Charles Darwin

“ But if we were to suppose the extreme anterior point of the body to be rapidly developed, the surface of attachment or basis, without it grew still more rapidly, could not possibly reach the dorsal surface, and would, consequently, be confined to the ventral or rostral surface. I have not seen the young of the ordinary or female Alcippe soon after its metamorphosis, but in the male the development of the extreme anterior end of the body is extraordinarily rapid, and from analogy we may fairly conclude that this is likewise the case with the female. ”

“ The membranes of the body, in the act of exuviation, split, I believe, only over the prosoma. How the neat separation of the opercular membrane, from all round the sheath and opercular valves, is effected, I do not fully understand; but it is, probably, analogous to the splitting of the thick carapace of the common crab. I suspect in Coronula, in which genus and its allies the opercular [Pg 158] membrane is not periodically moulted, that the membrane lining the sack is not always thrown off at the same exact time with that of the body. ”

“ After having given up several years to the study of this class, I must express my deliberate conviction that it is hopeless to find in any species, which has a wide range, and of which numerous specimens from different districts are presented for examination, any one part or organ,—which from differing in the different species is fitted for offering specific characters,—absolutely invariable in form or structure. ”

“ All the species, of the habits of which I know anything, are littoral; and in many parts of the world are excessively numerous, quite covering large spaces of the coast-rocks, and often coating the coast-shells. Chthamalus dentatus is littoral, like the other species; but it often lives attached on Balanus tintinnabulum and amphitrite, on the bottoms of ships arriving in British ports from the west coast of Africa. I do not know of any instance of more than two species occurring in the same region. ”

“ In a young Balanus, immediately after the metamorphosis, the operculum is attached by the opercular membrane all round to the summits of the compartments, and there cannot be said to be any orifice to the shell itself, but only an orifice or slit between the opercular valves; but during growth, as the compartments are added to at their basal edges, their upper ends are deserted, and cease to enclose the sack, within which lies the animal’s body. Hence the upper ends come to project freely, either quite separately as in some species of Pollicipes, where they cannot be said to form an orifice ”

“ The Terga differ considerably in outline in the different genera and species: their shape approaches more nearly to a triangle than to any other regular form; but there is generally a projection or spur on the basal margin, on the side towards the scutum. In some species of Pyrgoma, the tergum is of so irregular a shape as to defy description. In most cases, a longitudinal depression or furrow runs down the valve, from the apex to the extremity of the spur; and it not rarely happens that the sides of this furrow become folded inwards and almost closed. ”

“ In several species the males are short-lived, for they cannot feed, being destitute of a mouth and stomach. As the females are longer lived, successive crops of males, at each period of propagation, become attached to her. It is the females in the above genera which retain the characters of the genus, family, and order to which they belong; the males often departing widely from the normal type. Some of the males are rudimentary to a degree, which I believe can hardly be equalled in the whole animal kingdom; they may, in fact, be said to exist as mere bags of spermatozoa. ”

“ Under the head of Variation, in the Family Balanidæ and under the Genus Balanus, I have enlarged on this subject, and have shown that there is scarcely a single external character which is not highly variable in most of the species. As whole groups of specimens often vary in exactly the same manner, it is not easy to exaggerate the difficulty of discriminating species and varieties. ”

“ As the new circumferential slip of basal membranes goes on increasing in width, A continues to be stretched, but does not split, till at least another circumferential slip of basal membrane has been formed and has been split, and till B has been also stretched. By this time, the cement-tissue A has assumed its normal structure, and has the power of adhering to the whale’s skin, which it has now reached, owing to the splitting of the under and older slips of cement. At the next period of growth, A itself will split, and B will touch the whale’s skin and adhere to it ”

“ In Ibla, the males are attached low down within the sack of the female; they may be said to consist of a mouth surmounted on a long peduncle, for there is no capitulum or general covering, and the whole thorax is in a rudimentary condition, the cirri being reduced to two distorted pairs. As these males certainly moult several times and grow a little, they must feed; and as they have no cirri fit for action, they must seize their food by the contortions of their peduncle, which we know homologically consists of the three first segments of the head. ”

“ In fact the mouth is a model of the outside of the mouth of the young Cirripede. I may remark that some little way beneath the membrane answering to the labrum, a pair of ligamentous apodemes, the use of which I do not know, slightly penetrate the body. The degree of prominence of the mouth varies, but it is far less than in the mature animal. On the limbs I have nothing particular to add: the drawing of the first pair of legs (Pl. 30, fig. 5) is, I think, very accurate: I observed all the spines here figured, on the corresponding leg of the pupa of Balanus Hameri. ”

“ At each exuviation, the external membrane with the dentated hardened orifice, the lateral bars, the inner tunic of the sack with its bars, are all moulted, together with the usual integuments of the animal’s body. New and sharp triturating points are thus periodically formed for the work of excavation. The whole animal increases during growth in every direction, and therefore, at its lower or basal end, as was the case with Alcippe. ”

“ Internally, the articular ridge is pretty well developed, its lower edge being very oblique; there is barely an adductor ridge: the pit for the lateral depressor muscle is deep. The spur of the Tergum is placed close to the basi-scutal angle of the valve; it is about half as wide as the valve, with the lower end truncated: sometimes it may be rather said to be bluntly pointed, owing to its carinal side sloping up to the basal margin. The articular ridge is pretty well developed. The crests for the depressor muscles are barely discernible. ”

“ Anatomical Structure.—The animal’s body is attached to the two scuta by the adductor scutorum, and by the other usual muscles running towards the mouth, and surrounding the prosoma. The whole external covering or shell has no other muscles; Verruca thus differing from the Balanidæ and Lepadidæ; but the shell is attached all round, near its circumference, to the basal membrane, by a band of very short fibres, appearing like muscles, but really ligamentous, as determined for me by Professor Quekett. Branchiæ are entirely absent. ”

“ The compartments are separated from each other by sutures, which are often so fine and close as to be distinguished with difficulty. The edge of a compartment, which can only be seen when disarticulated from its neighbour, I have called the sutural edge (acies suturalis) .
Each separate compartment consists of a wall (paries) , or parietal portion (pp in woodcuts) , which always grows downwards, and forms the basal margin; and is furnished on the two sides either with alæ (fig. 4) , or with radii (fig. 2) , or with an ala on one side and a radius (fig. 3) on the other. ”

“ The soft, rounded, bag-like portion of the body, which I have called the prosoma, is usually separated by a notch from the five posterior thoracic segments; at its upper end it may be said to carry the mouth and first pair of cirri. The prosoma includes the main part of the stomach and the broad ends of the vesiculæ seminales. It is always clothed by very thin membrane, which in Chthamalus dentatus, is hairy. In Tubicinella and Xenobalanus, the prosoma is much elongated, being produced far down the deep sack. ”

“ Balanus balanoides and Chthamalus stellatus, I have observed that, when left uncovered by water, they kept the orifice of their operculums a little open, with a bubble of air within their sacks, so that the orifice was in fact closed by a thin septum of water, with air beneath; when disturbed, they closed their operculums with force, and expelled the bubble of air with a clicking noise, which has been noticed by Dr. Coldstream, [27] and has been thought to be made by the movement of the operculum itself. Bal. crenatus, a deep-water species, when out of water, keeps its operculum closed. ”

“ I have as yet not entered in detail on the sex of the supporting Cirripede: in Cryptophialus, Alcippe, and in one species of Ibla, I was able to demonstrate in many specimens, that all the male organs, internal and external, were entirely absent; and consequently that these Cirripedes were exclusively female. In Scalpellum ornatum, also, there is no trace of external male organs (the state of the four dried specimens [Pg 29] not allowing the internal organs to be examined) , and there cannot be any reasonable doubt that this species likewise is exclusively female. ”

“ The radii, when developed, are broad, square on the summit, and covered by brownish hirsute epidermis: internally they are formed of tubes like those forming the parietes; in this respect differing from all the species except the following, T. costata. The tubes in the radii run obliquely down towards the basis; instead of in a transverse line, directly towards the opposite compartment, as might have been expected from the structure of the radii in Balanus. The alæ have their edges finely crenated. The sheath in all the specimens which I have observed is colourless; its lower edge is not free. ”

“ It is obvious that these males must be very short-lived: they perform their masculine functions and then perish. We have seen, however, that after the act of metamorphosis they do grow a little, and I have reason to suspect that this is effected, as with other Cirripedes, by moulting. The growth must be absolutely dependent on the store of nutriment laid up within the pupa. The young male, immediately after the exuviation of the integuments, thorax, natatory legs, abdomen, and eyes of the pupa, consists of a pulpy cellular mass, without any internal organs as yet formed. ”