A Monograph on the Sub-class Cirripedia · Charles Darwin
TABLE OF CONTENTS.
Quite independently of the effect produced by the surface of attachment, the degree to which the longitudinal folds and ribs are developed on the parietes, is variable in most of the species, as in _B. tintinnabulum_, _vestitus_, and even in _B. porcatus_; the presence or entire absence of these ribs often surprisingly alters the whole aspect of the shell. The persistence of the so-called epidermis is in some degree variable; and in _B. lævis_ we have groups of specimens absolutely naked, and others uniformly clothed with a brown membrane. Again, some species in certain localities are all subject to the disintegration of the entire outer lamina of the walls; and in such cases (as with _B. perforatus_) there is not the smallest resemblance between the corroded and perfect specimens. The size of the orifice, and consequently of the operculum, compared with the shell itself, varies accordingly as the shell is more or less conical or cylindrical; in the latter case, the summits of the radii are generally more oblique and the aperture consequently more deeply toothed than in the more conical varieties. Size is a serviceable character in some cases, but very many specimens are required to ascertain the average or maximum size of each species, for there is no method of distinguishing a half-grown from a full-grown specimen; and I believe, as long as the individual lives, so long does it go on moulting and growing. _Colour_ is of very considerable service; though the precise tint varies greatly in almost every species; and what is a far more serious evil, the majority of the species have their white or nearly white varieties, the latter sometimes as numerous as the coloured ones: in _B. perforatus_, _lævis_, _flosculus_, _amphitrite_, and in several other species, the common white varieties are eminently deceptive.
Besides the slight variation in the obliquity of the summits of the radii and alæ, dependent on the more or less cylindrical form of the shell, in some species, as in _B. tintinnabulum_, _amphitrite_, _improvisus_, _trigonus_, and _porcatus_, their obliquity also varies occasionally from unknown causes, and thus greatly affects the general appearance of the shell. In some few species, as in _B. perforatus_, the radii are often either not at all developed, or are of very variable width; in others, when the shell has become cylindrical, or when very old, the radii cease to grow, and from the disintegration of the whole upper part of the shell, with the continued growth of the lower part, the radii at last come to exist as mere fissures: I have seen instances of this in _B. psittacus_, _nigrescens_, and _porcatus_. Nevertheless, the obliquity of the upper margin, and the breadth of the radii are useful characters; and still more useful is the fact whether the upper margins are smooth and arched, or straight and jagged. The fact of the terga being more or less beaked is useful: as is, likewise, the presence of striæ, or furrows, or rows of pits, radiating from the apices of the scuta; but to ascertain the presence of these marks, it is almost invariably necessary to take out the scuta, clean, and examine them with a lens; these ridges and furrows, moreover, in some species, as is strikingly the case with _B. tintinnabulum_, and in less degree with _B. trigonus_, _lævis_, and _concavus_, appear and disappear, and vary without any apparent cause.
Now if we reflect that form, size, state and nature of the surface, presence of epidermis, relative size of the orifice, presence of longitudinal ribs, tint, and often the existence of any colour, are all highly variable in most of the species; and that the obliquity of the summits of the radii, and the presence of longitudinal striæ on the scuta, are variable in several species; we shall perceive how difficult it must ever be to distinguish the species from external characters. As some evidence of this, I may mention that, after having described nearly 40 species, and when my eye was naturally able to appreciate small differences, I began carefully to examine varieties of _B. tintinnabulum_, _amphitrite_, _improvisus_, _porcatus_, _vestitus_, &c., without even a _suspicion_ that they belonged to these species, at that time thoroughly well known to me; yet in the cases here referred to, there could be no doubt, when a perfect series was examined, that the specimens were only varieties. From this cause the labour of naming a collection is great. Let no one attempt to identify the species of this genus, without being prepared to disarticulate, clean, and carefully examine with a microscope the basis and parietes, and both the under and upper surfaces of the opercular valves; for I feel convinced, that he will otherwise throw away much labour. Moreover, in many cases, it is almost necessary, on account of the variability of the characters, to possess several specimens. From these facts, I have not hesitated to form my sections on characters which require close examination, though I would gladly have seized on external characters, could I have found such even moderately constant.
The least varying, and therefore most important characters, must be taken from the internal structure of the parietes, radii, and basis: not that these characters are absolutely invariable; thus the porosity of the parietes is slightly variable in _B. glandula_, and highly variable in the fossil _B. unguiformis_; it is also highly variable in _B. balanoides_, but under a systematic point of view this is unimportant, as the section including this latter species is well defined by the membranous basis. The porosity of the basis is in some degree variable in _B. nubilus_, _improvisus_, and _patellaris_; and in _B. flosculus_ we see signs of a passage from a calcareous to a membranous basis. Characters derived from the general shape, and from the ridges and depressions on the under side of the scuta and terga, especially of the scuta, are highly serviceable. The terga, indeed, in many species, as in _B. amphitrite_, vary considerably, and are affected by the general shape of the shell. Unfortunately the differences are not very great between the scuta of the different species. The cause of the opercular valves offering more useful characters, as far as outline is concerned, than do the walls of the shell, is no doubt due to their being almost independent of any influence from the nature of the surface of attachment.
Even the ridges and depressions on the under side of the scuta, which are in direct connection with the muscles and soft parts of the animal, vary to a certain extent: thus the length and prominence of the adductor ridge is decidedly variable in _B. concavus_ and _tintinnabulum_, and in a less degree in _B. lævis_; the size and form of the little cavity for the lateral depressor muscle varies in many species; so does the exact shape and degree of prominence of the articular ridge. There is one character in the terga, which at first would be thought very useful, namely, whether an open longitudinal furrow, or a closed fissure runs down the valve from the apex to the spur; but it is found that the furrow almost always gradually closes up during growth; and as a consequence of this, the width of the spur compared to that of the whole valve, as well as its distance from the basi-scutal angle, and the form of its basal extremity, all vary in some degree. The length of the spur sometimes varies considerably, as in _B. concavus_ and _amphitrite_.
The parts of the mouth are only occasionally serviceable; for the teeth on the labrum, and the state of the lower teeth on the mandibles, and the presence of a step-formed projection at the lower angle of the maxillæ, are all often variable. The relative lengths of the two rami of the first pair of cirri, the degree of protuberance of the segments, and the number of pairs of spines on the segments of the posterior pairs of cirri, are sometimes useful; but the relative lengths of the cirri, and more especially the numbers of pairs of spines on the posterior cirri, are apt to vary. Finally, I must express my deliberate opinion, that every part and organ, internal and external, in Cirripedes, is liable to some amount of variation in some of the species.
I must now point out the principal changes which supervene during growth, and which cannot properly be called variations. In the first place, I think, it is scarcely possible to recognise a species when under the 1/10th of an inch in diameter. In some cases, as in _var. d'Orbignii_ of _B. tintinnabulum_, the shell is invariably coloured when old, but quite white when very young. Generally the tints become very much darker with age. Some species, which usually or invariably have, when mature, longitudinally folded walls, as _B. flosculus_ and _balanoides_, are perfectly smooth in youth. The walls in _B. eburneus_, when young, have white, hyaline, longitudinal lines, and are naked; whereas, with advancing age, these lines disappear, and the subsequently formed shell becomes covered with membrane. The summits of the radii are apt to be oblique in the young of _B. Capensis_, _psittacus_, and _tintinnabulum_, whereas they are generally quite square in old specimens. In _B. eburneus_, _cariosus_, and in a lesser degree in _B. psittacus_, the scuta become longitudinally striated only with age. On the other hand, in very young specimens of _B. tintinnabulum_, the scuta sometimes are deeply impressed by little pits placed in rows.
I would gladly have divided this genus, already including 45 species, into smaller genera; but so far from being enabled to do so, I have been compelled to form my Sections (immediately to be given) on characters not absolutely invariable, and far from obvious. I was particularly anxious to separate the elongated species with a boat-formed basis, which are attached to Gorgoniæ, and which form the genus Conopea of Say, but I was unable to effect their separation even as a sub-genus; for _B. navicula_ and _cymbiformis_ graduate in the most insensible manner through _B. galeatus_ (the type of Say's genus) and _B. calceolus_ into _B. stultus_, and this into _B. Ajax_; yet this latter species has even been described as a mere variety of the typical _B. tintinnabulum_! Indeed, so insensible is this graduation, that the first and second sections of the genus are hardly distinct. I fully admit, that if _B. stultus_ and _Ajax_ had never existed, _B. calceolus_ and its three allies might have formed as natural a little group, though difficult to be characterised, as does the sub-genus Acasta; or perhaps this group and Acasta might have been combined together. These same species, viz., _B. calceolus_ and its allies, are intimately allied to _B. terebratus_ and _inclusus_, which are contained in the last section (F) of the genus; and this shows that Dr. Gray's proposed genus Chirona, including the species with non-porose parietes in sections (E) and (F), could hardly have been instituted, even if the porosity of the parietes had not been variable in _B. unguiformis_, _balanoides_, and _glandula_. My fourth section (D), founded on the basis not being porose, is perhaps the weakest of the divisions, as may be seen in the list of exceptions appended to the sectional headings.
The arrangement of the species is, I think, as natural as a linear one could be made: every one knows how irregularly and in how many directions the lines of affinity in every natural genus branch out. Some few species stand rather isolated, as _B. declivis_; and _B. allium_, _cepa_, and _quadrivittatus_ in a little group by themselves. I have already shown how the species in the first and second sections (A and B) blend into each other; and that the blending species are likewise allied to some in the last section (F); furthermore, I shall have occasion to show that these same species can hardly be separated naturally from the sub-genus Acasta. The first section, moreover, passes into the third (C) by _B. tulipiformis_; and the third into the fourth (D) by _B. improvisus_, _nubilus_, _corrugatus_, and _patellaris_: the fifth and sixth (E and F) sections are closely connected together by _B. cariosus_ and _flosculus_; and these two sections blend into the fourth (D) through _B. unguiformis_ and _balanoides_, and lastly, into the third (C) section by _B. dolosus_ and _improvisus_. The genus, as we have just seen, is hardly separated from the sub-genus Acasta; by _B. allium_ it tends to pass through the sub-genus Creusia into Pyrgoma; and by _B. imperator_ and _flosculus_ it graduates into Elminius and Tetraclita.
_Geographical Distribution._--This, which is much the largest genus of sessile cirripedes, has its species scattered over the whole world, from the arctic regions, in lat. 74° 48′, where we have _B. crenatus_ and _porcatus_, throughout the tropical seas, to Cape Horn, where _B. flosculus_ adheres to the coast-rocks. Many of the species have individually very wide ranges; thus _B. tintinnabulum_ and _amphitrite_ are found throughout the warmer seas; but the wide distribution of these species may be partly due to their frequent adhesion to ships' bottoms: _B. crenatus_ ranges from the frozen seas, in lat. 74° 48′ north, to the West Indies and Cape of Good Hope--a wonderful endurance of the most opposite climates. _Balanus improvisus_, again, extends from Europe to Nova Scotia, thence southward to Patagonia, and up the western coast of S. America, someway north of the Equator. Most of the species have considerable ranges; thus of the six species found on the eastern shores of northern America, five of them occur in Great Britain. Of the thirty-six species of which the habitats are known, exactly one third, or twelve, inhabit both the torrid and temperate zones, these being divided by the isocryme of 68°; nine are found exclusively in the torrid, and fifteen exclusively in the temperate zones.
Within the warmer latitudes, and especially in the southern hemisphere, Tetraclita and Elminius to a certain extent supplant Balanus. In depth, the species range from the upper limits of the tidal zone to even fifty fathoms. _Balanus improvisus_ and _eburneus_ are able to survive in brackish water. The different species are attached to various surfaces--rocks, shells, timber, floating objects, sea-weed, lamelliform corals, Milleporæ, Gorgoniæ, and even to sponges. Mr. G. B. Sowerby has remarked[85] that in the species from the southern hemisphere it is the basis, and in the species from the northern hemisphere it is the parietes, which are elongated, when the individuals, from being crowded together, become cylindrical; but this is erroneous; _B. perforatus_, in the northern hemisphere, sometimes has an elongated basis; but no doubt the basis of our commonest species, as _B. balanoides_, _crenatus_, and _porcatus_, from being either membranous or thin, does not become cup-shaped; whereas this structure is conspicuous in _B. psittacus_ and _lævis_, the two commonest species in southern South America. [85] Darwin's 'Geology of S. America,' p, 264.
_Fossil Species._--Having already given, under the Family, some account of the geological history of sessile cirripedes, short as it is, I here only allude to the subject in order to state my conviction that species cannot be satisfactorily distinguished in a fossil state, and rarely in a recent state, without an examination of the opercular valves. Nothing, indeed, could have been easier than to have affixed names to many groups of specimens, having different aspects, but to feel sure that these were really distinct species requires better evidence than can be afforded by the shell, without the operculum. No doubt, in some of the smaller sections of the genus--for instance, in that characterised by a membranous basis--it would have been possible to have distinguished some or several fossil species; but such have not as yet been found. When the specimens are much fossilised, it is, indeed, difficult to make out the primary points of structure--namely, whether the parietes, radii, and basis are porose: to do this it is sometimes necessary to rub down, polish, and carefully examine, a transverse section of a piece of the shell. _Sections of the Genus._ _Parietes, and basis, and radii permeated by pores._ _Parietes and basis sometimes permeated by pores, sometimes not; radii not permeated by pores; shell elongated in its rostro-carinal axis; basis boat-shaped, attached to Gorgoniæ and Milleporæ._ _Parietes and basis permeated by pores; radii not permeated by pores._ _Parietes permeated by pores. Basis and radii not permeated by pores._ _Basis membranous._
Ideas discussed in this chapter
Text from Project Gutenberg, public domain in the United States.