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 have already alluded to the longitudinal furrow on the tergum so entirely changing its character, owing to the edges becoming, during growth, folded inwards; this likewise causes a decrease in the proportional breadth of the spur. In old specimens of [B. flosculus, var. sordidus], the whole tergum is much more elongated than in young specimens. The basal margin of the sheath is hollow beneath in the young of [B. cariosus] and of some other species, but in the old it is continuous with the inner surface of the walls. The inner lamina of the parietes generally loses, to a certain extent, its longitudinally ribbed character in old age. The basis is solid, instead of being porose, in very young specimens of [B. improvisus]. In all the species, the carino-lateral compartments, in early age, are very narrow in proportion to the width of the lateral compartments; and in all, at this early period, the operculum is large in proportion to the whole shell. The number of spines on the edge of the maxillæ, the number of segments in all the cirri, and the number of spines on each segment, are few in early youth, and go on increasing with each successive exuviation: the pedicels of the cirri are long in proportion to the rami at this same early age.[84]
[84] In some specimens of [Balanus perforatus] I made the following enumeration of the number of segments in the cirri:—
| Basal diameter of shell 1/20th of an inch. | Basal diameter of shell 1/5th of an inch. | Medium sized specimen about 3/4ths of an inch in basal diameter. | |
|---|---|---|---|
| First cirrus shorter ramus | ? | 11 | 17 |
| Second cirrus | 4 or 5 | 9 | 13 |
| Sixth cirrus | 9 or 10 | 19 | 31, in another 36 |
In the specimen 1/5th of an inch in basal diameter, each segment of the posterior cirri carried five pairs of spines; whereas, in full-grown specimens, there are six or seven pairs. In the 1/20th of an inch specimen, on the inner maxillæ, there were no spines between the upper large and the lower large pair of spines; whereas, in the 1/5th of an inch specimen, there were five intermediate spines, and in larger specimens nine or ten spines.
Notwithstanding the difficulties now enumerated, I hope that, owing to having examined a vast number of specimens of the most varying species, I have not fallen into very many errors. I have endeavoured to err on the side of making too few instead of too many species. In those cases, however, in which I have seen only a few specimens, I have been sometimes compelled to decide without sufficient evidence.
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].