CETOPIRUS (SED NON CORONULA). Ranzani. Memoire di Storia Naturale (1820).
POLYLEPAS. J. E. Gray, (Klein). Annals of Philosophy, (new series), vol. 10, 1825.
Compartments six, of equal sizes: walls thin, deeply folded, with the folds forming cavities, open only on the under side of the shell: opercular valves much smaller than the orifice of the shell.
Distribution, mundane, imbedded in Cetacea.
This genus and the three following, namely, [Platylepas], [Tubicinella], and [Xenobalanus], have very considerable claims to be separated as a sub-family, as has been proposed by Drs. Leach and Gray. Although in structure and habits they are certainly closely related together; yet only few characters can be predicated of all four in common,—some characters failing in one genus and some in another. All four, however, differ from the foregoing genera in the opercular valves not being articulated together, being simply united by tough horny membrane; but [Chelonobia] makes some approach in this respect. All four have the branchiæ composed of a double fold; but this can hardly be considered a character of much weight, as may be inferred from the remarks made on this subject at p. [153] of the [Introduction], where the differences of these four genera from the other genera of the family are discussed at some length. Altogether I have been led to conclude, though with much hesitation, that these genera had better not be separated as the sub-family of Coronulinæ.
Our present genus, [Coronula], is closely related to [Platylepas], and likewise to [Xenobalanus], though this latter genus is so very different in external aspect that it might easily be mistaken for a pedunculated Cirripede. Lamarck and some other authors have placed the species of [Chelonobia] under [Coronula], but this has arisen, as explained under that genus, from a misapprehension of their structure; the folded very thin walls in [Coronula] having been compared with the radiating septa of the very thick walls in [Chelonobia]. I may further add, that [Coronula] has been divided into two genera by Ranzani, on palpably insufficient grounds.
General Appearance.—The three recent species of this genus have a very handsome and striking appearance. The shell is highly symmetrical, owing to the six compartments being of the same size and having exactly the same outline. The general shape is either depressed or, as expressed by the name, like a crown. The walls are longitudinally and slightly ribbed, owing, as we shall presently see, to their wonderfully folded structure; and the surface is marked by very fine longitudinal striæ, crossed by finely beaded lines of growth. Hence the walls offer a strong contrast in appearance with the six, smooth radii, of equal breadth. The symmetrically hexagonal, or rounded-hexagonal orifice of the shell is closed by a thick, nearly horizontal membrane, supporting, towards the rostral end, the small opercular valves, with a slit, having protuberant lips, in the middle, for the protrusion of the cirri. The opercular membrane is attached all round, but a short distance beneath the summit of the shell. In regard to size, I have seen a specimen of [C. diadema] two inches in height and two and a half in diameter.
Structure of the Shell.—The structure at first appears singularly complicated, and quite unlike that of any other Cirripede; but the whole results simply from the folding of the very thin walls, which in all essential respects are constructed like those in [Balanus]. In a young specimen of [C. balænaris], having the orifice of the shell 2/10ths of an inch in diameter, I found the upper part of each compartment only slightly sinuous, not more so than is common in many varieties of [Balanus], but more symmetrical, for each compartment had three slight furrows, making, for the whole shell, eighteen furrows. The ensuing changes during the growth of the shell will be best understood by looking at the diagrams a, b, c, in fig. [10], Pl. [15], which are supposed to represent the basal margins of the walls of a single compartment: (a) shows the simply sinuous wall of the young shell. As the shell grows, the furrows rapidly grow deeper and deeper (b), and wider and wider; at the same time, the folds or ridges between the furrows gradually become drawn out at their ends into transverse loops (c), the extremities of which ultimately become closely pressed together—the furrows being thus converted into cavities, extending from the top to the base of the compartments. This structure in the mature shell will be best understood by looking at the transverse or horizontal section (Pl. [16], fig. [7]) of the rostral end of the shell of [C. diadema], in which species the folding of the walls is simpler than in [C. balænaris] or [C. reginæ]: the walls (e) are represented by a double line connected by little cross lines,—(h) being one of the transverse loops at the outer end of one of the folds, and (f′) one of the cavities between the folded walls, open at the bottom of the shell, and occupied by the epidermis of the Whale. The walls of the compartments, as here represented, (A being the rostrum, C C the lateral compartments, D D the alæ of the carino-lateral compartments,) are separated from each other by the broad radii; but if the section had been taken low down near the basis, the end of the folded wall of one compartment would have been separated from that of the adjoining compartment only by the close suture: this will be understood by a glance at the entire shell, given in fig. [3], Pl. [15]. The rostrum of the same species, viewed from the inside, is shown at fig. [1], Pl. [16]; here it may be observed, that the basal margin (e e′ e′′) of the folded wall is extremely oblique, the outer portion having extended downwards much more than the inner portion: this obliquity is more clearly shown in the lateral view (fig. [2]) of a lateral compartment, for this figure will equally well serve for a lateral view of the rostrum (fig. [1]), if the ala (a′) be supposed to be removed. The section of the rostrum in fig. [7], (A), will now be intelligible in relation to the view of the rostrum given in fig. [1], if it be borne in mind that the section has been taken high up, near the letters a, b, c′′ in fig. [1].
As above stated, the folds or ridges in the young shell become more and more transversely drawn out at their ends into the transverse loops, till the latter join and touch each other. In [C. balænaris] the lines of junction are simple, though very close; in the other species, the ends of the transverse loops, where touching, are finely and elegantly toothed, and thus locked together. These teeth appear single when the shell is viewed either externally or internally, but when the walls along the lines of junction are forced apart, they are seen really to consist of transverse rows of minute teeth. These teeth are less distinct, forming only sinuous ridges in [C. barbara] (fig. [6], Pl. [15]); in all cases the teeth are formed by the modification of the very minute beads, which ornament the lines of growth on the external surface of the shell. In the rostrum of [C. diadema], as seen internally (fig. [1], Pl. [16]), the serrated and closely-fitting lines of junction (f) between the ends of the transverse loops of the folded walls are plainly shown. It is the more or less rounded surfaces of the transverse loops which give to the external surface of the parietes its longitudinally ribbed structure: the ribs are plainest in [C. diadema], fig. [3], Pl. [15]. The shell, in fact, as seen externally, consists of but an extremely small portion of the external surface of the whole length of wall, being exclusively formed of the transversely looped ends of the radiating folds, together with the radii. Owing to the ends of the transverse loops being so closely pressed together, the furrows are practically converted (as already remarked) into cavities, open only on the under side of the shell, and extending from the oblique bases of the compartments up to their apices; and these are invariably filled by the black epidermis of the Whale. Owing to this circumstance, the skin of the Whale has been mistaken by some authors for parts of the Cirripede! In [C. diadema], in which the summit of the shell is often a little disintegrated, the whale’s skin is often there exposed, forming three black spots at the top of each compartment. It should always be remembered that these flattened and deep cavities are furrows, which homologically ought to be open in longitudinal lines along the external surface of the shell, from the top to the bottom.