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.