaller ones.
_Cirri._--The first and second pairs are always very short compared
with the four posterior pairs. The rami of the first pair are slightly
unequal. The third pair, in length and arrangement of the spines, very
closely resembles the three posterior pairs; in _C. intertextus_,
however, the few basal segments, chiefly on the anterior ramus, are
thickly clothed with bristles, like the segments of the second cirrus.
In _C. antennatus_ (Pl. 29, fig. 2), the anterior ramus of this same
third pair is usually (one single specimen being excepted) much
elongated, having at least twice as many segments as the posterior
ramus, but the number is variable; and these segments, either all,
or only the upper ones, instead of having their spines regularly
arranged in pairs, in a double row, are surmounted each by a circle of
spines: I suspect that these elongated rami of the third cirrus act
as antennæ. It can hardly be an accidental coincidence, that certain
genera, as Lysmata and Pandalus, amongst the Macrourous Crustaceans,
have the same leg (homologically the second thoracic limb) elongated
and antenniformed. Certain varieties of _C. stellatus_ and _cirratus_,
also, have the anterior ramus of this same third cirrus considerably
elongated. We are thus reminded of the remarkable variability in the
numbers of the segments, and in the arrangement of their spines, in the
cirri of _Tetraclita porosa_; in that species, however, it was chiefly
the terminal segments of the _posterior_ ramus of the third cirrus
which were so highly variable. The three posterior pairs of cirri in
Chthamalus support from three to five pairs of main spines on each
segment, the number often varying in the same species, with some minute
intermediate bristles. The dorsal surfaces of these segments, in some
of the species, are serrated in an upward direction.
_Body, &c._--The body does not present any particular character: in
_C. scabrosus_ there is a slight ridge running from the base of the
first cirrus towards the adductor scutorum muscle: this ridge is
clothed with a few hairs; there are also some hairs at the carinal
end of the sack. In _C. dentatus_, also, there are hairs on the outer
tunic of the prosoma. In two species which I opened, there were no
cæca to the stomach. The ova vary in length from 13 to 14/2000th of
an inch in length; they are packed in two lamellæ lying on each side
of the animal's body. The larva just escaped out of the egg, in _C.
stellatus_, _scabrosus_ and _dentatus_, had a large probosciformed
mouth.
_Branchiæ._--These present a very singular amount of difference within
the limits of the same genus. In _C. stellatus_ and _antennatus_ we
have a simple fillet, tapering a little, barely plicated, and about
half, or more than half, as long as the sack; in _C. scabrosus_ the
branchiæ are entirely aborted, or are perhaps represented by the slight
hairy ridge at the carinal end of the sack: in _C. dentatus_, on the
other hand, each branchia consists of two large folds, barely plicated,
almost covering the whole side of the sack, so that here the branchiæ
are developed to an unusual degree, more than in Balanus, and as in
Coronula and its allies: in the same manner as in these latter genera,
the outer fold is considerably larger than the inner fold.
_Affinities._--_C. intertextus_ is the most distinct species of the
genus, as shown by the peculiar radii and alæ, by the scuta and terga
being calcified together, by the character of the third pair of cirri,
and by the inflected parietes forming a ledge round the membranous
basis; but in this latter respect _C. intertextus_ resembles _C.
Hembeli_. _C. Hembeli_, in its serrated radii, is closely related to
_C. dentatus_; and this latter species differs in the structure of
its radii only in degree from certain varieties of _C. stellatus_.
Lastly, _C. intertextus_, in its peculiar radii, closely resembles _C.
scabrosus_, and this latter species does not differ much from the other
species. Hence the genus Chthamalus has no claims to be subdivided into
smaller genera.
_Range._--The species are found all round the world, from (as far as
I have seen) 54°-55° north, to Cape Horn, in 55°-56° south. All the
species, of the habits of which I know anything, are littoral; and
in many parts of the world are excessively numerous, quite covering
large spaces of the coast-rocks, and often coating the coast-shells.
_Chthamalus dentatus_ is littoral, like the other species; but it
often lives attached on _Balanus tintinnabulum_ and _amphitrite_, on
the bottoms of ships arriving in British ports from the west coast
of Africa. I do not know of any instance of more than two species
occurring in the same region. Some of the species have large ranges:
_C. scabrosus_ extends from the Falkland Islands and Tierra del Fuego
to Peru; and _C. stellatus_ has an enormous extension over almost the
whole world, excepting the west coast of South America and Australia.
I do not believe any species of the genus, owing probably to their
littoral habits, have hitherto been found fossil.