Under the Order it has been stated that the [Balanidæ], are, on the cirripedial type, the highest in the class; that is, they are the most complicated, but not (to use Professor Owen’s term) by mere vegetative repetition. Amongst the [Balanidæ], the first section of the genus [Balanus] may be taken as typical; here we have the structure of the shell extremely complicated, yet beautifully adapted for strength, and for the protection of the included body. The cementing apparatus is here, also, most complicated. I have divided the [Balanidæ] into two natural sub-families, the [Balaninæ] and [Chthamalinæ], in accordance with certain differences in the structure of the shell and of the animal’s body: that this division is natural, might almost be inferred from the one fact, that all the characters by which the [Chthamalinæ] differ from the [Balaninæ], are those by which the former approaches the family of the Lepadidæ; moreover, certain anomalous characters in the [Chthamalinæ], as the supplemental whorls of compartments in [Catophragmus], and the presence of caudal appendages in this same genus and in [Pachylasma], reveal this same affinity. The only objection which I can see to the separation of the sixteen genera into the above two sub-families, may be drawn from the degree to which they blend together; thus, as far as the shell is concerned, [Chelonobia], in one important internal point of structure, tends to assume the character of the [Chthamalinæ]; and on the other hand, [Pachylasma], a member of the [Chthamalinæ], has a shell, which if not examined during its earliest growth, would be placed without doubt amongst the [Balaninæ]. But it fortunately so happens, that in no one character of the body does [Pachylasma] approach the [Balaninæ] more nearly, than do the other members of its sub-family; or [Chelonobia] approach, in the same respects, the [Chthamalinæ]. It is only in [Chthamalus], of which the shell clearly places it in the sub-family bearing its name, that in some of the species, the less bullate labrum,—the larger palpi,—the lower teeth of the mandibles being laterally double,—and the lower segments of the third pair of cirri being thickly clothed, like the lower segments of the second pair, with bristles—all show that these species make an approach in structure to the [Balaninæ].
It will be seen that I have divided the [Balaninæ] into two little groups, according as whether the branchiæ consist of one or of two plicated folds of membrane, and as whether or not the scutum and tergum are articulated together. I have been greatly tempted to follow Drs. Leach and Gray, who have separated the second of these groups, containing the genera [Coronula], [Tubicinella], [Xenobalanus], and [Platylepas], into the sub-family of the Coronulinæ. Certainly these genera have a peculiar aspect in common, and agree in being parasitic and imbedded in the skin of Cetaceans, as is the case with the first three genera, or in that of turtles, manatee, and sea-snakes, as in [Platylepas]. Though these genera possess several peculiar characters, yet I can find none common to all four, excepting their imbedment in the skin of Vertebrata, their double branchiæ, and their non-articulated opercular valves; and these I do not think of sufficient importance to serve for the separation of a sub-family; for in [Chthamalus], one species has double branchiæ, one species has no branchiæ at all, and the other species have single branchiæ; so again in [Chelonobia], the scutum having only a horny articular ridge, makes an approach to [Coronula] and its allies. I may further specify that the folded walls, a singular character common to [Coronula], [Platylepas] and [Xenobalanus], fails in [Tubicinella]; the open tubes and the imperfect outer lamina of the parietes towards their bases, are characters which fail in one species of [Platylepas]; the muscles running to the opercular valves being thinly spread out, and partially without transverse striæ, is also a character which fails in [Platylepas]; the simplicity of the cement-ducts partially fails in [Tubicinella]; and lastly, the existence of small intermediate teeth on the mandibles, fails in [Xenobalanus]: hence, I repeat, I have not thought it prudent to admit the sub-family of the Coronulinæ though in many respects a very natural group.
The genera in the [Balaninæ] and [Chthamalinæ] are founded chiefly on the number of the compartments (the number being apparently due, as previously explained, to the fusion or abortion of certain of the eight typical compartments); and secondarily, on the nature and even form of the basis, and on the porosity of the walls. In [Coronula] and its allies, the non-articulated opercular valves and deeply folded walls come into play. As a justification for using these characters in distinguishing the genera, and even to a certain extent in separating the two sub-families, I must call to mind that the shell, with the basis, is not merely a dermal envelope, as amongst Molluscs, but actually consists of the first three segments of the head. The parts of the mouth and the cirri are of very little service in distinguishing the genera,—a singular fact, considering that most of the genera amongst the Lepadidæ could be distinguished by these organs,—though trifling details in their structure sometimes come in useful as specific characters. [Balanus], with the sub-genus [Acasta]; [Pyrgoma], with the sub-genus [Creusia]; [Tetraclita], and [Elminius], are genera of about equal value; though perhaps the two latter are rather more nearly related together than to the others. [Chelonobia] is more distinct; it shows some little affinity to the [Chthamalinæ], and likewise to the four following genera. [Coronula], [Platylepas], [Tubicinella], and [Xenobalanus], are genera quite distinct from the foregoing, and from each other; yet, as we have just seen, palpably allied together. Amongst the [Chthamalinæ], [Pachylasma], [Octomeris], and [Catophragmus], are more closely related to each other than to the other two genera of the sub-family; yet [Pachylasma], as far as the shell is concerned, leads into the [Balaninæ], and [Catophragmus] into the Lepadidæ; [Octomeris] leads towards [Chthamalus], and [Chthamalus] towards [Chamæsipho].
Variation.—The discrimination of the species in most of the genera, offers very great difficulties. I cannot too strongly impress on any one intending to study this class, not to trust to external characters: he must separate and clean, and carefully examine the internal structure and form of the compartments, and more especially of the opercular valves. After considerable experience, when numerous varieties of a species have been carefully examined, the eye acquires a sort of instinctive knowledge, by which it can recognise the species, though the character cannot be defined by language; but I have found that no amount of experience with some of the commonest species, will save frequent and grave errors, as long as external characters alone are trusted to. Not only does every external character vary greatly in most of the species, but the internal parts very often vary to a surprising degree; and to add to the difficulty, groups of specimens not rarely vary in the same manner. 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. I may in one respect even go further, and affirm, that, if in a species, any part or organ differs remarkably from the same part in its congeners, then if many specimens are examined, especially when collected from different districts, such part or organ will be found eminently variable. I may instance the antenniformed third pair of cirri in [Chthamalus antennatus], the teeth on the posterior cirri in [Acasta sulcata], the terga in [Pyrgoma dentatum], the adductor ridge of the scuta in [Pyrgoma cancellatum] and in [Creusia], and other such cases: hence it will not do to found a species on a slight, or sometimes even on a considerable difference, in any single point or organ. On the other hand, I am far from asserting, that if only half-a-dozen specimens of some rare species of Cirripede be brought from some one quarter of the world, characters beautifully defined may not be readily discovered. 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.
[74] In Daphnia, Straus (‘Mém. du Museum,’ tom. vi, p. 151) found that the individuals moulted every five or six days.
[75] Linnæus speaks of the included body (inhabitant as he calls it) of other Cirripedes, as a Triton; and this, I think, shows that Lesson’s conjecture that the Triton was an Alepas cannot be correct; for Linnæus could hardly have supposed that a pedunculated cirripede inhabited another pedunculated or sessile cirripede.
I have seen a few specimens showing that when the shell has been broken it can be repaired; and this I believe is effected by the growth of a crest of corium between the broken edges, and the subsequent calcification of this crest. Mr. Stutchbury possesses a monstrous specimen of [Chelonobia testudinaria], in which one of the lateral compartments on one side has not been developed. The cirri not rarely get cut off, but are, as it appears, soon repaired. I have observed a singular number of examples of the act of reparation in a group of the Australian [Balanus vestitus]. The manner in which the cirri are repaired seems to me curious: the cut-off end is closed by a rounded scale of yellowish chitine, and then the corium, in the four or five subjacent segments, separates from the external articulated membrane, which now serves only as a case or capsule. The tube of corium thus enclosed, with its contained muscles, shrinks a little, and soon can be perceived to be in process of dividing into new and smaller segments, in one instance ten in number,—which at the next exuviation would, no doubt, be invested with an external membrane, and be freely exposed. 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.