-------- De Blainville. Dict. des Sciences Naturelles.
ASTROLEPAS. J. E. Gray. (Klein) Annals of Philosophy, (new series), vol. 10, (1825).
Compartments extremely thick, six; but one of them, the rostrum, internally is composed of three rudimentary compartments, united together: basis membranous: scuta narrow, united to the terga by a horny articular ridge.
Distribution, throughout the tropical, and warmer temperate seas of the whole world; attached to turtles, crustacea, or smooth gasteropod molluscs.
This is a distinct and well-defined genus. Several authors have confounded it with [Coronula], but this has been owing to an entire misapprehension of the structure of the shell in the two genera. In [Coronula], the parietes are very thin, and are so deeply folded as to appear like rays or septa connecting the outside of the shell (the expanded ends of the folds) and the internal surface of the shell, but the open spaces between the folds are occupied by the epidermis of the Whale, and are external to the cirripede. In [Chelonobia], the parietes are remarkably thick; hence the plates or septa, connecting the outer and inner lamina, are of unusual length; and the spaces between them, though of course internal with regard to the cirripede and occupied by the ovaria, have been compared to the spaces external to and between the folded walls of [Coronula]. There is but little special affinity between these genera; and I regret that they have come to be placed one after the other in this work: but the elongated opercular valves,—the thick and double opercular membrane,—the weak depressor muscles,—and the peculiar manner in which the scutum is articulated by the aid of a horny projection to the tergum, may indicate some real but slight affinity to [Coronula]; the many points of difference, however, in the structure of the shell and of the opercular valves, and especially in the cementing apparatus of the basal membrane, and in the branchiæ, all prove that the genera are very distinct. The singular structure of the rostrum, which, in fact, consists of three compartments externally blended together, and which three correspond in all essential respects to the rostrum and two rostro-lateral compartments in the [Chthamalinæ], offers a very striking point of identity with that sub-family; but neither in the mouth, cirri, or other part, can I detect any other evidence of this relationship. Having so far discussed the affinities of the genus, I may add, that the three species, though decidedly distinct, are closely and nearly equally related to each other.
General Appearance.—The shell is generally depressed, and broadly oval or almost circular; in [C. testudinaria] and [caretta], it has a massive appearance: the surface is generally smooth, or, when disintegrated, finely striated: the colour is white. The six compartments do not differ much in size: the rostrum is rather larger than the carina, and the lateral compartments, than the carino-lateral compartments. It is remarkable that in [C. caretta] (Pl. [14], fig. [2]), even in specimens which have not grown crowded together, the compartments are almost invariably placed rather unsymmetrically, the rostrum and carina not exactly facing each other.[121] The shell, though so thick and massive, yields easily along the lines of suture. The radii are moderately wide, or narrow, or not at all developed, being represented by mere sutures: in this latter case, in [C. caretta], the orifice of the shell is enlarged, in the same manner as we have seen in some species of [Balanus] and [Tetraclita], by the gradual wearing away of the upper part of the shell. In [C. testudinaria], the radii have a singular notched structure (fig. [1 a]), and the whole shell a star-like appearance. The orifice is not filled up by the elongated opercular valves,—a considerable extent of opercular membrane being visible on the two sides. The largest specimen which I have seen, namely, of [C. testudinaria], was nearly two and a half inches in its longer diameter.
[121] In Mr. Stutchbury’s collection there is a specimen of [C. testudinaria] in which there are only five compartments, one of the lateral compartments having been aborted; of this I have seen no other instance in any genus.
Structure of the Parietes.—The parietes are of unequalled thickness, especially in the first two species of the genus. From the outer lamina (see Pl. [14], fig. [4], and the section in Pl. [15], fig. [1]), numerous vertical plates extend inwards, alternately to a less or greater distance, some of them reaching to the inner lamina: these answer to the longitudinal parietal septa in other genera, and the elongated cavities between them (which occur in [C. testudinaria] and [patula]) answer to the parietal tubes or pores. The radiating plates or septa have their sides finely channelled, and their basal edges generally slightly sinuous and always finely toothed. The interspaces between the plates in the uppermost part of the shell are filled up solidly, and, in [C. caretta], even down to near the basis: in this latter species, the plates are irregular and much broken up, so as in parts to consist of little, separate, flattened points. In [C. patula], the inner lamina of the parietes (b, in fig. [4]. Pl. [14]) can be best made out to be distinct from the sheath (e c e in fig. [4], and c′ in the section of [C. testudinaria], fig. [1], Pl. [15]). The sheath in this genus descends in a very remarkable manner to the basal membrane, and has its basal edge toothed like the basal edges of the radiating septa. The inner lamina itself does not descend to the basal membrane. In [C. testudinaria], the inner lamina is of great thickness; but in the section, (fig. [1]) owing to its having been taken high up, the inner lamina, (b), is not distinct from the shelly matter deposited between the septa. In [C. caretta], the line of separation between the inner and outer laminæ can in no part be distinguished, owing to the interspaces between the septa having been solidly filled up, close down to the basis.
Sheath.—The layer of shell surrounding the internal cavity (e c e, fig. [4], Pl. [14]), and extending down to the basal membrane, I have called the sheath, owing to its being distinctly continuous with the innermost layer in the upper part of the shell, to which the opercular membrane is attached: this can be best seen by examining the alæ in the separated compartments of [C. patula]. The sheath is not only remarkable from thus descending to the basal membrane, but in [C. testudinaria] and [patula] from its lower edge being perforated by arched channels (under c, in fig. [4]), allowing thick ribbons of corium to reach the interspaces between the radiating septa. There is one central arch or channel in the middle of the lower part of the sheath of each compartment, and one on each line of suture, the sheath being a little hollowed out on both sides of the sutures. As the rostrum, as far as its internal structure is concerned, consists of three compartments, we have altogether in the shell eight compartments and eight sutures, and consequently altogether sixteen arches through the lower part of the sheath, allowing sixteen thick ribbons of corium to penetrate the parietes, and thus likewise reach the radii. There is, however, sometimes a little variation in the number of these arched channels. The upper part of the sheath is transversely marked by zones of growth, to the lower one of which the opercular membrane is attached. The line of attachment is not low down the sheath.
Radii and Alæ.—The radii, when the compartments are disarticulated, present a remarkable structure, from appearing to consist of a distinct inner and outer portion. The radius normally consists of an inner and outer lamina, united by septa parallel to the basis; but here the inner portion is formed by a central ridge (a a, fig. [5], Pl. [14]), sending off on both sides little septa, often sub-branched; it is of nearly uniform width; and there is no distinct inner lamina. The outer portion (b), which often equals or exceeds in thickness the inner portion, is, in fact, the normal outer lamina, developed to an unparalleled degree. In most Cirripedes the edge of the radius is received in a slight furrow in the opposed compartment, the lid of which furrow is narrow, and matches the outer lamina of the radius; here the lid of the recipient furrow is very broad, and resembles the outer lamina of the radius in all its characters. In order to allow of growth between the thick opposed edges of the outer lamina of the radius and the lid of the recipient furrow, the two surfaces are finely dentated (look in fig. [5], under the pits, marked b), almost like the crown of a molar tooth; thus allowing films of corium to enter. The structure here described is common, in a greater or less degree, to all three species, but is best seen in [C. testudinaria]. In this species, moreover, (fig. [5], b), the outer lamina, instead of being smooth and of either equal or gradually increasing thickness from top to bottom, is generally, but not always, (fig. [1 a]), deeply pitted or notched in transverse lines, the outer lamina being thus rendered alternately thicker and thinner, and so formed into transverse ridges and valleys. Hence the lines of suture become toothed, the points of the teeth facing each other, and not interlocking. In the transverse section, fig. [1], Pl. [15], of the same species, taken high up across the shell, (f) is the pitted outer lamina, and (e) the inner portion of the radius. Although the radii are thus specially added to in thickness, they are not so thick as the very thick walls, and hence the lines of suture form furrows more or less deep. The alæ are of moderate thickness, and have their sutural edges crenated by fine transverse septa.