As the shell increases in diameter, each of the original eighteen transverse loops, forming the exterior surface of the shell, increases in breadth; and they would have had to increase extremely, had not some of the transverse loops become, during growth, divided into two or three new transverse loops, in a manner strictly analogous with the first formation of the eighteen folds in the young shell. In Pl. [15], fig. [7], [8], [9], we see how one of the circumferential transverse loops, by the formation of a medial furrow, or rather bay, becomes developed into two transverse loops; and it is rather important to observe that three new loops might equally well have been contemporaneously formed. By the repeated formation of new circumferential loops and the consequent formation of new folds, the wall of the shell, when old, especially in [C. balænaris], becomes folded in a wonderfully complicated manner, as may be seen in Pl. [15], fig. [5], which is an exact tracing of the extreme basal edge of the wall of a shell of [C. balænaris]; to perceive the full amount of complication, it is advisable to trace the wall of any one of the compartments, from one suture (s) to another. In this figure the sutures are purposely drawn a little open. It may be seen that the new transverse loops, and consequently the new folds of the walls, have been, in this species, mostly formed in symmetrical order, on both sides of the six sutures; this results from the transverse loops on the sutures almost always giving rise contemporaneously to three new transverse loops. In [C. diadema] the transverse loops on the sutures usually divide into only two new loops, one on the rostral side and one still remaining at the suture; hence the folding of the walls in this species is much less symmetrical. The number, however, of the transverse loops and the exact pattern of the folding is variable in all four species of the genus. I may further add, to show the complication of the folds, that in a shell of [C. balænaris], having a basal diameter of two inches, and which had the walls as little folded as ever they are, yet I found, by careful measurement, that the entire basal edge of the wall, if stretched straight, would have extended for a length of fifty-two inches! Therefore, if the wall had not been folded, but had been simply circular, as in ordinary cirripedes, the basal diameter of the specimen would have been between sixteen and seventeen inches!
The central membranous basis is flat, but the bottom of the folded walls of the shell is concave, which is caused by the outer ends of the folded walls having grown at a greater rate than the inner ends. The concavity is deep in [C. diadema]; in [C. balænaris] it is much less so, and here the inner hood-like ends of the folded walls are rather abruptly, but in a variable degree, produced downwards, generally even slightly beneath the level of the circumference of the shell; this fact is of interest in relation to the peculiar, depending, spur-like processes in the genus [Platylepas]. A lateral view of a compartment in both these species, is given in Pl. [16], figs. [2] and [3]; and by supposing in each case a compartment to stand opposite, at a distance which may be judged of from fig. [5], the vertical sectional outline of the whole shell will be understood: in fig. [3] of [C. balænaris], however, the inner hood-like ends of the folded walls are not produced so much downwards as is usual.
In the same manner, as the outside of the shell consists of the transversely expanded ends of the folded walls, pressed closely together, so the cavity in which the animal’s body is lodged, is formed by the inner and less closely joined ends of the folds, lined by the thick sheath (a, fig. [1] and [7], Pl. [16]), which latter extends down very near to the basal membrane. The cavity for the body, is small compared to the whole shell; in [C. diadema] it is deeply cup-formed, with a small, flat, membranous bottom or basis; in [C. balænaris] it is wider and shallower, with a broader bottom, and with the upper edges of the walls more inflected. In both species, the thick membrane connecting the opercular valves to the shell, is attached all round near the summit of the sheath. The uppermost portion of the sheath is not marked by concentric lines, as in most of the [Balanidæ], owing to the opercular membrane not being, as we shall presently see, regularly moulted. A portion of a single wall, when closely examined, is found to be formed of an outer and inner lamina, united by longitudinal septa, and is thus permeated by minute, square, longitudinal pores,—exactly as in the normal structure of [Balanus]. The walls are extremely thin, and are striated longitudinally, owing to the slight projection, on both the inner and outer surfaces, of the longitudinal septa; they are thicker in the part forming the external transverse loops, being here, in [C. balænaris], as much as 15/1000ths of an inch in thickness; but when forming one side of the spoke-like folds, the thickness is only 6/1000ths of an inch. The inner lamina is thicker, contrary to what is usual, than the outer lamina; the sharp tips of the longitudinal septa project a little beyond either lamina, giving to the basal edge of the wall a serrated outline. It is singular that the thin outer lamina is first formed as a rim or ledge on each side of the longitudinal septa; these ledges being not closely united for some little space up the wall, as is represented in the enlarged drawing of a bit of the basal wall of [C. diadema], Pl. [16], fig. [6]. The open clefts thus left are, of course, covered by the so-called epidermis, for otherwise the included threads of corium would have been exposed. Each fresh period of growth, in the case of [C. diadema] (fig. [6]) and [reginæ], and to a certain extent in [C. balænaris], is marked by little knobs on the longitudinal, slightly prominent, septa, and this prefigures an analogous strongly marked structure in [Tubicinella]. A fine thread of corium runs up each pore to the summit of the compartment; for these pores are not, as in [Balanus], cut off by transverse calcareous septa, or have their upper ends solidly filled up with shelly matter. As, however, the summit of the shell in [Coronula] is sometimes disintegrated, the threads of corium within the pores would have been exposed, had not each thread formed for itself, as I suppose, a transverse membranous septum near the summit of the shell; at least this is the case with the larger pores of the radii. The walls, where closely pressed together in the spoke-like folds, are disunited at the extreme base, but above this they are firmly calcified together. A ribbon of corium runs along the basal edge of each spoke, and sends threads of corium up the parietal pores on each side, and its upper edge serves to deposit calcareous matter (homologous with the layers of the sheath) and thus to unite the two walls firmly together. In [C. diadema], the walls of the terminal transverse loops are simply calcified together like the spoke-like portions; but in [C. balænaris] the opposite sides of the loops are united by septa (see the transverse section in Pl. [15], fig. [2 a]), making from five to eleven longitudinal tubes within each transverse loop; these tubes being larger than the parietal pores. When a piece of the shell is dissolved in acid, no tubuli can be discovered, which may be accounted for by the thinness of the walls; nor are there any spines on any of the external membranes. The number of the pores, in the parietes of a moderately-sized specimen of [C. balænaris], I calculated was at 3400, each occupied by a thread of corium springing from the eighteen branched ribbons, diverging from the corium, surrounding the base of the sack. To this number must be added between 300 and 400 larger threads of corium running up the tubes in the transverse loops; and no less than about 2300 fringes and threads occupying the pores in the six radii: thus we see that the dermal system in [Coronula] is wonderfully complicated.
Radii.—The radii are very wide in [C. diadema]. In all the species their summits are square or parallel to the basis. Their internal structure is remarkable: as the walls in this genus are extremely thin, so are the proper radii, for in fact they consist in this and all cases, as we know, of one margin of the wall modified by its lateral growth against the opposed compartment. But as the radii in [Coronula] are not plicated, like the walls, the shell would have been excessively weak along the six lines of suture, had not the radii been strengthened by numerous sinuous plates, springing from the inner lamina of the proper radius, and running downwards, attached to the folded wall of the compartment to which the radius belongs, and with their free edges pressed against the folded wall of the opposed compartment. These plates give out short transverse denticuli, making altogether a beautiful structure, as is best seen in Pl. [16], fig. [3], but also in fig. [2] and [4], and d in fig. [1]. In the section, fig. [7], the proper radius (d) is seen to be continuous with the wall (e), and to be very thin, in fact forming but a small portion of the compound radius: it is formed of an outer and inner approximate lamina, separated by septa, which are nearly horizontal, and which consequently cannot be shown in the transverse section fig. [7]. The outer lamina of the radius is imperfect, or does not reach quite to the suture, leaving the septa a little exposed (imperfectly shown in Pl. [16], fig. [3]), exactly as is the case with the outer lamina of the parietes, at the basal margin of the shell. In all common [Balanidæ], a ribbon of corium runs up each of the six sutures, and sends in fine threads between the septa of the radii, but here a thread of corium runs up a minute, cylindrical pore, situated on the line of junction between the radius and the wall whence it arises; and from this longitudinal thread the finer threads spring which pass between the horizontal septa of the proper radius: this cylindrical pore is rather large in [C. balænaris], but excessively small in [C. diadema] (see a black dot (d′) in section, fig. [7]), and is solidly filled up in the upper part of the shell. The plates (fig. [3]) which run down from the inner lamina of the proper radius, and form the greater part of its thickness, are occupied by fringes of corium, extending up from a ribbon of corium, running along each suture, like those which run along the bases of the spoke-like folded walls. This difference in the origin of the ribbons of corium, occupying the interspaces between the plates and the pores in the proper radii, shows the essential difference between the latter and the thick inner portions of the compound radii. In [C. balænaris] (fig. [3]), the compound radius extends from the outside of the shell to the sheath and to near the basal edges of the folded walls: in [C. diadema] (fig. [2]), it does not extend so far inwards and downwards; and in [C. reginæ] (fig. [4]), even still less so. It ensues from this circumstance that when, in these two latter species, a transverse section is made across the middle of the shell, a large chamber (v, fig. [7]), occupied by the ovaria, is found on each line of suture (s s), separating the radii (d) and alæ (a).
The Alæ in [C. diadema] (section, Pl. [16], fig. [7], a′) and [C. reginæ], are of remarkable thickness, nearly equal to that of the radii, and in [C. balænaris], of considerable thickness; this is evidently to give strength to the shell, which is weakest along the lines of suture, notwithstanding that the radii have been specially thickened. The edge of each ala presents a miniature resemblance of the edge of the radius, namely, a central ridge sending off on both sides sinuous plates, themselves denticulated. In [C. balænaris], the ala rests almost entirely on the inner surface of the compound radius; but in the other two species, in which, as already stated, the radii and alæ are separated by chambers, the ala rests on a plate (c′′ in fig. [1], [4], and [7]), which extends from the top of the radius to the bottom of the sheath, narrowing downwards (c′′, fig. [1]), and is a specially developed portion of the sheath for the radius to rest on. In [C. diadema] and [reginæ], the sides of the folded walls, at the ends of the chambers (v, in section, fig. [7]), are strengthened by the deposition of layers of shell in connexion with the sheath. The sheath extends close down to the basal membrane in [C. balænaris], and does not project freely: in the other two species it depends freely, but does not run quite so low down. I have only further to remark, that the sutures, though very strongly united, are not calcified together; for they easily separate after the action of caustic potash.
The Basis (Pl. [28], fig. [1 a]-[1 c]) is membranous and is formed of concentric slips: each slip has eighteen angles, corresponding with the open ends (see Pl. [16], fig. [5]) of the eighteen folds of wall. A ray of membrane runs under each of these folds, being prolonged from the basal membrane; but these rays can hardly properly be called parts of the basal membrane, for they split at each period of growth along the middle, and the two halves are drawn from under the basal edges of the walls, and thus come to invest their outer surfaces. The basal membrane, and the whole cementing apparatus, which is much simpler than in [Balanus], has been fully described (p. [135]) in the [Introduction].
Opercular Valves.—In the sessile cirripedes hitherto examined, the four opercular valves are inclined towards each other, and nearly fill up the orifice of the shell, being united to the walls by a more or less narrow circular border of membrane; this membrane being attached rather low down to the sheath. In [Coronula] the opercular membrane is stretched like the skin of a drum, almost horizontally across the top of the shell; it is, however, generally attached to the sheath rather lower down at the carinal than at the rostral end: hence the animal’s body, as remarked by Burmeister, is attached almost horizontally; but this we shall see, under [Xenobalanus] and [Tubicinella], cannot be considered a character of much importance. The valves are quite small compared with the opercular membrane, and certainly are of little functional importance. The scuta in [C. balænaris] and [diadema] (Pl. [15], fig. [3 b]) are sub-triangular; but the under or growing surface is elongated and arched. These valves stand almost at the rostral end of the orifice, instead of on each side of it. In [C. balænaris] the terga are small, with the under surface oval: in [C. diadema] they are either quite aborted, or are represented by a barely visible plate of shell, parallel and close to the tergal margin of the scutum. The aperture leading into the sack is formed by a nearly medial slit of considerable length, furnished with irregularly protuberant, inwardly inclined lips. These lips are formed by the development of an inner fold or crest of membrane, which can be just detected in most Cirripedes: the lips include a double fold of corium, and are covered by a delicate tunic, continuous with that lining the sack, and homologically continuous with the opercular membrane. The opercular membrane is very thick, tough, and yellowish; it is, in parts, finely plicated in lines radiating from the apices of the valves: these plicæ consist of membrane in an altered condition, being harder, more horny, and of a browner yellow; the plicæ are large at the rostral end of the scuta, and projecting beyond these valves, they afford attachment to the rostral depressor muscles. Rims of similarly modified membrane (Pl. [15], fig. [2 b]) connect the scuta and terga together. The rims and plicæ are occasionally moulted together with the opercular membrane. In almost all hitherto described [Balanidæ], a new opercular membrane is formed at each period of exuviation, and as soon as formed, the old one is generally moulted, together with the other membranes of the body: the case is very different in [Coronula], in which it is evident, from the lines of growth on the valves and sheath, that a new membrane is formed only at rather long intervals, and that it is formed in some degree extensible, so as to allow of some growth in the shell. Two or even three of these membranes are retained at the same time, one over the other; and thus, by their joint thickness, they afford protection to the included animal’s body, and compensate for the smallness of the opercular valves. In a large specimen of [C. balænaris], two inches in diameter, there had been formed, since its existence as a very young shell, not more than eight opercular membranes, whereas the other membranes must have been moulted within this same period at least thirty times. In a young specimen of this species, having the orifice of the shell only two tenths of an inch in diameter, I found the opercular membrane, as usual, double. This membrane is not furnished with spines, nor is it penetrated by tubuli as in most other genera. The tissue left after the opercular valves have been dissolved in acid, presents no tubuli, or any other recognisable character, and is not divided (as is usual) into layers.
Muscles of the Sack.—These muscles differ considerably from those of other sessile cirripedes hitherto described. There are the usual five (or properly six, the two tergal muscles being here, as elsewhere, confluent) bundles of fasciæ; but they hardly can be called bundles, they are so much spread out and thin. In [C. balænaris], each rostral muscle consists, in different specimens, of from three to five principal fasciæ; these, at their upper ends, are attached to the cartilaginous plicæ at the rostral extremities of the scuta; at their lower ends, they do not reach (as in all the previous genera) to the basal membrane, but after converging, they diverge again into a little fan of fibres, which are firmly attached to the corium low down on the sides of the sack. The lateral depressores of the scuta consist each of about three fasciæ, and they terminate downwards like the rostral muscles. The tergal depressores are spread out into a thin sheet; upwards they reach to the basal edges of the lips of the sack-aperture, and downwards they curve a little towards the rostral and opposite end of the shell, and extend nearly to the basal membrane. These tergal muscles include two fasciæ, larger than the others, which extend rather further, both upwards and downwards, than the other fasciæ. But the most novel character in these several muscles is that in their lower portions they do not exhibit transverse striæ, thus showing a tendency to become involuntary as in pedunculated cirripedes. This circumstance, and their feebleness, is easily accounted for by the thick unyielding nature of the opercular membrane, and the feebly developed character of the opercular valves. In [C. diadema], the tergal muscles are much spread out, having, as in [C. balænaris], a larger fascia on each side; but the lateral depressores of the scuta form a well defined nearly cylindrical bundle; the rostral pair are extremely weak and spread out: I could perceive only feeble transverse striæ on some of these muscles, and on others there was not the least trace of striæ. I may add that the adductor scutorum muscle is well developed, as are the eight pairs of muscles which unite the animal’s body to the under surfaces of the scuta. The action of the adductor scutorum serves to close the sack-aperture, but towards the carinal end necessarily with very small force; protection in this part can only be afforded by the protuberant, valvular lips, and by the dorsal surfaces of the inwardly curled cirri, with their tufts of bristles.
Mouth.—The mouth (Pl. [26], figs. [3], [4]) is much elongated transversely, and does not differ essentially from that of most other [Balanidæ]. The labrum is notched and not in the least bullate, though in [C. diadema] there is a slight prominence outside close beneath the notch. The palpi are of large size, with their bristly apices touching each other. The mandibles are very strong, with from four to five main teeth, which are remarkable by presenting only rudiments of being laterally double; but between the second and third, and between the third and fourth main teeth, there is a small intermediate tooth,—these I have not met with in any sessile cirripede hitherto described. The maxillæ are small, with the two upper spines remarkably strong. The outer maxillæ are on their inner faces bilobed. Between these organs there is a minute projection, or mentum, flattened in the longitudinal axis of the body; I have not noticed this in any previous Cirripede.