The Cirri are short and extremely much flattened: the three anterior pairs have their rami unequal in length by two or three segments; the posterior edges of their pedicels are fringed by tufts of extremely fine hairs. The pedicel of the first cirrus is very broad; its rami are short, with the segments very broad. The rami of the second and third cirri are short, with the segments protuberant in front and thickly clothed with spines; the terminal segments have some short, thick, claw-like spines. The three posterior pairs have protuberant segments, each supporting three or four pairs of short, strong, main spines, with a small intermediate tuft: the dorsal tuft is small.
The prosoma is of large size. The stomach is large, without cæca, but with some internal longitudinal plaits (in [C. balænaris] at least), showing a tendency to the formation of cæca. In the stomach of [C. balænaris] I found a considerable quantity of a conferva, too much, I think, to have got in accidentally.
Generative system.—The vesiculæ seminales are large; and at their broad blunt ends, in [C. balænaris], four separate vasa deferentia enter, of which fact I have seen no other instance. The ovarian tubes do not extend up the sides of the sack, but lie at the bottom, over the basal membrane; in [C. balænaris] they send six short ray-like prolongations into the six sutures; in [C. diadema] they send similar prolongations into the sutures, and fill up, as I believe, the six chambers (Pl. [16], fig. [7], v) lying between the radii and alæ: I have examined only one specimen of [C. diadema] in spirits, and this had the ovarian tubes in an early state of development, when they can with difficulty be distinguished from the pulpy corium; the orange-coloured masses, however, which filled the six cavities, resembled the layer which certainly consisted of undeveloped ovarian tubes and cæca, lying over the basal membrane. The ova are wonderfully numerous; their length is 3/400ths of an inch. The larvæ have been noticed in the anatomical introduction.
Branchiæ.—These are immensely developed, covering almost four fifths of the area of each side of the sack. Each consists of two nearly equal folds, attached vertically to the carinal end of the sack, and transversely across the upper end extending to the animal’s body. The outline of the free part is rounded. Both folds of both branchiæ are deeply plicated on both sides; hence the superficies of the whole is very great. We shall find that this structure is common to the three following allied genera, but with these exceptions, I have observed double branchiæ only in one species of one other genus, namely, in [Chthamalus dentatus].
Attachment.—The shells adhere with remarkable strength to the whale’s skin. Having, until recently, examined only separated specimens, and observing portions of the whale’s skin adhering to the outside, and solidly filling up the cavities on the under side, I did not doubt that the shell had the power of forming, by its own action, a deep cavity in the skin of the whale. Inspection of the basal outline of the walls of the shell (Pl. [16], fig. [5]) of [C. balænaris], will show how singularly unfitted its structure is for any burrowing process; and I was led to speculate on the possibility of the pupa being able to bury itself deeply in the skin, but rejected this view as opposed to what is known of the habits and structure of the pupæ of other Cirripedes. Having now examined several specimens of [C. diadema] adhering, in a group, to a large piece of skin, in Mr. Stutchbury’s collection, it has become evident that the attachment is as much owing to the upward growth of the whale’s skin, as to the downward growth of the [Coronula]. In Pl. [15], fig. [4], a vertical section is given of the whale’s skin, taken through the place whence a shell of [C. diadema] has been removed; consequently we here see nothing but the whale’s skin: the upper black layer is the dark horn-coloured epidermis, forming the general surface of the whale’s body, and resting on an orange-coloured fibrous layer, which is lightly shaded in the drawing. The two horns in the section are two of the eighteen projections, formed entirely of the dark epidermic layer, which fill up the eighteen flattened cavities produced by the folding of the walls. Outside the horns we see the section of a circular furrow, in which the circumferential margin of the shell was lodged; and between the horns, there is the central hollow, within which, when lined with shell, the cirripede’s body was included. The circular furrow is formed in main part by the epidermic layer being thinner there than on either side, and partly by the orange-coloured, underlying fibrous layer curving a little downwards, from having apparently yielded to the pressure of the circumferential margin of the shell. With respect to the cause of the thinness of the epidermis under the circular furrow, I do not know how much to attribute to mere mechanical compression or stretching, and how much to the pressure of the shell, having checked[124] its formation. In the case of very young and small shells, it is hardly possible that their pressure can have in any way influenced the formation of new epidermic layers under the thick old layers; and we must believe, at least in these cases, that the whole effect is mechanical, the sharp basal edges of the shell having indented the epidermis; but this is not more surprising than that the radicle of a plant should penetrate hard ground. Whether the indented epidermis in the circular furrow becomes ruptured, I am not sure; ragged layers may commonly be observed outside the shell, but it is very possible that these may be the ends of layers of epidermis which have been preserved by the covering of the shell, whereas, on the surrounding parts of the whale’s body, these same layers have been removed by disintegration. To return to the section,—the outline of the boss of orange-coloured fibrous tissue, under the central hollow, clearly shows that it must have been formed by its own upward growth, for it stands above the general surrounding level of the corresponding layer. This same conclusion is still more obvious with respect to the eighteen flattened prominent horns, formed of the dark epidermis; the manner in which the epidermis has been forced, moulded, and packed into the eighteen flattened and curved cavities of the shell, so as to adhere to them with considerable tenacity, is extremely curious. The prominence of these horns is so great that it appears to me quite impossible to account for them, excepting by a special formation of epidermis beneath each cavity. The basal membrane of the [Coronula], which lies at the bottom of the central hollow, adheres by its own cementing apparatus; and when the larva first attaches itself, this adhesion must be very important, as it allows the basal edges of the shell, during their slow downward growth, to press firmly on the whale’s skin, and so slowly indent it with the circular furrow. The final cause, probably, of the cavities on the under side of the shell in this genus, formed by the singularly convoluted parietes, is to allow of the upward growth into them of the epidermis of the whale, thus securing a firm attachment and allowing the shell to exert a strong downward pressure, and thus effect its partial imbedment, and protection from the enormous force of the waves to which it must be exposed.
[124] Formerly I was inclined to believe, as stated in my former volume on the Lepadidæ, that the cement injured the true skin of the supporting animal, but this, at least in such cases as the present, I do not now at all believe.
With respect to [C. balænaris], I have seen only specimens, preserved on shrunk and twisted whale’s skin, with the underlying fibrous layer not preserved; but the cavities in the shell were filled by horns of epidermis, exactly as in [C. diadema]. There is, however, this difference in the attachment of the two species, that in [C. balænaris], owing to its depressed form, the circumference of the shell indents the whale’s skin, not vertically downwards, as in [C. diadema], but very obliquely outwards; and, consequently, buries itself much more completely, but less deeply, under a folded and apparently ruptured flap of the epidermis. In young specimens, of the size of a shilling, the entire shell, with the exception of the operculum, is thus covered up and protected whilst young and tender.
Geographical Distribution.—The genus is found wherever whales occur, that is, from the Arctic to the Equatorial regions, in both hemispheres. It is asserted that sometimes as many as a couple of hundred specimens will adhere to a single whale. [Coronula barbara], a form closely allied to [C. diadema], existed during the Red Crag period; and Bronn has described some fossil specimens from Italy.
Affinities.—In the wonderfully convoluted shell,—in the parietal tubes not being either filled up by calcareous layers or being crossed by calcareous septa,—in the outer lamina of the shell between the longitudinal septa near the basis being imperfect, [Coronula] differs from all the foregoing genera; in the two latter respects it agrees with the three following allied genera, viz., [Platylepas], [Tubicinella], and [Xenobalanus]. The equal size of all six compartments of the shell, has been observed in very few genera besides [Coronula]. In a new opercular membrane not being formed at each exuviation, and in two or three of these membranes being persistent, and in their being attached high up the sheath, this genus agrees with the three following genera alone. In the valves tending to be rudimentary, and in the protuberant lips of the sack-aperture, we have a close alliance with [Xenobalanus]. The muscles of the sack being spread out, and tending to lose their transversely striated character, are great peculiarities in [Coronula], [Tubicinella], and [Xenobalanus]. The simplicity of the cement-ducts is a remarkable character, observed in two of the following genera, but not in [Tubicinella]. The double branchiæ is a peculiarity common to all four genera. Neither the mouth nor cirri offer any new characters of much importance in [Coronula] or in the three allied genera: the lower teeth of the mandibles not being laterally double, but having small intermediate teeth, is the newest feature in the mouth.