A Monograph on the Sub-class Cirripedia · Charles Darwin
TABLE OF CONTENTS.
In determining what forms to call varieties, I have followed one common rule; namely, the discovery of such closely-allied, intermediate forms, that the application of a specific name to any one step in the series, was obviously impossible; or, when such intermediate forms have not actually been found, the knowledge that the differences of structure in question were such as, in _several allied forms_, certainly arose from variation,--for instance, in the case of two shells otherwise identical, one being longitudinally ribbed and the other smooth, a character which we know to vary,--but I have always used this argument from analogy with great caution. Finally, as in the large genus Balanus, there is an especial amount of variation, I have there entered in detail on this subject; and I hope that those interested in it, will refer to that discussion, which is almost verbally applicable to some other genera of the family, as Tetraclita and Chthamalus. _Rate of Growth, Exuviation, Powers of Repairing Injuries._
In my former volume I have shown that the pedunculated cirripedes grow rapidly; this is likewise the case with the Balanidæ. Mr. Stutchbury informs me that he has seen numerous specimens of _Balanus tintinnabulum_ from 2 to 3 inches in height and from 5 to 6 inches in circumference (and this is nearly the full size which the species attains), on a vessel which had been to sea only during one year. At Coquimbo, in Chile, I have seen a specimen of _B. psittacus_ 1.3 of an inch in basal diameter, and .8 in height, adhering to a chain that had been only six months under water. Poli, also, gives the case of some Balani (probably _B. perforatus_), which, in about four months, had attained a basal diameter of 1 inch, and a height of 1-1/6th of an inch. _Balanus balanoides_ is a smaller species, and of slower growth; for the late Mr. W. Thompson, of Belfast, found that in three months from July 3d, certain marked specimens had increased from 2-1/2-3 lines to 4-1/2 lines, which is the usual maximum size attained in that locality. From other observations, Mr. Thompson believes that the extreme duration of life of this species is about two years: whether the other and apparently quicker-growing species, are shorter-lived, I have no means of judging.
In accordance with this rapid growth is the frequency of the periods of exuviation. Mr. Thompson kept twenty specimens of _B. balanoides_ alive, and on the twelfth day he found the twenty-first cast-off integument, showing that all had moulted once, and one individual twice, within this period.[74] This frequency of exuviation, together with the durability of the cast-off integuments, explains the astonishing masses of exuviæ, which Mr. Peach assures me he annually has observed off the coast of Cornwall; they are most abundant in April and May, but he has seen quantities also in September; he could easily, as he tells me, have filled several quart-measures with them. The specimens sent to me consisted chiefly of _Balanus balanoides_, _perforatus_, and _Chthamalus stellatus_. The opercular membrane, with a narrow strip from between the two scuta, and another narrow strip from between the two terga, are moulted together, in connection with the more delicate membrane lining the sack, and investing the plicated branchiæ. This membrane, likewise, is continuously connected with that covering the whole body and its appendages. As I have stated under the Lepadidæ, the inner tunic of the œsophagus, of the rectum, of the olfactory pouches, and that which enters a little way into the acoustic meatus, and the apodemes of the maxillæ, are all moulted.
On the cirri and jaws, new spines are formed with their upper ends enclosed within the old spines, but with their lower ends projecting inwards, beyond the bases of the old spines, and inverted like the fingers of a glove hastily pulled off. 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 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. In _Chthamalus stellatus_, in the act of moulting, the opercular membrane is the last part that separates from the new underlying membranes: I find that this species can moult when kept in a damp box out of water. The new membranes of the body, immediately after the exuviation, are not lax in any extreme degree. The exuviæ of the genus Chthamalus, and of some other genera amongst the Chthamalinæ, can at once be recognised by the divergence of the posterior _four_ pairs of cirri: in the case of _Chthamalus stellatus_ I have also noticed that the animal generally dies with these cirri in the same divergent position. Finally, I cannot doubt[75] that the Triton described by Linnæus was only the exuviæ of some Balanus (probably _B. porcatus_); Linnæus mistaking the probosciformed penis for the mouth of his imagined distinct animal.
In another instance, the pedicel of a posterior cirrus had been cut off and subsequently closed; in this instance, a whole, immature, miniature cirrus, with the two rami, each having fifteen minute segments, was thus enclosed in what had been the single lower segment of the pedicel. I have seen several specimens of _Balanus balanoides_, as described under that species, with several of the cirri and the penis truncated; but I believe this was owing to monstrosity, which seemed particularly to affect the male organs of generation; for no reparation seemed to be in progress. In a specimen of Coronula, however, the penis appeared to have been really cut off by accident; it had been closed, by a scab, with concentric lines, like the articular rings on the penis itself; and within the case thus formed, the corium had healed, and had become pointed, but _inverted_; I presume that the point would, after another exuviation, have been everted, and its length thus increased. _Geographical Range and Habits._
With respect to range, the results arrived at have no particular interest, for the species are not sufficiently numerous; and, what is still more adverse, the genera, with unimportant exceptions, range over the world; so that there is no scale of differences, and it cannot be said that these two regions differ in their genera, and these two only in their species. In all the following remarks, I have trusted exclusively to my own specific identification; and have rejected all assigned localities which appeared from any cause to be doubtful. Sessile cirripedes are found in every sea, from lat. 74° 18′ north to Cape Horn. The area included between the north point of the Philippine Archipelago and the south point of Australia, extending on the right hand to New Zealand, and on the left to Sumatra,--an area, which, though including two distinct Cirripedial regions, is small compared with the surface of the globe, yet includes a greater number of species, especially of peculiar species, than the whole rest of the known world. This is, probably, in chief part due to the broken nature of the land, affording diversified habitats, and to much of the coast being rocky.
Cirripedes, from requiring to be attached, cannot live where the shores and adjoining bottom are sandy or muddy or formed of moving shingle; hence, no doubt, it arises, that there is such a remarkable contrast in the great number of the species inhabiting the bold rocky western coast of South America, and the few species living on the sloping, and often sandy or muddy or shingly, eastern shores of this continent. Hence, also, I believe, it is that not many species have been brought from India. Coral-reefs are not favorable to Cirripedes, consequently but few are known to inhabit the islands of the Pacific Ocean. Where Cirripedes can live, though the species in no district are numerous, the individuals abound in infinite numbers: I have walked over the coast-rocks of the Falkland and Chonos Islands, of Chile and Van Diemen's Land, fairly encrusted over wide spaces with a continuous layer of Cirripedes, consisting of only two or three species; in the same manner as may be observed on many parts of the shores of Great Britain, and, I believe, of North America.
With respect to the effects of temperature on the range of Cirripedes, no genus (having more than one species) is confined to the torrid zones. Pyrgoma, from being always attached to corals, is, of course, ordinarily found in the hotter seas; but one species ranges from the Cape de Verde Islands in the torrid zone to the southern shores of England and Ireland. Tetraclita is not found in the colder seas, but is numerous in species and in individuals, on the southern shores of Australia and at the Cape of Good Hope. I may here add, that the two genera with the most confined ranges, are Chamæsipho and Elminius; the former has only two species, one inhabiting Australia, and the other the East Indian Archipelago; Elminius has four species, confined to the southern hemisphere, and inhabiting Australia, New Zealand, and South America. To return to the effects of temperature; in Mr. Dana's great work on Crustacea, an excellent chart is given, in which the _isocrymal_ lines, or those exhibiting the mean temperature of the waters along their course, for the coldest thirty consecutive days in any season of the year, are given; and which lines Mr. Dana has shown are the most influential in governing the distribution of marine animals.
At the isocryme of 68°, Mr. Dana divides the torrid and sub-torrid zones from the several temperate zones; and at 44°, the temperate from the sub-frigid and frigid zones; but as no Cirripedes are exclusively confined to these frigid zones, we may here disregard them. From Mr. Dana's[76] table of the areas of the torrid and temperate ocean-zones, on both sides of the equator, it seems that they are nearly as 337 to 278, in relative area; and consequently, he remarks, that the marine species in any class, if distributed equally over the two, would be one fifth more numerous in the torrid than in the temperate zones. Now of Cirripedes, taking all the orders, there are at present known 147 species; of these, 7 have doubtful habitats, leaving 140 for comparison. Of these 140, nearly one quarter, or 37, inhabit both the torrid and temperate zones, as above defined; 46 are found exclusively in the torrid, and 57 exclusively in the temperate zones; so that the temperate zones, though less in area, and having, proportionally, even a considerably lesser length of coastline, nevertheless have a preponderance in the number of species. But it should be borne in mind, that there are _two_ great temperate districts, separated from each other by _one_ great torrid district; and, inasmuch as the number of species in any region seems to depend in some degree on the isolation of the sub-regions, we might have expected (the other conditions now being, and the _past_ conditions having been alike), that the two great temperate areas would have contained more species, perhaps doubly more, than the single great torrid area.
With respect to the relation between the size acquired by the different species of sessile cirripedes, and the temperature of the localities inhabited by them, the genera Chthamalus, Tetraclita, and Balanus, alone can serve for comparison: in Chthamalus much the largest species is found in the temperate zone: on the other hand, the two largest species of Tetraclita are from the torrid zone, though one of them also sometimes ranges into the temperate seas: in Balanus, the largest species, and six other species having a basal diameter sometimes over two inches, inhabit the temperate regions; and two out of these seven species, flourish even in the Arctic seas; whereas, within the torrid zone, there are only three species with a diameter sometimes exceeding two inches, but two of these frequently become very large and massive; so that Balanus, judging from the size of the species, as well as from their range, does not require for its highest development the temperature of the torrid zones.
The greater number of the species of the Balanidæ have wide ranges, as might be inferred from the fact of between one third and one fourth of the total number inhabiting both the torrid and temperate zones; but it should not be overlooked, that those species, as _Balanus tintinnabulum_, _amphitrite_, _improvisus_, and, in a lesser degree, _B. trigonus_ and _Tetraclita radiata_, which seem to range over nearly the whole world (excepting the colder seas), are species which are habitually attached to ships, and which could hardly fail to be widely transported. Indeed, it appears to me surprising, that such species as _Balanus psittacus_ and _eburneus_, which often become attached to vessels, should still be confined, the one to Southern, and the other to Northern America. But some other Cirripedes, which I have never seen attached to vessels, have likewise immense ranges: thus _Tetraclita porosa_ is found in every tropical and warmer sea, and _Chthamalus stellatus_ ranges round the world in the northern hemisphere, and, along the eastern side of America, far south of the equator: _Balanus spongicola_, and _Acasta spongites_, extend from the shores of Britain to the Cape of Good Hope: _Balanus lævis_ ranges from Tierra del Fuego to California. I may further remark, that the only two other species of Balanus, and the one Chthamalus, inhabiting Tierra del Fuego, are, also, found on the shores of Peru. But to show the powers of endurance in some species, I may specify the case of _Balanus improvisus_, which flourishes on the coast of Nova Scotia, amongst the West Indian Islands, in Southern Patagonia, and near Guayaquil. Even more striking is the case of _B. crenatus_, of which I have seen specimens from latitude 74° 48′ north, from the West Indies, and the Cape of Good Hope! In these two latter localities, however, it seems to be rare, and may have been first transported to them from the shores of Europe, on the bottoms of vessels, to which it sometimes adheres.
The several species of Balanidæ live attached either to coast-rocks, or to objects at various depths, down to, as in the case of _Balanus crenatus_, 50 fathoms. _Balanus balanoides_ sometimes adheres to rocks or wood so high up, that it is not covered by water during the neap tides. Mr. Thompson has informed me, that he once accidentally kept some specimens of this species out of water for seven days in a warm room, and that they were then quite lively. This species, is very easily killed by brackish water, as are some other species, whilst _B. improvisus_ and _eburneus_ can flourish in it; and at the Falkland Islands, I saw _Elminius Kingii_ attached to rocks at the mouth of a fresh-water brook, so as to be covered by pure water during the ebb of each tide. Sessile cirripedes adhere to all sorts of objects, floating and fixed, animal and vegetable, living and dead, organic and inorganic. Chthamalus is, perhaps, more commonly attached to rocks than are the other genera. Living Mollusca are, I think, the most frequent objects of attachment: Mr. Cuming has remarked to me, that shells covered by an epidermis, as Patella, Haliotis, and Mytilus, are the greatest favorites. Acasta is always imbedded in sponges, or in the sponge-like bark of Isis; Pyrgoma and Creusia in corals; Chelonobia is attached to turtles, and one species to crabs or very smooth shells; Coronula, Tubicinella, and Xenobalanus, are imbedded in the skin of Cetaceans; and Platylepas in that of manatee, turtles, or sea-snakes.
If we attempt, with our present not very imperfect materials, to divide the globe into provinces, according to the amount of difference in their Cirripedial inhabitants, including all orders and families, and disregarding entirely, as I think we ought, all probabilities or conclusions deduced from the distribution of other tribes of animals, we find that the globe may be divided into the four following great provinces and one sub-province. I should premise, that in the following remarks and tables,[78] the species of Lepas, Conchoderma, Chelonobia, Coronula, Platylepas, and Tubicinella, are excluded, owing to their being attached to floating or swimming objects, and being consequently widely and irregularly distributed. [78] As the number of Cirripedes in the whole class is not very great, I have given lists of the species in the four main provinces and in the one sub-province.
The first, or North Atlantic province, is that of Europe and the eastern shores of North America, from the arctic regions to lat. 30°: the island of Madeira, part of the north-west coast of Africa, and the whole Mediterranean being included. In this province (the above-named genera being excluded) we have 31 species, of which 22 are not found in any other distant quarter of the globe. As some few of these species range into the West Indies, I have not, on this account, excluded them from the 22 peculiar forms. Had I included the West Indies[79] in my first province, the total number of species would have been 42, of which 28 would have been peculiar. The coast of Brazil, even as far south as the Rio Plata, might, also, have been included, for I have not seen from it a single species not included in the above 42 West Indian species. So also, by adding a single species, might the west coast of equatorial Africa. The two coasts of South America and Africa, which face each other within the torrid zone, seem to be remarkably barren in Cirripedes. Europe has several more species than the United States, which is inhabited by only ten species, including even the probably imported _Balanus tintinnabulum_ and _amphitrite_. Of these ten United States species only two are not found in Europe; and both these two range into the West Indies, and as far as the northern shores of South America, and therefore cannot be considered as peculiar to the United States.
As Cirripedes belong to the great class of Crustacea, and as the distribution of the latter has lately been fully discussed by Mr. Dana, it may be worth while briefly to compare my results with his; more especially as they are so very different. I should premise, as perhaps accounting to a certain extent for this difference, that, owing to the wide range of many species, and the almost universal extension of the same genera, my provinces are founded merely on a certain proportion of the species, namely, two thirds, being peculiar or confined to a region of considerable dimensions: whereas, in the case of ordinary Crustaceans, the greater number of the species are distinct even in the sub-provinces, and the provinces are founded mainly on generic differences. Mr. Dana divides the surface of the globe into three great sections, or provinces, the _Africo-Europæan_, the extent of which is shown by its title; the _Occidental_, which includes both the east and west coast of both Americas; and the _Oriental_, including the Indian and Pacific Oceans, with the East Indian Archipelago, and Australia.
Thus Mr. Dana entirely separates the Eastern shores of North America from Europe; whereas, as far as their Cirripedial inhabitants are concerned, they are most intimately allied, and form my first or North Atlantic province; and to this, as I have shown, even the West Indies, the coast of Brazil, and equatorial West Africa might have been added. It follows, from this similarity in the Cirripedes on the two sides of the Atlantic, and from their dissimilarity with those on the shores of the Pacific, that the east and west coasts of the two Americas form two quite distinct Cirripedial provinces; though, in the northern half, some connection is shown by a few representative species: on the other hand, Mr. Dana unites both sides of the whole American continent, into his single Occidental province. The South-African province is not brought out by Mr. Dana so prominently, as I have found necessary. Mr. Dana joins the East Indian Archipelago and Australia into his single Oriental province, and makes New Zealand, as a sub-province, apparently as distinct from Australia, as Australia is from the East Indian Archipelago: whereas I find that the Cirripedes of New Zealand clearly belong to Australia; and that the Australian Cirripedes, especially if the temperate shores be alone considered, are as distinct from those of the East Indian Archipelago, as from those of any other quarter of the whole world. I believe that the provinces deduced from the distribution of Cirripedes, accord better with the Molluscan provinces, than with those given by Mr. Dana for the rest of the great class of Crustaceans.
Here we have 31 species, of which 22 are not found in any other great region of the world. (2.) SUB-PROVINCE: _Africa, South of lat. 30°._ Balanus tintinnabulum (1 to 5). Capensis* spongicola (1) and West Indies (?). amphitrite (1 to 5). crenatus (1) and West Indies. Acasta spongites (1). Tetraclita serrata* rosea (4). Chthamalus dentatus* also on West Coast of Africa. Octomeris angulosa* Scalpellum ornatum* In this small region we have only 11 species, of which five are peculiar: _Balanus Capensis_ and _Tetraclita serrata_, seem to be representatives of _B. psittacus_ of S. America and of _T. porosa_ of that and several other regions. (3.) SECOND PROVINCE: _West Coast of South and North America, from Tierra del Fuego to Behring's Straits._ Balanus tintinnabulum north and south of the equator (1 to 5). psittacus* south. vinaceus* " trigonus north and south (4 and 5). lævis* " concavus " (4 and 5). pœcilus* south. improvisus south (and north?) (1). nubilus* north. glandula north, and Southern Pacific Ocean. cariosus* north. Tetraclita porosa north and south (4 and 5 and W. Indies). Elminius Kingii* south. Chthamalus stellatus north. cirratus* south. scabrosus* " fissus* north and south. Hembeli* north. Verruca lævigata* south. Pollicipes elegans* south and north. polymerus* " Cryptophialus minutus* south. Here we have on this long line of coast, 22 species, of which 15 are peculiar.
(4.) THIRD PROVINCE: _Indian Archipelago (including the Philippines, Malacca, Borneo, Sumatra, Java, and New Guinea, and eastern coast of India)._ Balanus tintinnabulum (1 to 5). Ajax* navicula* stultus and West Indies. trigonus (3 and 5). concavus (3 and 5). amphitrite (1 to 5). patellaris* amaryllis (5). quadrivittatus* Acasta lævigata and Red Sea. fenestrata* purpurata* sporillus* Tetraclita porosa (3 and 5) and West Indies. costata* vitiata (5). cœrulescens Pacific Ocean. radiata (5) and West Indies. Pyrgoma cancellatum* grande* milleporæ* crenatum* monticulariæ* Creusia spinulosa and West Indies. Chthamalus stellatus (1 and 3). intertextus* Chamæsipho scutelliformis* Octomeris brunnea Pœcilasma fissa* eburnea* Dichelaspis Warwickii* Ibla Cumingii* Scalpellum rostratum* Pollicipes mitella* Lithotrya Nicobarica* truncata and Pacific Ocean. Here we have 37 species, of which 24 are peculiar to this province. (5.) FOURTH PROVINCE: _Australia (including New Zealand)._ Balanus tintinnabulum (1 to 5). nigrescens* decorus* trigonus (3 and 4). concavus (3 and 4). amphitrite (1 to 5). amaryllis (4). allium* vestitus* imperator* Acasta sulcata* glans* Tetraclita porosa (3 and 4). rosea (2). purpurascens* vitiata (4). radiata (4) and West Indies. Elminius plicatus* simplex* modestus* Chthamalus antennatus* Chamæsipho columna* and Pacific Ocean (?). Pachylasma aurantiacum* Catophragmus polymerus* Alepas tubulosa* Ibla quadrivalvis* Scalpellum Peronii* Pollicipes spinosus* sertus* Lithotrya cauta* Here we have 30 species, of which 21 are peculiar. _Geological History._
The ancient history of the Balanidæ is a brief one. No secondary species has hitherto been discovered; in my monograph on the fossil Lepadidæ[80] I have shown that the negative evidence in this case is of considerable value, and consequently that there is much reason to doubt whether any member of the family did exist before the Eocene period. The existence of a Cretaceous Verruca is an apparent exception to the rule, as this genus has hitherto always been ranked amongst sessile cirripedes; but Verruca, as we now know, must be placed in a family by itself, quite distinct from the Balanidæ. Balanus is the oldest genus as yet known; it first appeared in Europe and North America, during the deposition of the eocene beds; and was at that time, as far as our information at present serves, represented by very few species. In South America, one species of Balanus abounds in individuals in the ancient Patagonian tertiary formation. I have seen, in the British Museum, specimens, said to have come from the eocene nummulitic beds near the mouth of the Indus, belonging to the second section of the genus. Generally, the extinct forms belong to the last section of this genus, which has the parietes not permeated by pores. During the miocene and pliocene ages, sessile cirripedes abounded. No extinct genus in this family has hitherto been discovered. It is singular, that though the Chthamalinæ approach much more closely than do the Balaninæ to the ancient Lepadidæ, of which so many species have been found fossil even in the older Secondary formations, yet that only one species of one genus of this sub-family has been hitherto found in any deposit; and that species is the still existing _Pachylasma giganteum_, in the modern beds of Sicily. During the epoch of the Glacial deposits in Scandinavia, Scotland, and Canada, the still existing species seem to have abounded; and they attained larger average dimensions than the same species now do on the shores of Great Britain, or even on the shores of the northern United States, where the average size seems larger than on this side of the Atlantic.
Ideas discussed in this chapter
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