Peculiarities in the Structure of the Radii.—In some of the species of [Tetraclita], in which genus the walls consist of several rows of tubes, the radii are likewise perforated by several rows; and in some of the other species (Pl. [10], fig. [1 h]), the edge, or disarticulated surface of the radius, is marked by irregularly branching ridges; and these evidently correspond with the branching septa or ridges of the wall. In [Chelonobia], the outer lamina of the radius, as well as of its recipient furrow, is of extraordinary thickness; and this lamina, in [C. testudinaria] (Pl. [14], fig. [1 a], [5], b, and Pl. [15], fig. [1], f), is modelled into sharp transverse ridges and valleys. In the [Chthamalinæ], the radii, like the parietes, are simply solid; and apparently in consequence, for the sake of strengthening the sutures, the edges of the radii, and of the recipient furrow in [Octomeris] (Pl. [20], fig. [3 a]) and in [Chthamalus dentatus] and [Hembeli] (Pl. [18], fig. [3 b], [5 a]), are neatly dentated. In some other species of [Chthamalus] (Pl. [19], fig. [1 a]), the radii present a slight modification of this structure, the sutures being formed by oblique interfolding laminæ. In the radii of [Coronula] and [Tubicinella], there is a peculiarity, in apparent connection with the fact, that in these genera the parietal tubes are not crossed by transverse calcareous septa, namely, that the pores by which the radii are permeated keep unclosed throughout their length, and open into a special longitudinal tube (Pl. [16], fig. [7], d′), which runs along that margin of the wall, whence the radius arises. In [Coronula] the wall is of extreme thinness, and in conformity so is the true radius, but that the shell might not thus be rendered very weak, complementary or pseudo-radii are developed on their inner sides (Pl. [16], fig. [7], adjoining the true radii A d, C d, and shaded by distant convex lines). Even in the allied genus [Xenobalanus], in which the whole shell tends to become rudimentary, traces of these pseudo-radii (Pl. [17], fig. [4 b], d) can be detected. In [Coronula], though the radii (Pl. [16], fig. [7], A d, C d) are, by the above special means, rendered thick, and though the alæ also are thick (C a′, D a′), yet together they do not equal in thickness the folded walls; and consequently, there is left between the radii and alæ square chambers (v), occupied by the branching ovarian tubes.
Structure of the Alæ.
These project, generally abruptly, from the sides of the upper part of the compartments; they appear from the first growth of the shell; they are overlapped by the radius and by part of the wall of the adjoining compartment; they are thinner, and have, owing apparently to being overlapped, a very different aspect from the parietal portion; but they do not differ from it in essential nature. They are solid, that is, they are never permeated by pores; but their edges are generally crenated, and there is, in some cases, as in [Chelonobia], sufficient evidence that these crenations answer to the horizontal septa on the edges of the radii (also often reduced to mere crenations), and consequently, likewise, to the longitudinal septa of the parietes. In [Coronula] the edge of each ala consists of a medial ridge, sending off denticulated septa on both sides, and is therefore anomalous as compared with the alæ in other genera, but corresponds in structure with the similarly anomalous radius of [Coronula]. In order to allow of the growth of the edge of the ala, a fine thread of corium runs up the narrow furrow in which the edge is lodged, proceeding from the corium of the sack. In proportion as this thread runs up higher or lower, so are the summits of the alæ rendered, during growth, less or more oblique.
Structure of the Sheath.
As the compartments overlap each other, the edges of the alæ would have projected, and the inner surface of the orifice of the shell would not have been smooth and rounded, had not that part of each wall, which does not overlie an ala, been thickened so as to allow of the formation of a shoulder or indentation, against which the edge of the ala fits and abuts. The thickened portions, and the alæ themselves, together form the sheath, of which the use seems to be to strengthen, like a broad internal hoop, the upper part of the shell round the orifice, where naturally it is weak. The sheath is composed of successive, fine, shelly layers, which extend, as the shell is added to at the basal margin, lower and lower down on the inner surface of the walls. The lower edge of the sheath either simply projects a little inwards, or more commonly is formed into a sharp depending ridge, as represented in fig. [1], K′, Pl. [25]. In some species of [Pyrgoma] (Pl. [13], fig. [2 b]), the sheath reaches nearly to the bases of the compartments; and in [Chelonobia] (Pl. [14], fig. [4] e c e), the inner layer of shell surrounding the sack, which seems to correspond more nearly to the sheath than to the inner lamina of the walls, actually rests on the basal membrane. The opercular membrane is generally, but not invariably, attached only a little way above the lower edge of the sheath: at each exuviation, a new opercular membrane is formed, and is attached to the next lower zone of the sheath; the old membrane being cast off, but a circular slip of it is left, investing the last zone. Hence the whole upper part of the sheath above the opercular membrane, comes to be thus invested; and is marked by circular lines, one above the other, caused by the successive exuviations. This investing membrane often supports rows of minute bristles, directed upwards. Generally, a film of shell is deposited, at the period of the formation of each new opercular membrane, on that part of the sheath which lies immediately beneath. This innermost film or thin layer of shell, on the lines of suture between the compartments, breaks joint, at least in some cases, with the underlying shelly layers,—that is, the suture in this last-formed film does not lie exactly over the suture in the subjacent layers of the sheath. In [Tubicinella], the sheath extends down close to the basis; and what is unique in this one genus, the opercular membrane, gradually thinning out downwards, closely adheres to the whole inner surface of the shell. In [Tubicinella] and in [Xenobalanus] (Pl. [17], fig. [4 b]), the sheath separates easily into separate successive rims of shell; and this structure evidently is for the sake of facilitating the breakage of the upper end of the shell, which, as we shall presently see, is necessary to allow of the increase in size of its orifice.
Structure of the Basis.
This, in several genera and species, is composed of simple membrane, and consists of successive, concentric, circular slips, added round the outside, at each period of growth. In some species of [Tetraclita] and [Balanus] the basis is calcareous, but diaphanous, very thin, smooth, or somewhat granulated. In other cases it consists of a single calcareous lamina, either smooth, or with ridges radiating from its centre; it is formed of two laminæ, (as is most usual in [Balanus],) separated by radiating septa. These septa, as well as the radiating ridges in the case of the single lamina, are homologous with the longitudinal septa of the parietes. The denticulated ends of the latter enter the mouths of the tubes formed by the radiating septa of the basis: threads of corium pass between the denticuli of the parietal septa, and thus enter the basal tubes. The ends of these threads of corium generally deposit transverse calcareous septa, exactly as within the parietal tubes. When the basis is thick the septa themselves (ccc) between the proper basal tubes, become porose, (or rather cancellated,) and they sometimes expand into a very thick, cancellated layer, separating the outer lamina (a) of the basis from the proper basal tubes, which always lie close under the inner lamina (b). This structure differs only slightly from that seen in the parietes of [Tetraclita], in which the branching of the longitudinal parietal septa, produces thick walls, formed of several rows of tubes or pores. With respect to peculiarities in structure of the basis, [Balanus lævis] offers the most remarkable case; for here, in specimens which have grown crowded together, the whole interior appears sometimes to have become too much elongated or too deep for the animal’s body, and consequently the lower part of the deeply-concave basis has been filled up (Pl. [4], fig. [2 a]) by thin, irregular, calcareous diaphragms. In elongated specimens, also, of [Balanus balanoides], the shell sometimes appears to have grown too long for the animal’s body; but in this case the membranous basis becomes extremely convex inwards; it still reaches the basal edges of the parietes all round, but in the middle it is raised high above the surface of attachment; yet sometimes threads of the cementing tissue depend from the middle part to the surface of attachment. In [Balanus terebratus] (Pl. [8], fig. [2 a], [2 b]), and in some species of [Acasta], the basis is riddled, as previously stated, by numerous, minute, membrane-covered orifices. In [B. declivis] the membranous basis is always extremely oblique, owing to the rostral end of the shell being twice as high as the carinal and opposite end.
Fig. 11.
