A Monograph on the Sub-class Cirripedia · Charles Darwin
SECOND SECTION OF THE SUB-FAMILY OF BALANINÆ.
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. 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.
_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.
1. CORONULA BALÆNARIS. Pl. 15, fig. 2, 2 _b_: Pl. 16, fig. 3, 5. LEPAS BALÆNARIS. _Gmelin._ Systema Naturæ (1789). ---- ---- _Chemnitz._ Conch., vol. 8, Tab. 99, fig. 845, 846 (1785). BALANUS ---- _Bruguière._ Encyclop. Method., Pl. 164, fig. 13-18 (1789). CORONULA ---- _Lamarck._ Annales du Museum, vol. 1, Tab. 30, fig. 2-4 (1802). ---- ---- _Chenu._ Illust. Conch., Plate, fig. 1 and 4. ---- ---- _De Blainville._ Dict. des Scien. Nat., 1818 and 1824, Tab. 117, fig. 3, 3 _a_. _Shell much depressed, with longitudinal flattened ribs, having simple edges; orifice rounded-hexagonal: radii very thick, almost equalling the shell in thickness: opercular valves four._ _Hab._--Attached to whales in the Southern Ocean. Having described, under the genus, in so much detail the structure of the whole shell, it will be sufficient here to point out the characters by which this species differs from the others.
_General Appearance._--The shell is generally much depressed, though sometimes, in large specimens, from the turning in of the basal edges, a considerable degree of convexity is acquired. The radii are moderately wide, and give a star-like appearance to the shell. The surface of the whole upper part of the shell is smooth; the broad and much flattened ribs (_i. e._ the terminal, transverse loops of the folded walls) of which each compartment is formed, generally divide at a short distance from the apex. The close sutures separating these ribs are straight, and not finely serrated, by which character alone this species can at once be discriminated from the others. The surface, when closely examined, is found to be finely striated longitudinally, and is transversely crossed, chiefly in the lower part, by minutely beaded growth-ridges. The orifice is rounded-hexagonal, and is small compared with the whole shell; it is also smaller than the basal edge of the internal cavity of the shell; consequently the whole basal edge cannot be seen through the orifice from one point of view,--the operculum of course having been removed. An unusually large specimen was 2.75 of an inch in diameter; this individual was one of the more convex varieties, and yet its entire thickness, from top to base, was only .9 of an inch.
_Operculum._--Having described, under the genus, the general structure of the opercular valve, I will here only add a few details on their shape. They are small compared with the whole extent of the opercular membrane. The scuta (Pl. 15, fig. 2 _b_, scutum to the left) stand near each other at the rostral end of the aperture leading into the sack, with their two rostral ends united by the yellowish-brown, longitudinally plicated, horny membrane, described under the genus. The terga (fig. 2 _b_) stand a little apart from the scuta, on the sides of nearly the middle of the sack-aperture. The lips are protuberant and moderately developed all round. The scuta are elongated, and a little curved; including the upper imbedded portion, they are almost sub-triangular: but the under growing surface is much elongated, nearly flat, with the two ends of nearly the same width, truncated and rounded. The _Tergum_, in rather small specimens is, if the upper imbedded portion be included, sub-triangular, with the growing surface oval, and between one third and one half of the length of the scutum; but in large specimens, the tergum becomes style-formed, lying parallel to the tergal margin of the scutum, with the growing surface proportionally much smaller, and not above one fifth of the length of the growing surface of the scutum. Hence we see some tendency in the tergum to become rudimentary, as it is in the next species. The brown, horny, plicated substance in which the terga are imbedded, extends considerably beyond the valves themselves.
_Structure of the Shell and Radii._--I have already so fully discussed this subject, that I will here only enumerate the points in which this species differs from _C. diadema_ and _reginæ_:--Firstly, the more symmetrically folded walls (Pl. 16, fig. 5), new folds arising on both sides of all six sutures. Secondly, the inner ends of the folded walls, which surround the internal cavity, being almost square, but with their angles rounded; their inner ends descend some little way beneath the basal edge of the sheath, as low, or lower, than the circumference of the shell. Thirdly, the external lines of junction between the transverse terminal loops being smooth or not serrated. Fourthly, these loops being flatter towards the outside, and being internally filled by septa (Pl. 15, fig. 2 _a_); tubes being thus formed larger than the proper parietal pores. Sixthly, the sutural edges of the compound radii (Pl. 16, fig. 3) being very much broader in the inner and upper part of each compartment than in the outer and lower part; for in the inner and upper part they stretch from the outside of the shell to the sheath, so that the alæ rest on them, and hence no large open cavity is left between the radii and alæ; in the inner and upper part, also, the radii extend down nearly to the basal edges of the folded walls. The septa, of which the radii are formed, stand further apart than in _C. diadema_ and _reginæ_. Seventhly, the alæ are only 1/5th or 1/6th of the thickness of the radii, whereas in the two other species they are very much thicker, being nearly as thick as the radii: the alæ are also here (fig. 3, _a′_) squarer than in those two species, that is, their basal margins are not so short compared with their upper margins; their edges present, also, a slightly different structure. Eighthly, the basal edge of the sheath does not here project freely, and does not descend quite so close to the basal membrane.
_Mouth._--The palpi are furnished with a row, extending along the whole basal exterior margin, of very long upwardly pointing spines; on their upper edges there is a brush of small spines. The edge of the labrum is clothed with extremely fine spines, and is furnished with a few minute teeth. The mandibles have four teeth, and the lower end is broadly rounded and coarsely spinose: the second and third teeth are double at their points, and between the second and third, and again between the third and fourth, there is a small intermediate tooth. Regarding the _Cirri_, I have nothing especial to remark. _Geographical Distribution._--I have examined nine sets of specimens, having localities attached to them: three from off New South Wales; two off the Cape of Good Hope; two from the west coast of South America; and two marked only "South Sea." Hence I am led to conclude that this species is confined to the southern hemisphere, or if it extends into the northern hemisphere, it is probably only in the Pacific Ocean. I do not believe (though so stated in some works) that this species occurs on the shores of Europe. It seems often to be associated with Tubicinella. Some specimens thus associated, sent by Mr. Bennett to Professor Owen, were said to have been attached to the _Balæna australis_.
2. CORONULA DIADEMA. Pl. 15, fig. 3, 3 _a_, 3 _b_; Pl. 16, fig. 1, 2, 7. LEPAS DIADEMA. _Linn._ Systema Naturæ, 1767. ---- ---- _Chemnitz._ Conch., vol. 8, Tab. 99, figs. 843, 844. BALANUS DIADEMA. _Bruguière._ Encyclop. Method., n. 164, fig. 13, 14 (1789). CORONULA ---- _De Blainville._ Dict. des Sc. Nat. (1824), Tab. 117, fig. 4. ---- ---- _Leach._ Encyclop. Brit. Suppl., vol. iii, 1824. ---- ---- _Chenu._ Illust. Conch., Plate, fig. 3. ---- ---- _Burmeister._ Beiträge zur Naturgeschichte der Rankenfüsser, 1834, Tab. 2, fig. 1-14, 18. _Shell crown-shaped, with longitudinal convex ribs, having their edges crenated; orifice hexagonal: radii moderately thick, very broad: terga absent or rudimentary._ _Hab._--Attached to whales, in the Arctic Seas; United States and Great Britain; Gulf-Stream, Atlantic Ocean; New Zealand(?). _General Appearance._--As previously remarked, owing to the fulness of the generic description, minute details on structure, excepting those characteristic of the present species, need not here be given. The crown-like shape of the shell is well expressed by its name of _Diadema_, but the crown tends to pass into a cylinder. The radii are extremely broad. The orifice is large, and neatly hexagonal: when the operculum is removed the whole inside of the cup-formed shell can be seen at once, for the flat membranous basis is much smaller than the orifice. The under side of the shell is deeply concave. The outside of each compartment is formed by broad, rounded, and somewhat prominent, rarely divided, ribs (_i. e._ the transverse ends of the folded walls); these ribs are closely united together by finely serrated lines of junction (Pl. 16, fig. 1, _f_). Their surfaces outside are delicately striated longitudinally, and plainly crossed (more plainly than in the foregoing and the next species) by irregular, transverse ridges, especially in the lower part of the shell. The largest specimen which I have seen was two and a half inches in diameter and two in height.
_Scuta._--These are placed close together at the rostral end of the orifice, and are imbedded in the brownish, tough, longitudinally plicated, horny substance, which extends far beyond both ends of the valves. In outline (fig. 3 _b_) they are mitre-shaped, or rounded and sub-triangular, a little curved, and more or less elongated, being most so in young specimens; they are, however, less elongated and rather more massive than in _C. balænaris_. _Terga_,--these seem entirely absent in most specimens; but in one (fig. 3 _a_) I found a rudiment, namely, a short thin plate of shell, barely visible to the naked eye, extending parallel and near to the tergal margin of the scutum. The lips of the aperture of the sack are prominent, and highly so towards the carinal end.
_Structure of Shell and Radii._--Owing to the shell not spreading much at the base, new folds in the walls are much seldomer formed, and therefore the external longitudinal ribs (_i. e._ the terminal transversely elongated loops), are much seldomer divided, than in _C. balænaris_ or _reginæ_; even rather large specimens sometimes having only the original eighteen folds.[125] When new folds are formed, only one is formed on one, viz., the rostral, side of each suture, instead of on both sides, as in _C. balænaris_. The inner ends of the folded walls, surrounding the basal membrane, are narrow, instead of being almost square, as in _C. balænaris_. The lower edge of the sheath, which projects freely, descends almost to the level of the basal edges of the walls. The outer ends of the folded walls, forming the transverse loops, are internally filled up solidly by calcareous matter, instead of by septa forming tubes, as in _C. balænaris_. The radii (Pl. 16, fig. 2) are a little thicker in the lower and outer than in the upper and inner part of each compartment; in the middle, they do not reach the sheath by about half the thickness of the compartment, and consequently they are separated from the plates (_c′′_ in fig. 1 and 7) on which the alæ rest, by large chambers (_v_ in fig. 7), which extend up to nearly the apices of the compartments: the extent, however, to which the upper ends of these chambers have been solidly filled up, varies a little. The sinuous plates forming the main portion of the compound radii are rather thinner and closer together than in _C. balænaris_. The alæ are thick, being thickest in the middle part, and there equal the radii in thickness; their lower margins are very short compared with their upper margins, hence they are almost wedge-formed.
Text from Project Gutenberg, public domain in the United States.