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.