Chapter XIX: Introduction (15)

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.

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.

_Diagnosis._--Finally, this species, if it be, as I believe,
distinct, differs from _A. spongites_ only in the internal surface of
the parietes being more strongly ribbed and brighter coloured,--in
the edge of the basal cup being more plainly crenated,--in the
articular ridge of the scutum being of a different shape,--in the
spur of the tergum being often broader,--in the segments of the
posterior cirri having only three pairs of main spines,--and, lastly,
in the occasional presence of the hook-like teeth on the anterior
ramus of the fourth cirrus.

3. ACASTA CYATHUS. Pl. 9, fig. 3 _a_-3 _c_.

_Carino-lateral parietes about one fourth of width of lateral parietes: radii wider than the parietes: basis nearly flat, small: tergum with the spur truncated, half as wide as valve._

_Hab._--Madeira, Mus. Lowe, and Bowerbank. West Indies, Mus.
Stutchbury. Hab. unknown, Mus. Brit.

I feel more confidence in this case, than in that of _A. sulcata_, that we here have a distinct species, though at one time I treated it only as a marked variety of _A. spongites_. I rely chiefly on the great proportional width of the radii of all the compartments, and on the width of the carino-lateral compartments, compared with the lateral compartments, and on the general shape of the shell, which differs considerably from that of the two previous forms: in consequence of Acasta being attached to and imbedded in a yielding substance, such as sponge, I believe external form to be of more value as a specific character in this genus, than in most sessile cirripedes. I have examined specimens taken out of the yielding _Spongia officinalis_ and out of an unusually compact sponge, and they resembled each other in every respect.

_General Appearance._--Colour pale pink, or that of flesh: basis
remarkably flat and rather small, with the walls above bulging out
a little. The radii are very wide, being wider than the parietes
to which they belong: the orifice is generally rather large. The
parietes of the carino-lateral compartments vary from one third to
one fourth of the width of the parietes of the lateral compartments.
Basal diameter of largest specimen .35 of an inch. Internally, the
parietes are generally more strongly ribbed than in _A. spongites_.

The _Opercular Valves_ are large, owing to the form of the shell. The
_Scuta_ present no particular character, and are not distinguishable
from those of _A. sulcata_; but the adductor ridge is perhaps rather
more developed. The _Terga_ (Pl. 9, fig. 3 _c_) are nearly as large
as the scuta, and this is an unusual circumstance; the spur is more
than half as wide as the valve; it is placed not quite close to the
basi-scutal angle; on the carinal side, the basal margin of the valve
slopes a little towards the spur. I may mention, that in several
specimens from Madeira, the scuta and terga, on one side, had grown
to a monstrous thickness.

_Cirri_: these resemble, in every respect, those of _A. spongites_,
with the remarkable exception that on the anterior ramus of the
fourth cirrus, several segments were furnished with the beautiful
downward curved, mandible-like teeth, as in _A. sulcata_; but
differently from in that species, there were none on the upper
segment of the pedicel. I should have thought this an excellent
specific character, had not these teeth been so extremely variable in
_A. sulcata_.

Finally, I think this species is more nearly related to _A. sulcata_
than to _A. spongites_.

4. ACASTA UNDULATA. Pl. 9, fig. 4.

_Shell, apparently, as in A. spongites, but larger: scutum marked by longitudinal ridges, often in pairs, with the intermediate furrows rather wide: spur of tergum nearly half as wide as valve._

_Fossil_ in Coralline Crag (Sutton), Mus. S. Wood.

I owe to Mr. Wood the inspection of a fine suite of separate valves. Owing to the shell never having been found entire, its general shape is not known, and, what is of more consequence, the relative proportional width of the parietes of the carino-lateral compartment is unknown. I have (but with doubt) given it a distinct specific name, owing to the peculiar character of the furrows on the scuta, and to the large size of the whole shell. In its other characters it comes nearest to _A. spongites_, excepting in the spur of the tergum, which resembles that of _A. sulcata_.

The compartments appear to have been rather smooth externally. The
radii are not wide, as in _A. cyathus_; and the basis is cup-formed.
Internally, the parietes are feebly ribbed, as in _A. spongites_.
Judging from the dimensions of the separated valves, this species
must have equalled and perhaps exceeded the size of the largest
living species, namely, _A. glans_, from Australia. Hence we may
infer, that the basal diameter probably exceeded .55 of an inch: I
may add, that the largest European specimens of _A. spongites_, from
Naples and Portugal, are only .3 of an inch in basal diameter.

_Scuta._--These seem to resemble the scuta of _A. spongites_ in all
respects, except in the longitudinal ridges standing much further
apart, and, consequently, in the furrows being much wider: each ridge
is generally double. Although there is a good deal of variability
in the character of these ridges in _A. undulata_, and likewise in
_A. spongites_, I have not seen any form intermediate between them.
It must, however, be confessed, that this is an extremely variable
character in many sessile cirripedes. In the _Terga_, the spur is
about half the width of the whole valve, and therefore rather wider
than in _A. spongites_.

5. ACASTA GLANS. Pl. 9, fig. 5 _a_-5 _c_.

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

_Parietes internally quite smooth, with the lateral margins of each compartment inwardly prominent: basis with the edge rarely crenated, but furnished with six inwardly prominent teeth: scutum strongly striated longitudinally._

_Var._ (_a_) _with the edge of the basal cup finely crenated_.

_Hab._--New South Wales, Southern Australia; Mus. Brit., Stutchbury,
&c.

This fine species seems to be extremely common, imbedded in an open porose sponge on the eastern and southern shores of Australia. It is very distinct from the other species, with the exception of the following _A. lævigata_, which, with some hesitation, I have allowed to remain specifically separated.

_General Appearance._--Excepting in its larger size, this species
differs in external appearance but little from _A. spongites_; its
colour is pale dirty reddish. The surface is generally studded
with small calcareous points. The parietes of the carino-lateral
compartments are about one fourth of the width of the parietes of the
lateral compartments, and therefore proportionally of the same width
as in _A. cyathus_. The largest specimen which I have seen, was .55
of an inch in basal diameter.

_Scuta._--These are slightly narrower, thicker, and more convex than
in _A. spongites_: they are strongly striated in longitudinal lines.
The articular ridge is very feebly developed. _Terga_: in full-grown
specimens, the spur is half the width of the whole valve, and is
truncated; its basal edge being parallel to the basal margin of the
valve. The articular ridge and crests for the depressor muscles are
very feebly developed.

_Internal structure of the parietes._--The inner surface of the
parietes is quite smooth, without even a trace of ribs or teeth. But
the most important character is that the internal lateral margins on
both sides of each compartment, from the sheath to the basis, project
inwards and form a rim; so that when the shell is viewed from within
(Pl. 9, fig. 5 _b_, representing the lateral and carino-lateral
compartments, and part of the carina), the six sutures are seen to be
strengthened by six double columns.

_Basis._--This is moderately cup-formed. The edge, in order to
meet the basal points of the inwardly projecting lateral margins
of the six compartments, has six knob-like teeth. These are placed
at unequal distances, for two on each side stand near each other,
owing to the narrowness of the carino-lateral compartments. The
degree of their development varies extremely; when most developed,
as in the specimen figured (Pl. 9, fig. 5 _a_), each tooth is bifid
and a little hollowed out, so as to receive the points of the two
inwardly projecting margins which form each suture. Ridges, more or
less prominent, running from each of the six marginal teeth, extend
towards the centre of the cup. These six teeth cannot be seen from
the outside. The edge of the cup is rarely crenated; but I have seen
two instances in which this was the case.

_Cirri._--In the first pair, the rami are not quite so unequal as
in _A. spongites_; the longer ramus being about twice as long as
the shorter. In the third pair, there are some very minute, thick,
upwardly-pointing spines, which I did not notice in _A. spongites_.
In the fourth pair, the spines are a little more crowded, with longer
dorsal tufts, than in the sixth pair; and they are mingled with some
very minute, thick, upwardly pointing spines. In young individuals,
there are only three pairs of main spines on the segments of the
sixth pair, instead of four pairs.

6. ACASTA LÆVIGATA. Pl. 9 fig. 6 _a_, 6 _b_.

ACASTA LÆVIGATA. _J. E. Gray_ (!). Annals of Philosophy, (new
series), vol. 10, Aug. 1825.

_Parietes internally quite smooth, with the lateral margins of each compartment inwardly prominent: basis with the edge strongly crenated, and furnished with six inwardly prominent teeth: scutum feebly striated longitudinally, or smooth._

_Var._ (_a_), _epidermis coloured dull orange_.--Red Sea.

_Hab._--Red Sea, Philippine Archipelago; Mus. Brit., Cuming, &c.

This species, of which I have examined many specimens from the above two and other unknown localities, agrees in all essential points of structure with _A. glans_, and consequently I for some time classed them together; but the characters, though usually of small value, by which this form differs from _A. glans_ being apparently constant, I have with some doubt allowed it to remain specifically distinct. These characters are, firstly, the much smaller size of the whole shell in _A. lævigata_; secondly, the edge of its basal cup being always crenated, which seems to be a rare accident in _A. glans_; thirdly, though of secondary importance, the scutum being here less plainly striated; and, lastly, the spur of the tergum being of less breadth, and of a more rounded outline; on the other hand, it must be confessed, that when small specimens of _A. glans_ are taken, there is hardly any difference in the spurs of the terga.

_General Appearance and Structure of Shell._--The surface of the
shell is often very smooth, but is sometimes studded with some small
sharp calcareous points. The colour is white, or pale reddish-brown;
but in the specimens from the Red Sea, the tint is more orange,
with the upper part of the shell white. The orifice of the shell is
unusually small. The largest specimen which I have seen was only .25
of an inch in basal diameter, and therefore less than half the size
of _A. glans_. The internal surfaces of the _parietes_ are smooth,
with the two lateral margins inflected, as in _A. glans_. The edge of
the basal cup has six knob-like teeth, like those in _A. glans_, but
smaller; and, in addition, it is finely crenated.

_Scuta_: these differ only in being less plainly striated in
longitudinal lines; indeed, some specimens show hardly a trace of
this structure. _Terga_; these valves, in some varieties (Pl. 9, fig.
6 _b_) can hardly be distinguished from those of equal size from
young individuals of _A. glans_; other varieties have the spur (Pl.
9, fig. 6 _a_) not truncated, but broadly pointed, and therefore of
considerably different shape.

Neither in the mouth, nor cirri could I detect any difference with
_A. glans_.

7. ACASTA FENESTRATA. Pl. 9, fig. 7 _a_-7 _c_.

_Shell reddish, with six large, membrane-covered apertures between the sutures, above the basal cup: carino-lateral parietes half as broad as lateral parietes; internally, parietes and edge of basis smooth; tergum with the articular ridge short and prominent; spur pointed._

_Hab._--Philippine Archipelago, Mus. Cuming.

_General Appearance._--Shell rather elongated or tubular; with the
upper part reddish, and the surface roughened with very minute
points. The basal cup is generally as deep as the shell is high,
ending downwards in a blunt point, often curved to one side. The
summits of the radii, as usual, are oblique. The parietes of the
carino-lateral compartments are about half as wide as the parietes of
the lateral compartments, and are therefore of greater proportional
width than in the foregoing or any other species of the genus.
The large membrane-covered openings, or, as they may be called,
windows, presently to be described, between the lower halves of the
compartments, is much the most remarkable character of this species.
The largest specimen which I have seen was only .23 in diameter, and
.6 of an inch in height, measured from the basal point of the cup to
the tips of the compartments.

_Scuta._--These barely exhibit a trace of longitudinal striæ. The
valve is rather thick and convex. The basi-tergal angle is much
rounded off. Internally, the articular ridge is thick and rather
prominent. _Terga_: the valve is furrowed in the line of the spur:
the spur is pointed and rather long; it is distinctly separated from
the basi-scutal angle of the valve, and the basal margin on the two
sides of the spur forms a straight line. The articular ridge is
prominent, and short.

_Structure of the Parietes, Radii, and Basis._--The parietes are
internally quite smooth down to its basis. The edges of the radii are
also smooth, as is the edge of the basal cup. The alæ project less
than usual. The radii are of moderate breadth, they extend downwards
only a little below the sheath, namely, about half way down the
shell, where they terminate, as usual, in a point. The increase in
width, during growth, of the radii, and their not extending down to
the basis, would necessarily cause a gap between the opposed edges
of the walls, in the portion beneath the radii; but besides this,
the edges of the walls themselves, beneath the radii, and on the
opposed side beneath the alæ, are hollowed out, but on the latter
side or beneath the alæ sometimes in a lesser degree. The result of
this is, that the compartments, in their lower halves, are separated
from each other by membrane-covered windows or apertures, arched at
their upper ends, and of considerable size, namely, about as wide as
the parietes of the carino-lateral compartments. I have only further
to remark, that during the downward growth of the parietes, the
apertures increase in size, but at the same time become closed up at
their upper ends; and the arched layers of shell added at these upper
ends, assume a very different aspect from the rest of the parietal
surface,--appearing like two wedges, with their points upwards, let
in, on one side of the suture, between the ordinary parietal surface
and the radius, and, on the other side of the suture, between the
ordinary parietal surface and the recipient furrow of the radius.

The _animal's body_ was in a bad state of preservation; but, as far
as I could make out, the cirri resembled those of _A. glans_.

_Affinities._--This species differs from _A. glans_ in not having the
internal margins of the compartments projecting inwards. It differs
from all the ordinary varieties of _A. spongites_, in the smoothness
of the basal edges of the parietes and of the edge of the cup; in
the greater width of the carino-lateral compartment, though this is
a variable point in _A. spongites_; slightly in the shape of the
scuta and terga; and, lastly, in the large, membrane-covered openings
between the compartments.

8. ACASTA PURPURATA. Pl. 9, fig. 8 _a_-8 _c_.

_Shell dull blueish-purple, with six small, membrane-covered apertures between the sutures, close above the basis: tergum with the articular ridge very short and prominent; spur very broad and rounded._

_Hab._--Sumatra; Philippine Archipelago; imbedded in the bark of an
Isis; Mus. Cuming, Stutchbury, Brit.

This species is perfectly distinct from the others, as shown by its general appearance, its habits, and the structure of its opercular valves: it is allied to _A. fenestrata_, in having membrane-covered apertures between the compartments, and in some respects in its opercular valves: it is also allied to _A. sulcata_ and _cyathus_ in the parietes being often internally ribbed, in the basal cup having a crenated edge, and in the anterior ramus of the fourth cirrus being furnished with the minute hook-like spines.

_General Appearance._--Sub-globular, slightly compressed, with a
rather small orifice; smooth, but sometimes furnished with sharp
shelly points; dull purple, more or less dark, with the upper parts
of the walls often white. The radii are rather narrow, and generally
white, with their summits only slightly oblique, but variable in
this latter respect. The parietes of the carino-lateral compartments
are narrow, being only one sixth of the width of the parietes of the
lateral compartments. In some specimens there are membrane-covered
apertures of considerable size, in others mere narrow clefts, between
the basal halves of the compartments. The basal cup is moderately
deep. The largest specimen was only .16 of an inch in basal diameter.

_Scuta_, rather broad, externally convex, not longitudinally
striated: articular ridge prominent, short, not extending down above
one third of the length of the valve. Depression for the adductor
muscle deep. On the internal surface, near to the rostral angle, a
rather large purple spot of corium adhered to the valve. _Terga_,
broad, externally rather convex: scutal margin protuberant: carinal
margin slightly inflected, or furnished internally with a rim:
articular ridge prominent, very short, not extending down above one
fourth of the valve. Spur very broad, rounded, confluent with the
basi-scutal angle of the valve.

_Internal Structure of the Parietes, Radii, and Basis._--The
parietes, internally, are either quite smooth, or more commonly
ribbed, with the basal edge in consequence crenated; the ribs
are either placed at an unusual distance from each other, and
consequently are few in number, or are pretty close together. The
edge of the basal cup is either quite smooth, or closely crenated,
or distantly toothed, in conformity with the state of the internal
surface of the parietes. The radii have nearly smooth edges, with
their summits more or less oblique. They sometimes extend down only
three fourths, or only two thirds, of the length of the shell, and
the margins of the parietes under the radii being a little hollowed
out, the sutures are converted into clefts or apertures (of course
covered by membrane) like, but not so large as, those in _A.
fenestrata_. The margins of the parietes are hollowed out only on the
side of the radius, and not on both sides of the sutures, as is most
usual in _A. fenestrata_. In some specimens the radii extended down
close to the basal cup, and only very minute clefts were left between
the opposed edges of the parietes.

In the _animal's body_ the only noticeable character was, that on the
anterior ramus of the fourth pair of cirri, some of the segments were
furnished with very broad and thick, small, downwardly curved, teeth
or hooks, like those described in certain varieties of _A. sulcata_;
but they are here stronger and thicker. The segments in the three
posterior pairs of cirri are not so much elongated, as in the other
species.

9. ACASTA SPORILLUS. Pl. 9, fig. 9 _a_-9 _d_.

_Shell purplish-brown, with the parietes internally strongly ribbed and reticulated: carino-lateral compartments extremely narrow, not extending down to the basis._

_Hab._--Sooloo Islands, East Indian Archipelago; Mus. Dana.

I am indebted to Mr. Dana, the distinguished naturalist of the United States Antarctic Expedition, for two specimens of this interesting species, which, in the singular reticulated structure of the inner surface of the walls, and in the almost rudimentary condition of the carino-lateral compartments, not extending down to the basal cup, is very distinct from the foregoing species. I have used Mr. Dana's very appropriate MS. name of _sporillus_. The specimens were dredged up, lying quite loose and unattached at the bottom of the Sooloo sea; the one which I opened, must have long lain dead; but Mr. Dana assures me that some were living, and he has sent me drawings of parts of the mouth and cirri: I am much surprised at this circumstance; for analogy would have made me believe that this species must have been imbedded in some sponge-like body, such as the bark of a zoophyte, and that it could not have lived unattached. I may add that a small fragment of a brown leathery substance adhered to the upper end of one of the two specimens, and this seems to indicate attachment.

_General Appearance._--Shell shaped like a pointed acorn; slightly
flattened; orifice extremely small; surface very finely punctured,
covered by a purplish-brown epidermis, with transverse stripes of
different shades, and with the apex dark; according to Mr. Dana, when
fresh, the colour was purplish-carmine. Radii narrow, white. The
carino-lateral compartments are extremely narrow; the wall-portion
(fig. 9 _b_) forming a mere linear rib, terminating downwards in
a sharp point, which does not reach the basal cup: hence this
compartment evidently tends to become rudimentary. The basal cup is
moderately deep and pointed. Basal diameter .16; height, from the
bottom of the cup to the top of the shell, .24 of an inch.

_Scuta_: narrow, with the upper part produced; not striated
longitudinally; coloured by a pale purple, longitudinal band.
Internally, there is scarcely a trace of an articular ridge, which,
in the other species, is always more or less developed. _Terga_ with
the spur bluntly pointed; nearly the whole basal margin, on the
carinal side, slopes towards the spur.

_Internal Structure of the Parietes, Radii, and Basal Cup._--The
parietes are strongly ribbed internally; and these ribs are connected
by very narrow, less prominent, transverse, slightly branched ridges,
giving a reticulated structure to the inner surface. Between several
of the main longitudinal ribs, in the lower part of the shell, new
ribs may be seen in process of formation, and these tend to convert
the reticulated structure into a double row of minute cells. I have
not met with an exactly similar structure in any other cirripede; but
I have no doubt that the little transverse ridges are homologous with
the transverse calcareous septa in the parietal pores of many Balani,
in the same manner as the internal longitudinal ribs, in this and
other species of Acasta, are homologous with the longitudinal septa
forming the above pores. The edge of the basal cup is pectinated with
teeth, which lock into the teeth formed by the ends of the internal
parietal ribs. The radii are narrow, and have smooth edges. The alæ
project beyond the parietes to a remarkably small extent. The sheath
is free, or hollow beneath. I have already described the almost
rudimentary condition of the carino-lateral compartments; this is
best exhibited in an internal view of the two compartments, as given
in Pl. 9, fig. 9 _b_.

_Animal's body_ unknown to me: from Mr. Dana's drawing the three
posterior pairs of cirri seem to have been much elongated: and the
rami of the first pair, as usual, unequal in length.

M. Deshayes has given an indifferent figure and imperfect description of ACASTA TUBULOSA (Guerin, Magasin de Zoologie, 1831, Tab. 39; and Guerin, Iconographie du Règne Animal, Mollusques, Tab. 38, fig. 4, but here by a misprint called _A. spinulosa_); it is utterly impossible to recognise the species of this genus from such materials.

3. _Genus_--TETRACLITA. Pl. 10, 11.

TETRACLITA. _Schumacher._ Essai d'un Nouveau Syst., &c., 1817.

CONIA. _Leach._ Journal de Physique, tom. 85, 1817.[105]

ASEMUS. _Ranzani._ Memoire di Storia Naturale, 1820.[106]

POLYTREMA. _De Ferussac._ Dict. Classique d'Histoire Naturelle, 1822.

LEPAS. _Gmelin._ Systema Naturæ, 1789.

BALANUS. _Bruguière._ Encyclop. Method., 1789.

---- _Lamarck._ Animaux sans Vertèbres, 1818.

[105] From a note by the Editor, it appears that Schumacher's essay
appeared before the number of the Journal containing Leach's paper,
so that Schumacher's name must be adopted.

[106] I have not seen a _complete_ copy of this work, and give the
title from a catalogue; the running heading of the part containing
the Cirripedia, is "Opuscoli Scientifici."

_Compartments four, sometimes externally calcified together: parietes permeated by pores, generally forming several rows. Basis flat, irregular, calcareous, or membranous._

_Hab._--Throughout the tropical and warmer temperate seas.

_General Appearance._--The shell is conical, more or less depressed, and very rarely, even when growing in crowded groups, becomes cylindrical or elongated. The orifice is seldom large, generally diamond-shaped or oval. The colour is either nearly white or purple, occasionally even inky black, or very dark green, and sometimes of a pale pink peach-blossom. The surface is sometimes smooth, but more commonly longitudinally ribbed; the outer lamina of shell is very often wholly corroded away, excepting close to the basis, leaving the solidly upfilled parietal tubes exposed: these give the shell a striated appearance, or they appear like flattened tapering points adpressed to its surface (Pl. 10, fig. 1 _b_): Lamarck attempted to express this appearance, by using the specific name of _stalactiferus_. The colour of the shell depends, to a considerable extent, on the colour of the shelly matter in these exposed parietal tubes. We shall presently see that the corrosion and disintegration, to which some of the species are so liable, plays an important part during their growth. The radii are either well developed, as in most of the species; or they are entirely absent, as in the great majority of specimens of _T. porosa_ and _serrata_. In many individuals of _T. porosa_ and _purpurascens_ not only are the radii absent, but the four compartments are calcified together without any trace, on the external surface, of the sutures. The largest specimen which I have seen of _T. porosa_, which is the largest species, was two inches in basal diameter, and nearly one inch and a half in height.

_Scuta._--These valves are sub-triangular, and either longitudinally or transversely elongated. Externally, the growth-ridges are moderately prominent, and in _T. costata_ and _cœrulescens_ they are crossed by longitudinal striæ. Along the occludent margin, the inflected extremity of each alternate growth-ridge is generally much thickened,--a set of teeth being thus formed, by which the two valves are locked together. In _T. porosa_, this character is variable, for sometimes every alternate ridge, and sometimes only two or three ridges, separated from each other by several growth-ridges, are thus developed into teeth. The _articular_ ridge is either moderately prominent, or is extremely prominent, as in _T. cœrulescens_; but the lower edge in no case depends as a free, hinge-like style, as sometimes in Balanus. The _adductor_ ridge is generally well developed and distinct from the articular ridge: in _T. purpurascens_ it is very blunt: in _T. serrata_ it is united to the articular ridge half way up it, thus forming a deep tubular cavity running up to the apex of the valve: in _T. cœrulescens_, the adductor ridge is very short, and is united to, or almost continuous with, the lower end of the articular ridge, a small sub-cylindrical tubular cavity being thus formed. Small crests exist for the attachment of the rostral and lateral depressor muscles, in most of the species, excepting _T. purpurascens_ and _costata_, in which, however, more especially in the former, there are, instead of crests, minute pits for the attachment. These crests vary much in prominence in the same species.

_Terga._--These valves present no essential differences from those of Balanus; they are sometimes beaked, and the beak is hollow and occupied by a thread of corium, as in that genus. The external surface of the valve is often depressed in the line of the spur, but there is never a longitudinal furrow with the edges folded in, as in Balanus. The spur is very short in _T. purpurascens_. In _T. radiata_, the articular ridge is remarkably prominent. The crests for the depressor muscles are well developed in all the species. The shape of the terga is variable in nearly all the species, and greatly so in _T. porosa_.

_Compartments._--Owing to there being only four compartments, the lateral pair are large; the size of the carina relatively to the rostrum varies, according as its alæ have been added to during diametric growth. The walls are very thick, and are composed of numerous tubes, in some species as many as fourteen or fifteen rows being exposed on the basal margin (Pl. 10, fig. 1 _g_). The tubes are generally angular, and slightly elongated in the ray of the circular shell; sometimes they are nearly circular and small. New tubes are formed only at the basal edge of the outer lamina, by the bifurcation of the septa which form the tubes. In very young specimens there is only a single row of tubes; and in _T. rosea_ this holds good throughout life: in this species (fig. 3 _d_) the tubes, in the single row, are large and quadrangular, and the outer lamina of shell is strengthened by numerous, small, internal, longitudinal plates. I believe the branching septa, which separate and form the parietal tubes, correspond with the longitudinal septa in the more simple walls of Balanus. The tubes become solidly filled up, in their upper parts, with hard, and generally coloured shelly matter. The degree to which they are filled up differs in the different species; the external side of each tube is always first thus coated. The thin outer lamina of shell, in several of the species, commonly disintegrates and disappears; the upfilled parietal tubes being thus exposed. The inner lamina of the walls is generally smooth, but in _T. radiata_ it is longitudinally ribbed, as in most species of Balanus. The sheath is generally dark-coloured; its lower edge does not project or overhang the inner lamina, as is usual in Balanus, excepting in _T. serrata_, and in some few varieties of _T. porosa_.