3. Genus—TETRACLITA. Pl. 10, 11.

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

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.

The Radii, when developed, are either narrow or broad, with their summits either oblique or extending in a straight line from the top of one compartment to that of another. In T. serrata, I have not seen a single specimen with the radii developed; in T. porosa, they are very seldom developed, and then, apparently, only in quite young specimens, in which they are narrow; in T. purpurascens, they seem to be about as often developed as not, and when present they are broad; in T. costata, cœrulescens, and radiata, they are always largely developed. In some specimens of the species, in which the radii are not developed, even the sutures do not reach the external surface; the outer lamina of the parietes being continuous all round, so that the shell seems formed of a single piece. Even in such specimens, the four compartments, viewed from within, can be seen to be distinct; and the sutures can generally be traced across the whole thickness of the parietal tubes; in this latter case, when the sutures are broken open, the radii are seen to be represented (Pl. 10, 1 h) by a few small sinuous ridges. Owing to the disintegration of the upper and outer part of the shell, and the consequent exposure of parts of the sheath and alæ, the radii sometimes appear as if developed, when such is not the case. With respect to the internal structure of the radii, they are formed, in T. purpurascens and costata, of tubes, like those of the parietes, and therefore according to the normal plan; whilst in the other species they are formed by longitudinal sinuous ridges, sending out on each side irregular denticuli; and the interspaces between the ridges are filled up solidly during the growth of the radii, in all the species, except in T. radiata, in which they are left to a considerable extent open. These sinuous ridges, with their denticuli, homologically represent the branching septa which form the parietal tubes. The edges of the alæ are crenated in all the species, except in T. costata.

Diametric growth.—When first examining groups of T. porosa, in none of which the radii had been developed, and in which, consequently, the shell could not have grown in diameter, but only at its basal margin, I was at first unable to comprehend, how the upper part of the shell and the orifice could have acquired their proper proportional width. The young shell, at its first formation, starts with an orifice so small that a pin could hardly be inserted in it; and this in many individuals is never increased in diameter by the diametric growth of the shell; but in place of this, as the conical shell is added to at its base, the whole upper part disintegrates and wears away, the orifice becoming thus enlarged. We thus see that the corrosion and wearing away of the upper part of the shell is a necessary element in its growth. The development of the radii, which in some of the species, as in T. purpurascens, at first seems to be quite capricious, really depends upon the fact, whether the specimens have been exposed to disintegration; for I have almost always found that when the outer lamina of shell has been well preserved, the radii have been developed, and the orifice has been enlarged by their growth, instead of by the wearing down of the upper part of the conical shell.

Basis.—This consists of a very thin, flat, though irregular, translucent, calcareous plate, which towards the edges is sometimes membranous. In T. purpurascens, the basis is entirely membranous. When a portion of the calcareous base is dissolved in acid, a tissue is left, composed of several laminæ, to which numerous bifurcating cement-ducts are attached: even before dissolution, these delicate bifurcating ducts can just be perceived by the aid of a simple lens.

Mouth.—The several organs present no particular characters. There are generally three teeth on each side of the notch in the labrum. The palpi usually have parallel sides, but are club-shaped in T. purpurascens and costata. The mandibles have generally four teeth, but there are five in T. vitiata, and only three in T. costata. The maxillæ are notched. The outer maxillæ are bilobed in front.

Cirri.—The segments of the three posterior pairs usually support only three pairs of main spines, but there are four pairs in T. vitiata and costata: between each pair, there is either a tuft of fine spines, or a single fine spine. The rami of the first cirrus are unequal in length. In the third cirrus, the posterior ramus is sometimes much elongated, but sometimes both rami are short and blunt. Some of the segments in the third cirrus often support very coarsely and doubly pectinated spines. Under the head of T. porosa, it will be seen to what a remarkable degree the relative numbers of the segments in the several cirri vary, even in specimens taken out of the same cluster.

The Branchiæ are well developed (at least in T. porosa and costata), as a large, plicated, tapering fold, with a small second fold on the inner side at the base. In T. porosa the stomach is destitute of cæca. The vesiculæ seminales in this same species are large, with their broad, blunt ends reflexed. The ovarian tubes surround the sack, and cover the basal plate.

Distribution and Habitats.—This genus is confined to the tropics, and to the warmer parts of the temperate seas: in the southern hemisphere, it ranges south, to the Cape of Good Hope and to Van Diemen’s Land: in the northern hemisphere it does not appear to range so far; I do not know of any authentic case of a species having been found in the Mediterranean, or on the shores of the United States, north of the West Indies. Tetraclita porosa is found round the whole world; T. radiata, also, has a very wide range, inhabiting the West Indies, the East Indian Archipelago, and New South Wales. This latter species, as well as T. cœrulescens, is often attached to the bottoms of ships, adhering to Balanus tintinnabulum. The several species live attached to tidal rocks, to littoral shells, or to massive corals. I have met with two instances, in the West Indies and the Philippine Archipelago, of T. porosa adhering to wood. Tetraclita porosa seems to feed chiefly on crustaceans: the number and the size of the amphipods, isopods, and entomostracans, together with an annelid, in the stomachs of some specimens from South America, was quite surprising. As many as five species occur in the same region, in the eastern half of the world; thus on the shores of New South Wales, we have T. porosa, vitiata, radiata, purpurascens, and rosea; in the Philippine Archipelago, we have T. porosa, vitiata, costata, and cœrulescens.

I have not seen any species of this genus fossil.

Variation.—The species vary in shape nearly, but not quite as much as in Balanus, for we very rarely here see cylindrical varieties; but in the same species, we have extremely depressed, steeply conical, and convex forms; the orifice varies in relative size; the state of the surface, whether ribbed or smooth, whether well preserved or corroded, the upfilled parietal tubes being thus exposed, varies more than in Balanus. The colour also varies; specimens of T. porosa (Pl. 10, fig. 1 a to 1 f) being dark purple, or even inky black, white, pale pinkish-purple, and green: as far as I have seen, the pinkish varieties of T. porosa are confined to the eastern half of the world. Even the ridges and crests on the under side of the scutum vary in some degree; and the tergum in T. porosa varies considerably, and in some of the other species, slightly, in general shape: I believe that the tergum becomes narrow and elongated, when the shell is steeply conical, with the orifice of small diameter. After having spent several days in disarticulating and closely examining the many specimens of T. porosa in my possession, I concluded that its varieties formed at least four species, these being in external appearance extremely distinct; but ultimately the many intermediate forms compelled me to unite all into a single species. Again, I may instance the conical, ribbed variety of T. purpurascens (Pl. 11, fig. 1 b), with the outer lamina of the shell preserved and with the radii widely developed, as having not the smallest resemblance to the other common depressed variety (fig. 1 a), having a granulated surface, produced by the exposed tips of the upfilled parietal tubes, and without a trace of the radii or even of the sutures; I may add that, according to the characters used by some authors, these two varieties would be classed in two separate genera.

To distinguish the species of this genus, the chief reliance must be placed (as in the case of most other sessile cirripedes) on the general outline of the opercular valves, and on the ridges and crests on their under sides: the internal structure, however, of the radii, and in a lesser degree of the parietes, must not be overlooked. I have not found the parts of the mouth, or the differences in the cirri, of much service; and it will be shown under T. porosa, that the relative numbers of the segments in the several cirri, and even their shape, is extraordinarily variable.

Affinities.—All the species are pretty closely allied, and there is no ground for making any sectional division of the eight species, more especially not on the ground whether or no the radii are developed. Tetraclita purpurascens, taking into account all the characters, including the animal’s body, is perhaps the most distinct species in the genus, but it is closely allied to T. costata. In the well-marked character of the parietes being formed of a single row of large tubes, T. rosea differs from all the other species. The genus is closely allied to Balanus; I can point out no difference in the animal’s body, nor any constant difference in the opercular valves. The ridges and crests on the under sides of the scuta are more prominent in Tetraclita; and all the species, excepting two, have crests (though sometimes very slight) for the attachment of the rostral depressor muscle, and these occur only in two species of Balanus: crests, also, for the attachment of the lateral depressor muscle are common in Tetraclita, but rare in Balanus. The main character, however, of the genus, as compared with Balanus, is derived from the existence of only four compartments, and in a lesser degree from the several rows of parietal pores; but, as just stated, T. rosea has only a single row, and, on the other hand, in Balanus crenatus, there is a slight tendency exhibited, in the dividing of the longitudinal septa near the outer lamina, to form a second row of parietal pores; and in B. cariosus, moreover, we actually have two or three rows of irregular and very short pores. The thin, yet solid calcareous basis which occurs in all the species of Tetraclita, excepting T. purpurascens, resembles the basis in Balanus flosculus and imperator, but I suspect that the structure of the cement-ducts is different in the two genera.

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