A Monograph on the Sub-class Cirripedia · Charles Darwin
SECOND SECTION OF THE SUB-FAMILY OF BALANINÆ.
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.
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.
Text from Project Gutenberg, public domain in the United States.