Part 35

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.

1. ACASTA SPONGITES. Pl. 9, fig. 1 _a_-1 _d_.

LEPAS SPONGITES. _Poli._ Testacea utriusque Siciliæ, 1795, Tab. 6, fig. 3-6.

BALANUS SPONGEOSUS. _Montagu._ Test. Brit. Suppl., 1808.

---- SPONGITES. _De Blainville._ Dict. Sc. Nat., Pl. 116, fig. 3.

LEPAS SPONGIOSA. _Wood's_ General Conchology, p. 47.

ACASTA MONTAGUI. _Leach_, in Lamarck, Animaux sans Vertèbres, 1818, et in Encyclop. Brit., Supplement, 1824, vol. 3, Pl. 57.[103]

---- ---- _J. E. Gray._ Annals of Philosophy, New Series, vol. 10, Aug. 1825.

---- SPONGITES. _Philippi._ Enum. Mollus. Siciliæ, vol. 2, p. 211, 1844.

BALANUS MONTAGUI. _Brown's_ Illustrations of Conchology (2d edit. 1844), Pl. 53, fig. 24-26.

[103] As the plate to the Supplement to the 'Encyclop. Brit.' is marked as engraved in 1817, I presume Dr. Leach gave a proof to Lamarck, thus enabling him to publish this species four years before the Supplement itself appeared.

_Carino-lateral parietes about one sixth of width of lateral parietes: inner surface of the parietes generally ribbed feebly: scutum with the articular ridge abruptly cut off at its lower end: tergum with the spur rounded-truncated, about one third of width of valve._

_Hab._--South coast of England; South Wales, (Tenby); Portugal; Naples; Sicily; Cape of Good Hope.

This species and the three following, have caused me much doubt and trouble. At first I took the view here adopted, namely, that they were distinct: more careful examination made me run them altogether under one name. Finally, after still further deliberation, and the examination of a few additional specimens, I concluded there was the least chance of error in classing them separately. I may here mention, that in some sponge from the Cape of Good Hope, this species was mingled with _Balanus spongicola_.

_General Appearance._--The shape is usually that of a cup, the orifice being, in most cases, rather large, and deeply notched, owing to the great obliquity of the radii and alæ. The surface is generally smooth, but furnished with sharp calcareous projections. The colour is pinkish, and chiefly in the upper part of the shell; the lower part is often yellowish from the preserved epidermis. The parietes in the carino-lateral compartments are always narrow, being only one sixth or one seventh of the width of the parietes in the lateral compartments. The radii are not very wide, never equalling in width the parietes. The basis is moderately deep, and sometimes very deep, being even occasionally curled like a horn on one side. The specimens from Lisbon and Naples are a little larger than any British specimen which I have seen; the former being .3 of an inch in basal diameter.

_Scuta_: these are striated longitudinally in close lines, generally plainly, but to a variable degree. The whole valve is flat, thin, and rather elongated, with barely a trace of an adductor ridge: the articular ridge is short and rather prominent: it terminates downwards abruptly, and this does not appear to be the case in the two following species. _Terga_: these are small compared with the scuta, they are slightly beaked: the spur is truncated but rounded, more especially on the carinal side; it is rather more than one third of the width of the whole valve. The articular ridge and crests for the depressor muscles are feebly developed.

_Structure of the Parietes and Radii._--The state of the inner surface of the parietes varies much; generally they are slightly ribbed close to the basis, the ribs sometimes extending up to the sheath; rarely the surface is quite smooth. The edges of the radii are slightly crenated. The upper internal surfaces of the radii, where overlying the alæ, are usually marked by feeble undulating lines, nearly parallel to the basis. The alæ have very oblique summits.

_Basis_: this is generally of a regular cup-like form, and about two thirds as deep as the shell is high; sometimes it is pointed at the bottom and distorted. The edge is feebly crenated, and rarely quite smooth. It is often penetrated by small rounded irregular holes; and I have seen specimens from the Cape of Good Hope with parts like a sieve.

_Cirri_: in the first pair, the anterior ramus is nearly thrice as long as the posterior ramus. The second cirrus is short, with one ramus longer by three or four segments than the other ramus; the terminal segments are truncated. The third cirrus is about one third longer than the second cirrus. In the anterior ramus of the fourth pair, the regular pairs of spines are rather crowded together in the upper part of each segment, and the intermediate little spines and dorsal tufts are rather long in comparison with those of the fifth and sixth pairs of cirri: moreover, amongst the regular pairs, a few very minute and thick spines, pointing upwards, could be perceived. So that we here have the very unusual case of the fourth cirrus not exactly resembling the fifth and sixth pairs; and we shall see, in the following species, that this same anterior ramus of the fourth cirrus presents in addition another very extraordinary character. In the sixth cirrus there are on each segment four pairs of spines.

2. ACASTA SULCATA. Pl. 9, fig. 2 _a_-2 _d_.

ACASTA SULCATA. _Lamarck._ Animaux sans Vertèbres, 1818.

---- ---- _Deshayes_, in Guerin, Magasin de Zoologie, 1831, Tab. 24.

_Carino-lateral parietes about one sixth of width of lateral parietes: inner surface of the parietes generally ribbed strongly: basis with the edge strongly crenated: orifice of shell rather small: tergum with the spur generally truncated and nearly half as wide as valve._

_Var._ (_a_) (fig. 2 _b_): _with the walls externally ribbed_.

_Var._ (_b_) (fig. 2 _a_): _with small membrane-covered clefts between the edges of the walls, close above the basal cup_.

_Hab._--Sydney, Port Fairy, Moreton Bay in lat. 27° S., New South Wales; Southern Australia; and, according to Lamarck, Western Australia, in lat. 25° S. Mus. Brit., Cuming, Bowerbank, &c.

I am almost ashamed to admit this species, so small are its differences compared with _A. spongites_; yet I think that it probably is a distinct form.

In general appearance and character this species comes very near to _A. spongites_. As in the latter, the parietes of the carino-lateral compartments are narrow.[104] The orifice seems always to be smaller. Internally, the parietes are generally much more strongly ribbed, and the edges of the basal cup more plainly crenated. The sheath is generally coloured of a brighter pink, sometimes tinged with orange. The average largest specimens (from .3 to .5 of an inch in basal diameter) are a little larger than the largest European specimens: I have seen one specimen from Moreton Bay .4 in basal diameter, and from the basal cup being very deep, actually .75 in height. The _scutum_ has the articular ridge not so prominent and not so abruptly cut off at the lower end, as in _A. spongites_: on the other hand, the adductor ridge seems rather more prominent; but these differences are trifling. In the _tergum_ the breadth of the spur (fig. 2 _c_, 2 _d_) varies in specimens taken out of the same branch of sponge; some can hardly be distinguished from the same valves in _A. spongites_, but generally the spur is broader and squarer.

[104] In Mr. Cuming's collection there is a specimen, from Sydney, which I fully believe to be the present species, but cannot be positively sure, as the opercular valves have been lost, which is very remarkable from the walls of the carino-lateral compartments being reduced to the thickness of a mere thread, not one twentieth of the width of the lateral compartments; we here see the structure of _Acasta sporillus_ prefigured.

This species presents some remarkable varieties: in one specimen, from Sydney, the parietes were externally ribbed longitudinally (fig. 2 _b_), the ribs being roughened with minute points, giving to the shell an elegant appearance; and this without doubt is the _A. sulcata_ of Lamarck, procured by Peron at Shark's Bay, lat. 25° S., on the opposite of the Australian continent: some specimens from Port Lincoln, in South Australia, were not ribbed, only smoothly striated in longitudinal lines; although both sets of specimens had almost smooth scuta, and were thus different from common specimens, yet there could be no doubt, from their similarity in all other points, that they did not differ specifically from them, though the latter had their scuta striated, but not their parietes. Hence we see that there is no relation between the striæ on the parietes and on the scuta. The Port Lincoln specimens, and some others, were remarkable from the radii not extending down to the basal cup, a minute cleft, covered only by membrane, being thus left along the sutures, low down between the parietes (fig. 2 _a_); we shall see this singular structure strongly developed in _A. perforata_. Owing apparently to these clefts, the edge of the basal cup, exhibited traces of six knob-like teeth, like those characteristic of _A. glans_.

_Cirri._--The cirri resemble those of _A. spongites_, with the exception that the segments on the posterior pairs bear only three main pairs of spines. With respect to the anterior ramus of the fourth pair of cirri, the following very singular facts were observed:--in a specimen from New South Wales (_var._ with the tergum having a narrow spur), on two or three of the lower, but not on the lowest, segments, the front margin was produced or developed into two or three minute, thick teeth, slightly curved like hooks downwards: in other specimens from New South Wales (_var._ with the tergum having a broad spur, and inhabiting the same branch of sponge with the last-mentioned variety), there was no trace of these teeth. But again, in two other specimens with the tergum having a broad spur (collected by different persons, near Sydney), and in another from South Australia, this structure was carried to an extreme, for in these (as represented, Pl. 29, fig. 2) there were beautifully formed teeth on the fourteen lower segments (the twelve upper being without them), and likewise on the upper segment of the pedicel. These teeth are graduated in size on each segment; they are admirably adapted for securing any prey; and, in fact, they convert each segment into a mandible-like organ. On the segments, on which these teeth are well developed, some of the regular pairs of spines are aborted.