ACASTA. _Leach._ Journal de Physique, tom. lxxxv, 1817.
_Compartments six; parietes and basis non-porose: basis calcareous,
cup-formed, not elongated, attached to sponges, or rarely to the bark
of Isis._
Distribution mundane; imbedded in sponges and the sponge-like bark of
Isis.
This sub-genus, in one sense, is a very natural one, inasmuch as all
the species are closely allied in essential structure, in general
appearance, and in habit. On the other hand, in the structure of the
shell, in all the characters derived from the opercular valves and
animal's body, Acasta cannot properly be distinguished generically from
some species of Balanus; thus _B. navicula_ and _cymbiformis_ agree
in the parietes and basis not being porose and in all other essential
respects, differing only in the shell being more elongated in the
rostro-carinal axis and in being attached to Gorgoniæ instead of to
sponges; yet we shall see that _Acasta purpurata_ lives imbedded in
the bark of Isis, so that even the habit of being imbedded in sponges
fails. _Balanus terebratus_ would have been ranked as an Acasta had it
inhabited sponges. On the other hand, some species of Balanus inhabit
sponges, as is often the case with _B. spongicola_, and always with
_B. declivis_: but both these species are distinguished easily from
Acasta, the former by its porose walls and basis, and the latter by
its membranous basis; it may, however, be reasonably doubted whether
such differences ought to be considered as even sub-generic. The most
important character of Acasta probably consists in the anterior ramus
of the fourth pair of cirri, differing slightly in the arrangement
of its spines, and in some other points, from the rami of the two
posterior pairs of cirri,--a character not as yet observed in any
other cirripede. Had not the genus Acasta been already founded and
extensively admitted, certainly I should not have formed it; but
considering the close similarity in habits, aspect, and structure, of
the nine species of Acasta, and considering the already large size of
the genus Balanus, I hope I may stand excused for admitting Acasta as a
sub-genus.
_General Appearance._--The shape varies from nearly globular to that
of a somewhat flattened acorn, the orifice being often a little
contracted from the inward curvature of the tips of the parietes.
In _A. spongites_, however, the orifice is generally widely open;
and, on the other hand, in _A. sporillus_, the orifice is reduced
to a mere pore. The usual tint is pale reddish, but _A. purpurata_
is purple, and _A. sporillus_ purplish-brown. The surface is either
smooth, or is shagreened with minute points, as in _A. sporillus_,
and _fenestrata_, and in some specimens of _A. sulcata_; and in all
the species, except _A. sporillus_ and _fenestrata_, many individuals
are furnished with elongated, curved, sharp, shelly points, like
those in _var. spinosus_ of _Balanus tintinnabulum_. The summits of
the radii, which are generally of moderate breadth, are more or less
oblique; their surface is often marked by lines parallel to the basis,
instead of by vertical lines corresponding with the lines of growth,
as in most species of Balanus. The carino-lateral compartments vary in
proportional breadth in the different species: in _A. sporillus_, they
tend to become rudimentary, and in this species (Pl. 9, fig. 9 _b_)
their basal margins, or rather points, do not reach down to the basis.
The species are all small, _A. glans_ and _undulata_ are the largest,
being sometimes .55 of an inch in basal diameter.
_Opercular Valves._--These differ in no generic respect from those
of Balanus. The _Scuta_ are striated longitudinally in several of
the species: the adductor ridge is barely developed in any, being
most prominent in _A. cyathus_. The articular ridge is prominent in
_A. fenestrata_ and _purpurata_. In the _Terga_, the spur is either
truncated and very broad, or moderately narrow and bluntly pointed:
the surface of the valve is often depressed, and in _A. spongites_
and _fenestrata_ it is furrowed in the line of the spur. The articular
ridge and furrow are well developed in _A. fenestrata_ and _purpurata_.
The crests for the tergal depressor muscles are either absent or very
feebly developed.
_Structure of the Parietes and Radii._--The parietes are not porose;
internally, they are either smooth, or slightly, or strongly ribbed in
longitudinal lines; the presence of these ribs, which are homologous
with the parietal septa in Balanus, is variable even in the same
species. In _A. sporillus_ the inner surface is curiously reticulated.
The sutural edges of the radii are either smooth, or very slightly
crenated by the septa, in lines parallel to the basis. The upper
margins of both radii and alæ are always more or less oblique. The
radii sometimes do not extend down to the basis; and in this case, as
will presently be described, apertures are left in the lower half,
between the compartments. In _A. glans_ and _lævigata_ the internal
margin of the wall of each compartment, from the sheath to the basis,
projects inwards, forming inside the shell as many double ridges (Pl.
9, fig. 5 _b_), as there are compartments, namely, six: a nearly
analogous structure occurs in certain species of Balanus. The basal
edge of the sheath, in most of the species (5 _b_, 9 _b_), depends
freely, and is hollow beneath, but this is always a variable point.
_Basis._--The base is either saucer or cup-shaped, but in _A. cyathus_
it is almost flat; it is generally symmetrical and smooth, with the
lines of growth closely approximate. In _A. fenestrata_ the basis is
commonly as deep, as the shell is high. The edge, in several of the
species, is crenated with minute teeth or notches; and these are so
large in some specimens of _A. sulcata_ and _cyathus_, as to make the
edge almost pectinated. In _A. glans_, and in a lesser degree in _A.
lævigata_, there are six knob-like teeth (fig. 5 _a_), corresponding
with the points of junction, between the basal edges of the inwardly
prominent margins of the six compartments, and the basis: in those
specimens, in which the six teeth are largely developed, six ridges
produced by their successive development, extend down towards the
centre of the basal cup. When the basal cup is dissolved in acid,
there is but a little animalised tissue and an external membrane,
formed as usual in slips, and furnished with blunt little external
points (apparently representing spines), each of which has a short
tubulus extending to the corium. Although I dissolved the basis of
three specimens, I could not distinctly make out any cement; nor did I
see any cement-ducts; yet these are readily distinguished, after the
dissolution of the basis in acid, in Balanus, Elminius and Tetraclita.
There can be no doubt that the young shell must at first be cemented
to a fibre of the sponge; but I suspect that the cementing-tissue is
not subsequently formed, owing to the support afforded by the growth
of the enveloping sponge. As some species of Balanus are habitually or
occasionally imbedded in sponges, it is important to observe, that the
species of Acasta are not only imbedded, but are attached to the fibres
of the sponge: but even this character, as we have already seen, is
not sufficient to distinguish the genus Acasta from Balanus, for _B.
declivis_ is attached exclusively to sponge.
_Perforations in the Shell._--Calcification seems often to fail to a
certain extent in this genus: the basal cup in most specimens of _A.
spongites_, and in some of _A. glans_ and other species, is irregularly
perforated by numerous minute orifices, closed only by the external
membrane, and filled up inside by pulpy corium. In some specimens of
_A. spongites_, from the Cape of Good Hope, parts of the basis were
riddled like a sieve. I have seen similar perforations in the parietes
of a few specimens of _A. glans_. In some specimens of _A. sulcata_,
the radii do not extend quite down to the basal edge of the walls
(Pl. 9, fig. 2 _a_), and in consequence a small cleft, closed only
by membrane, is left between the compartments, for a little space
above the basal cup. In _A. fenestrata_ (fig. 7 _a_), and in a lesser
degree in _A. purpurata_ (8 _a_), not only do the radii not extend
to the basal cup, but the parietes either on one or both sides of
each suture are hollowed out, so that six, large or small, elongated,
membrane-covered openings are formed, which extend from beneath the
sheath down to the basal cup. These openings, which I have not seen
in any other genus, will be more fully described under the respective
species.
_Mouth._--The parts of the mouth are identical in the several species,
and present no generic differences from those in Balanus. The outer
maxillæ and palpi appear unusually prominent: the labrum is deeply
notched, with no teeth, or very obscure teeth on each side. The
mandibles have five teeth, but the fifth is sometimes confluent with
the inferior angle. The maxillæ are not notched; and carry one or two
spines, near their inferior angle, nearly as large as the upper pair.
The outer maxillæ are bilobed.
_Cirri_: in the first pair, the rami are very unequal in length, the
one ramus being from half to one-third of the length of the other.
The segments in the second and third pairs, are not so much broader,
or so much more crowded with bristles, in comparison with the three
posterior pairs, as is the case with most species of Balanus. The
three posterior pairs, except in _A. purpurata_, are much elongated,
and the long thin segments bear four, and sometimes only three, pairs
of spines, which are generally doubly and finely serrated, or even
feathered. The most remarkable fact respecting the cirri, is, that in
_A. spongites_, _sulcata_, _cyathus_, and _glans_, the fourth pair,
instead of being identical in structure, as in all other genera, with
the fifth and sixth pairs, has, on its anterior ramus, the pairs of
spines more crowded together, with the little intermediate spines, and
those in the dorsal tufts, a little longer than in the sixth cirrus;
and between the pairs of spines, there occur some straight, upwardly
pointed, very minute, and very thick spines or teeth. And, what is
still more remarkable (as will hereafter be described in detail), in
certain specimens, but not in all, of _A. sulcata_, the front surfaces
of the lower segments on the anterior ramus, are developed into thick,
small, downwardly curved, hook-like teeth; this likewise is the case
with the upper segment of the pedicel (Pl. 29, fig. 2),--a most
elegant, mandible-like organ for the prehension of prey being thus
formed. The variability of such beautifully contrived teeth is very
surprising. Some similar teeth occur on the segments of the anterior
ramus of the fourth cirrus, but not on the pedicel, in _A. cyathus_. A
few teeth resembling the above, but thicker, occur on the segments of
the anterior ramus of the same cirrus, in _A. purpurata_.
_Branchiæ, &c._--In _A. spongites_, I found the branchiæ rather small,
with transverse plications. The muscles of the sack, which run to the
opercular valves, seemed rather feeble in most of the species. The
penis in several species was remarkably long, and in _A. spongites_ I
noticed the straight projecting point at its dorsal base, as is common
in Balanus.
_Affinities, &c._--At the commencement of the description of the genus,
I gave my reasons for keeping Acasta distinct as a sub-genus from
Balanus. The species are particularly troublesome to identify, not only
from the great variability of the most obvious characters, but from
the very close general external appearance of most of the species, and
the consequent necessity for cleaning and disarticulating at least one
specimen in every group. The shape, however, of the shell and basis,
and the state of their disarticulated edges, whether smooth, crenated,
or toothed, here offer more serviceable, though still very variable,
characters for the identification of the species, than is usual with
sessile cirripedes; and this, probably, is in part due to the almost
free or unattached condition of the whole shell, suspended, as it were,
in the midst of sponges, which they inhabit. The opercular valves, on
the other hand, are less serviceable than usual.
_Range, Habitats, &c._--The species are found all over the world,
excepting in the very cold latitudes.[102] _Acasta lævigata_ ranges
from the Red Sea to the Philippines; _A. spongites_ from the south
of England and Wales to the Cape of Good Hope; and _A. cyathus_ from
Madeira to the West Indies; most of the other species seem to have
rather confined ranges. The East Indian Archipelago seems to be
the metropolis of the genus, for here _A. lævigata_, _fenestrata_,
_purpurata_, and _sporillus_, are all found. Of these four species,
_A. purpurata_ lives imbedded, not in sponges, but in the sponge-like
bark of an Isis; and I think it probable, that _A. sporillus_ may
have somewhat analogous habits. The same species often lives imbedded
in different kinds of sponge; thus, I have seen _A. lævigata_ and
_cyathus_ in apparently three kinds, and _A. spongites_ in, as I
believe, four kinds. The genus existed during the miocene period, in
the Coralline Crag, under a form closely allied to _A. spongites_.
[102] I am greatly indebted to Mr. Bowerbank for his great
kindness, in looking over his immense collection of sponges from
all parts of the world, and sending to me all the specimens of
Acasta he could find imbedded in them.