Chapter VI: Introduction (2)

The males of _Scalpellum vulgare_, _ornatum_, and _rutilum_, resemble each other in all essential points, and differ wonderfully in appearance and structure from all ordinary Cirripedes. They consist of a minute flattened bag with a small orifice at the summit, and at the lower end attached by the cemented pupal antennæ. On each side of the orifice, there is a pair of calcareous beads, representing the two scuta and two terga of ordinary Cirripedes; and between the scuta a minute black eye is generally conspicuous. In _S. ornatum_ the beads, I may remark, on the two sides are not equal; those either on the right or on the left side, being larger than those on the opposite side, so that the animal externally is asymmetrical. Inside, within a tubular sack, the thorax is lodged, supporting four (instead of six) pairs of limbs; and these, instead of forming biramous, multiarticulated, captorial cirri, are reduced to almost a rudiment, supporting a few long sharp spines, which apparently act only as defensive organs. At the end of the thorax there is seated a large abdominal lobe, which does not occur in the other sex. Hence the thorax, though rudimental, has been specially modified. Of the mouth and stomach there is not a vestige. Constructed as these males are, assuredly they have no claim to be ranked amongst the Lepadidæ or pedunculated Cirripedes; nor is it possible to class them in any group whatever of ordinary Cirripedes. In _S. vulgare_ the males are attached, often several together, to the extreme edges of the two scuta, and therefore immediately over the orifice leading into the sack; in _S. rutilum_ and _ornatum_, they are attached in concavities on the under side of both scuta, just above the depression for the adductor scutorum muscle. In the former of these species, the pit for the reception of the male is formed by shelly matter not having been deposited over a certain space on the under side of the valve; and the pit is converted by a covering of membrane into a pouch. As there are two scuta so there are two pouches, in each of which a male is lodged; hence, according to the Linnean nomenclature, _Scalpellum ornatum_ may be said to belong to Diandria monogynia. As these males, from being mouthless, soon die, they are succeeded by successive pairs; the pupa being led by a wonderful instinct to crawl into the pouch, and there undergo its metamorphosis.

Lastly, the males of Alcippe and Cryptophialus (Pl. 23, fig. 19, and Pl. 24, fig. 19) are remarkable for their similarity to each other, considering the essential dissimilarity of the two females. The females live in cavities which they excavate in the shells of Molluscs, and within which they are attached by a horny disc; this disc is the only part of the outer integument which is not frequently moulted, and, apparently in consequence, the males are attached to its edges. It results from this position, that the males are protected by being enclosed within the cavity excavated by the female; and it further results, that the males are attached at a considerable distance from the orifice of the sack of the female, into which the spermatozoa have to be conveyed; and to effect this, the probosciformed penis is wonderfully developed, so that in Cryptophialus, when fully extended, it must equal between eight and nine times the entire length of the animal! These males, like those last mentioned of Scalpellum, consist of a mere bag, lined by a few muscles, enclosing an eye, and attached at the lower end by the pupal antennæ; it has an orifice at its upper end, and within it there lies coiled up, like a great worm, the probosciformed penis, and beneath it a single testis, with a single vesicula seminalis. These organs complete the whole organisation of the male; for there is no mouth, no stomach, no thorax, no abdomen, and no appendages or limbs of any kind. Yet all these parts are present in the female. I know of no other instance in the animal kingdom of such an amount of abortion. The whole exterior of these males evidently is composed, as in all ordinary Cirripedes, of the three first cephalic segments; of the fourteen succeeding segments of the archetype Cirripede we have not a vestige, excepting the probosciformed penis, which, from analogy, should arise from the ventral apex of the seventeenth segment, the first three segments of the head being counted in the seventeen. Here, then, fourteen out of seventeen segments have aborted, the tip of the seventeenth having coalesced with the third cephalic segment! I am tempted just to notice the case of Proteolepas, in the order Apoda, as showing, within the limits of the same sub-class, a wonderful amount and diversity in abortion; for in Proteolepas, the three anterior cephalic segments are reduced to the merest rudiment, encasing the cement-ducts, the fourteen succeeding segments being unusually well developed; whereas in the above described males, we have just seen the three anterior segments fully developed, whilst the fourteen succeeding segments are lost or have coalesced with the others; so that within the same sub-class all seventeen segments of the archetype have almost disappeared.

It may be asked how I know that the several above described rudimentary epizoons are really the males of the Cirripedes to which they are attached. Even if the whole course of the metamorphoses had not been known in three of the cases, the mere fact of these epizoons being cemented by the three terminal segments of their peculiar, pupal antennæ, would have been sufficient to have shown that they belonged to the class of Cirripedes. In nearly every case, I was able to demonstrate, and not in one or two but in many specimens, that these epizoons were males; and as in several instances the spermatozoa were developed, and as, notwithstanding, in no instance was there a vestige of ova or ovaria, it may safely be concluded that they were not hermaphrodites, and therefore required females of some kind. If these epizoic Cirripedes had been independent animals, as they all belong to the same sub-class, and all have such peculiar habits, it might have been expected that they would have shown some special affinity towards each other; but this is not the case; the epizoon of Ibla is more nearly related to Ibla, and the epizoon of Scalpellum more nearly related to Scalpellum, than are these epizoons to each other. If the several epizoons were classed by themselves, they would be grouped in divisions, corresponding with those of the Cirripedes on which they are attached, which is just what might have been expected if these latter were their females. There are, also, many special relations between the male epizoons and the Cirripedes to which they are attached; thus, the mouth of the epizoon of Ibla, is so like the mouth of Ibla, which is peculiar in several respects, that I should easily have recognised it as belonging to a member of that genus. _Scalpellum villosum_ is remarkable as one out of only two or three members of the whole Family, which is destitute of caudal appendages, so is its male epizoon; again, _S. villosum_ is unusually spinose, so is its male epizoon; on the other hand, _Scalpellum ornatum_ is remarkably smooth, so is its male epizoon; and I could give other similar instances. Will it be believed that these coincidences are accidental, and that the epizoons have no special or sexual relation to the Cirripedes to which they are attached?

One other instance of coincident structure is so important, that it must, even in this sketch, be noticed; the prehensile antennæ of the pupa are most important and complicated organs, and differ in the different genera of the same family; they are preserved in a functionless condition throughout life, and in two instances I was able accurately to compare these organs in the epizoon and in the Cirripede to which it was attached, and they were identical in every particular. The full force of the excessive improbability of this resemblance, and of the above coincidences in structure, on the supposition of the epizoon and its support not being sexually related, will hardly be perceived without referring to the facts given in detail in my former volume.

Lastly, in the case of Cryptophialus (and indirectly in that of Alcippe) the nature of the male epizoon is, I think, actually demonstrated; for I traced both it and the female or ordinary form of Cryptophialus, through the same several larval stages, from the egg, enclosed within the sack of the female, to the pupa and mature animal. Moreover, if the male nature and sexual relation to the supporting Cirripede, be admitted in any one of these epizoons, then so close is the agreement in habits, and to a certain extent in structure, in all the foregoing epizoons, that probably no one admitting one instance would dispute the others, and further evidence would even be superfluous. Indeed, had it not been for the following facts, I should not have brought forward, either here in abstract, or in other places in detail, so many arguments and so much evidence.

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 not allowing the internal organs to be examined), and there cannot be any reasonable doubt that this species likewise is exclusively female. It should be borne in mind that the male organs, external and internal, are most easily discovered, and that in the above cases I had an abundant supply of excellent specimens. On the other hand, in _Ibla Cumingii_, and in four species of Scalpellum, I was able to demonstrate in the supporting Cirripede the presence of all the male organs, as well as of the female; and in the vesiculæ seminales of several specimens, both in the Ibla and in _Scalpellum vulgare_, spermatozoa were contained; the male organs, however, not being very amply developed. These species, consequently, are not exclusively female, but are hermaphrodite, though having male epizoons attached to them. This statement, I am well aware, is enough, at first, to cast a doubt on all that I have said; but let any one reflect on the evidence, of which I have here given a summary, and which has been elsewhere given in full, and I think he must admit that at least those epizoons which are exclusively male, and which are attached to Cirripedes exclusively female, are sexually related and form one species; but if he admit this, he cannot possibly escape from the conclusion that some of the other epizoons, for instance that of _Ibla quadrivalvis_, are the males of the hermaphrodites to which they are attached,--these epizoons not exclusively impregnating the ova of a female, but aiding the self-impregnation of an hermaphrodite. Hence I have called these males _Complemental Males_, to show that they do not pair with a female, but with a bisexual individual. Nothing strictly analogous is known in the animal kingdom, but amongst plants, in the Linnean class, Polygamia, closely similar instances abound.

In the series of facts now given, we have one curious illustration more to the many already known, how gradually nature changes from one condition to the other,--in this case from bisexuality to unisexuality. Finally, in the four genera so often named, we meet the following several cases, some of them even the most diverse, occurring in closely allied species. (1st), a female, with a single male (rarely with two) permanently attached to her, protected by her, and capable of seizing, by the movements of its pedunculated body, any minute animals or substances found within her sack; (2d), a female with successive pairs of short-lived, mouthless males, inhabiting pouches on each side under her scutal valves; (3d), a female with many, in one instance fourteen, short-lived males, destitute of mouth, thorax, and appendages, but furnished with a stupendously long male organ, attached to a thickened portion of her outer integuments, but lying within the cavity which she has excavated; (4th), an hermaphrodite with a male attached within the sack, capable of feeding itself, as in the first case; (5th), an hermaphrodite with from one to three males, organised like ordinary Cirripedes, and apparently capable of seizing prey in the common way; and attached between the scuta, and thus protected; (6th and lastly), an hermaphrodite with from one or two up to five or six, short-lived, mouthless males, like those in the second case, attached in one particular spot, on each side of the orifice leading into the sack.

ORDER I.--THORACICA.

_Cirripedia having a carapace, consisting either of a capitulum on a peduncle, or of an operculated shell with a basis. Body formed of six thoracic segments, generally furnished with six pairs of cirri; abdomen rudimentary, but often bearing caudal appendages; mouth with the labrum not capable of independent movements; larva firstly uniocular, with three pairs of legs, lastly, binocular, with six pairs of thoracic legs._

In the sketch of the three Orders given in the Introduction, it will have been seen that the differences in their structure are so great, that it would have been hardly possible to have given a single blended account of the whole Class. But as all common Cirripedes are included in the present Order, here would have been the natural place for a full description of their external and internal structure. Having, however, been necessarily, yet perhaps unfortunately, led to give, in my former volume, a description of this kind of the Lepadidæ; and as it is necessary to give a similar account of the other great family of the Order, namely, the Balanidæ, I have found it more convenient to make this latter account comparative and supplemental to the former one on the Lepadidæ, and so serve for the Order, rather than attempt to give a separate description in full of it. For this latter plan would have involved much useless repetition, as, on account of the many exceptions and limitations necessary to almost every statement, there is little choice between a description of great length and a mere diagnostic character of the Order, such as I have given above.

The Thoracica may be divided into three very natural Families, of nearly equal value; firstly, the Balanidæ, or sessile Cirripedes, which may be subdivided into two sub-families, also very natural, the Balaninæ and Chthamalinæ; secondly, the Verrucidæ, containing only one genus; and thirdly, the Lepadidæ, or pedunculated Cirripedes. These three families differ from each other, besides in mere external appearance, almost exclusively in the relation of the different portions of their external covering or carapace, and of the muscles moving such portions. In the Balanidæ, the four opercular valves surrounding the orifice leading into the sack, are capable of other movements, besides being opened and shut; whereas all the other valves are immoveably united together. In the Lepadidæ, the valves answering to the opercular valves, are furnished with a muscle only for shutting them; whereas the peduncle answering to the basis in the Balanidæ is capable of various movements. In the Verrucidæ the shell is singularly asymmetrical; only half of the operculum (either the right or the left side, this varying even in the same species) being moveable; the other half being immoveably united to the remaining valves; and the whole shell has only one muscle serving to shut the moveable half of the operculum. All the internal parts and organs are very similar in the above three Families. If, however, the internal structure of one of the two sub-families, into which the Balanidæ may be divided, namely, of the Balaninæ, be compared with that of the Lepadidæ, several important differences may be detected;--on the one hand, in the Balaninæ, the presence of branchiæ, the extremely complicated cementing apparatus, the difference in structure between the third and succeeding pairs of cirri, the large palpi, the notched labrum, and the laterally double teeth of the mandibles;--and on the other hand, in the Lepadidæ, the presence of ovigerous fræna, caudal appendages, bullate labrum, and often prominent olfactory orifices. But if the Lepadidæ be compared in these several respects with the other sub-family, or Chthamalinæ, which cannot possibly be removed out of the family of Balanidæ, many of these differences break down and disappear, in some or all of the species.

The Lepadidæ include, as has previously been noticed, a much greater range of difference than the Balanidæ; and this is what might have been expected, for it is the most ancient family, and extinction has done its work, separating genera, which, in accordance to analogy, we may suppose were once more nearly connected by intermediate forms. The Lepadidæ, in one sense, may be taken as the type of their order; for they have undergone less "morphological differentiation;" that is, they differ the least from the last larval stage, and seem to give the most _general_ idea of a Thoracic Cirripede. On the other hand, if we mean, as some authors do, by the word type, that form which, in the group in question, has been most modified, and illustrates every peculiarity of its class in the strongest manner, then we must look to the Balaninæ, and to its typical genus, Balanus, for the most Cirripedial form. In this genus the different portions of the carapace differ most, and subserve to a certain extent different ends, and in minute structure are most complicated; here the cementing apparatus, which offers the main characteristic of the whole sub-class, is most complex; here the several pairs of cirri differ most from each other in structure and action; here the peculiar branchiæ (organs apparently derived from the modification of another organ, itself confined to Cirripedes, viz., the ovigerous fræna) are best developed; here the nervous system is most highly concentrated; and, lastly, here we meet with the largest and most massive species of the whole group.

1. Family BALANIDÆ, (or Sessile Cirripedes).

_Cirripedia without a peduncle; scuta and terga furnished with depressor muscles; other valves united immoveably together._

TABLE OF CONTENTS.

Page

Structure of shell 34
" of the individual compartments 43
" of the radii 45
" of the alæ 47
" of the sheath 48
" of the basis 49
" of the opercular valves (scuta and terga) 51
Growth of whole shell and microscopical structure 54
Muscles of sack 61
Branchiæ 63
Thorax and body 65
Muscular system 68
Movements and muscles of the cirri 71
Mouth 74
Cirri 81
Caudal appendages 85
Alimentary canal 85
Circulatory system 87
Nervous system 88
Eyes and vision 93
Acoustic organs 95
Olfactory sacks 97
Male organs of generation 97
Female organs of generation 100
Metamorphoses and homologies 102
Larva, first stage 103
Larva, second stage 109
Larva, last or pupal stage 110
Act of metamorphosis 126
On the homologies of the carapace 131
Cementing apparatus 133
Affinities, classification, variation 152
Rate of growth, exuviation, &c. 156
Geographical range and habitats 159
Geological history 172

Almost every one who has walked over a rocky shore knows that a barnacle or acorn-shell is an irregular cone, formed generally of six compartments, with an orifice at the top, closed by a neatly-fitted, moveable lid, or operculum.[19] Within this shell the animal's body is lodged; and through a slit in the lid, it has the power of protruding six pairs of articulated cirri or legs, and of securing by their means any prey brought by the waters within their reach. The basis is firmly cemented to the surface of attachment. The whole shell, basis, and operculum consists, as we have already seen, of the first three segments of the head, modified into a singularly constructed carapace, which encloses the mouth and rest of the body. The anterior extremity of the shell is situated in the centre of the basis, where indeed, by due care, the antennæ of the pupa may be always detected; and the posterior extremity is directed vertically upwards.

[19] The best published description of the structure of the
shell of a sessile Cirripede, is given by Dr. Coldstream, in the
'Encyclopædia of Anatomy and Physiology,' article 'Cirrhopoda.'

_Structure of Shell._

When the shell of a sessile Cirripede or barnacle, for instance, of a Balanus, is first examined, the structure appears extremely complicated; but this can hardly be considered as really the case. The structure will, I think, be best understood by recalling to mind that of Pollicipes,--the oldest known genus, from which, in one sense, all ordinary Cirripedes, both sessile and pedunculated, seem to radiate. I must premise, and the fact in itself deserves early notice, that the homologies of the several parts in the pedunculated and sessile Cirripedes admits of no doubt,--that is, if amongst the pedunculated, the genus Pollicipes, or certain species of Scalpellum, be taken as a standard of comparison.[20] The peduncle corresponds with the basis, as may be clearly seen, if a Pollicipes with a short peduncle, and a Balanus, with a deep cup-formed or cylindrical basis be compared, for the contained parts are similar, and both grow at their upper edges upwards and outwards. Secondly, the valves round the lower part of the capitulum of a Pollicipes, though generally much more numerous, and forming more than one whorl or circle, and not so closely packed together, answer to the compartments forming the shell of a sessile Cirripede; this is shown by their lateral and downward growth, by their upper ends generally projecting freely above the cavity in which the animal's body is lodged; and in the case _Pollicipes mitella_, by an actual resemblance in outline, some being triangular, some broad at the upper end, and some sub-rhomboidal, and, lastly, in the manner in which they slightly overlap and indent each other: moreover further resemblances in the relative position and even in the size of the several valves, will hereafter be pointed out between certain sessile genera amongst the Chthamalinæ and certain genera of the Lepadidæ. Thirdly, the scuta and terga in Pollicipes, so strikingly resemble in manner of growth in position relatively to the animal's body--in shape--and even in being articulated together, the valves which form the operculum or lid of sessile Cirripedes, that their identity is at once obvious.

[20] Dr. J. E. Gray long ago observed these homologies. If Lepas be
taken, the comparison is not quite so simple, owing to the growth
of all the valves in that genus being upwards; but in several
species of Scalpellum we may see the intermediate steps between
the normal downward growth of the valves in Pollicipes, and the
abnormal upward growth in Lepas.

It may be well here further to premise, that apparently none of the sutures in the shells of Cirripedes correspond with the articulations between the three archetype cephalic segments, of which the whole shell is formed; or with the eight elemental pieces, of which each separate segment in the archetype crustacean is known to consist. But, as I believe, the several valves in the shell of a Cirripede are homologous, or at least analogous, with the sclerodermic plates,[21] of which the carapace of the Podophthalmia is formed; with this difference, that in the latter they become, after their first formation, united together into a single piece, and are thus moulted as a whole; whereas in Cirripedes, the valves or sclerodermic plates are not moulted, but continue to be added to throughout life.

[21] Milne Edwards, 'Annales des Sciences Naturelles,' tom. xviii,
(1852), p. 236.

In Pollicipes, there is no difficulty in understanding the growth of the lower valves of the capitulum, especially if a species be taken in which these valves stand a little way apart: at each period of growth, they are added to at their basal edges and a little way up both sides; at the same time, a new membrane connecting them together is formed, the old membrane disintegrating, or being left hanging in tatters to the last zone of growth. Now if we look at the shell of a sessile Cirripede, there is no essential difference in the growth of the compartments or valves; all grow downwards and laterally; but they overlap each other much more laterally than in Pollicipes, and the connecting membrane is in most parts reduced to a mere film jammed in between the valves; but, in the case of the opercular membrane, it still remains wide, and is periodically moulted.

In the annexed woodcut (fig. 1), of the rostrum of _Balanus Hameri_, the downward growth and the lateral growth on both sides is plain. The modified sides (_rr_) for convenience sake, have been called the _radii_; they invariably overlap the adjoining compartments. The middle part (_p_), has been called the wall, or parietal portion: in the specimen figured, the walls and radii are distinctly separated, but in some cases, especially amongst the Chthamalinæ, the lines of growth are absolutely continuous from one to the other. In fig. 2 of a Lateral compartment of the same Balanus, we have the same essential structure; but the left side (_a_) is more protuberant, and is hollowed out in its lower half; it is, also, more distinctly separated from the parietal portion: this side has for convenience been called the _ala_; it is invariably overlapped by the adjoining compartment: in some few cases, as in Pachylasma, the ala is not hollowed out in its lower part, and from being added to in a straight line along its whole edge, with the lines of growth continuous with those on the wall, it differs hardly at all in appearance from a radius. Lastly, in fig. 3 of the carina, or compartment facing the rostrum, we have alæ (_aa_) on both sides; these being, as in all cases, overlapped by the adjoining compartments.

Fig. 2.

Fig. 3.

_p_, _p_, Parietes; _r_, _r_, Radii; _a_, _a_, Alæ.

Fig. 1, Rostrum with two radii, serving in the Chthamalinæ for rostro-lateral compartments.

Fig. 2, always serving for lateral and carino-lateral compartments.

Fig. 3, Carina, serving in the Chthamalinæ, also, as a rostrum.]

Now, the compartments in the shell of every sessile Cirripede, are without exception constructed on the above three simple patterns. In number, they are 8, 6 or 4, or all confluent together.

Considering this simplicity in growth and form of the separated compartments, it seems at first surprising that the construction and enlargement of the whole shell in Balanus, should long have been viewed as a difficulty. But the radii, from growing against rectangular indentations, or rather furrows, in the opposed compartments, come to be set a little inwards; and their external surfaces assume a very different appearance from the wall-portions of the compartments, which grow against the surface of attachment. In different species, the summits of the radii (and of the alæ) grow either very much more obliquely than in the species figured, or more squarely--that is, they extend from tip to tip of the adjoining compartments, parallel to the basis. In this latter case, and when the surfaces of the radii differ considerably in appearance from the walls, as in _Balanus tintinnabulum_ (Plate 1), I am not at all surprised that the radii should have been described as separate elements, and called "areæ interjectæ," or "compartments of the second order:" for the shell of this Balanus seems to be composed of six wedges with their points upwards, namely, the parietal portions of the compartments, and of six other narrower wedges, the radii, with their points downwards; and the fact that these latter wedges consist simply of the sides of the parietal portions, modified by growing against the adjoining compartments, is completely masked. I should add, that sometimes the radii are not developed, which simply means that the overlapping lateral edges of the compartments have not been added to during growth.

The alæ are originally developed at the period of the metamorphosis, as slight lateral protuberances in the upper part of the compartments; from being overlapped, and therefore not exposed to external influences, and from growing (as in the case of the radii) against rectangular indentations or furrows in the adjoining compartments, they generally assume an extremely different appearance from the parietes, and might naturally be thought to have a very different nature. But the alæ in all cases (as is obvious in Pachylasma) are nothing but the protuberant lateral edges of the compartments, rendered thin and modified during growth. In order that the margins of the alæ should be received in an indentation, the upper internal surfaces of the walls of the recipient compartments are thickened all round, excepting where they receive the alæ. This thickened, upper, internal portion of the walls or shell, together with the alæ themselves, form the part called the sheath. The sheath sometimes blends insensibly into the lower parts of the compartments, and then perhaps it would not be thought to be a distinct element; but often it is abruptly separated by an overhanging edge (see Pl. 9, fig. 5 _b_, 9 _b_; Pl. 20, fig. 1 _b_; Pl. 25, fig. 1, K′) from the lower part, and then the sheath greatly complicates the internal appearance, but not the essential structure of the shell. The sheath acts beautifully, like an internal hoop, in strengthening the shell round the orifice, where it is naturally weaker than at the lower or basal end, where it adheres to the surface of attachment: in the upper part of the shell, moreover, the sutures between the compartments do not go straight through, but owing to the alæ projecting and being overlapped, are extremely oblique; or the joints, in the language of carpenters, may be said to be broken.

There is one other point of structure in the shells of the Balanidæ, more especially of species like _Balanus tintinnabulum_, which adds to their apparent complexity, namely, that the rim or orifice of the shell formed by the upper ends of the compartments, projects considerably above the opercular valves. 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; or more or less united into a ring so as to form an orifice, as in the different species of Balanidæ. It follows, that to understand the real shape of a Balanus, or rather of the cavity enclosing the animal's body, all that part of the shell which projects above the opercular membrane, may, in imagination, be removed as something extraneous, like so many spines; not that I mean to say that these points of shell are dead; on the contrary, they are often porose and penetrated by numerous threads of corium. This upper part of the shell, thus produced so as to form an orifice, no doubt serves to protect the less strong and moveable operculum.

Fig. 5. Chthamalus.

Fig. 6. Chamæsipho.

Fig. 7. Balanus.

Fig. 8. Tetraclita.

_a_, Rostrum; _b_, Rostro-lateral, _c_, Lateral, _d_, Carino-lateral compartment; _e_, Carina.

Horizontal sections through the Shells of the principal genera of Balanidæ, showing the arrangement of the Compartments. Genera 4, 5, and 6 belong to the Chthamalinæ; 7 and 8 to the Balaninæ.]

_Number and Arrangement of the Compartments._--I have already stated that the shell, in every one of the Balanidæ, consists of eight, six, or four compartments, or of all fused together into a single piece; and that the compartments themselves are all constructed on the three simple patterns of which woodcuts (figs. 1, 2, 3) have been given. They are arranged in a certain definite order. The type arrangement is found amongst the Chthamalinæ, as might have been expected, inasmuch as this sub-family is so closely related to the ancient genera Pollicipes and Scalpellum, whence all the Thoracic Cirripedia may be said to radiate. In Octomeris (fig. 4) the type-arrangement of the compartments, eight in number, is well shown; the rostrum and carina resemble each other, and have alæ on both sides, and therefore are overlapped on both sides: the rostro-lateral compartments have radii on both sides, and therefore overlap the adjoining compartments on both sides; the lateral and carino-lateral compartments have radii on their carinal, and alæ on their rostral sides; and therefore overlap on one side, and are overlapped on the other side. Now the shell of every other sessile Cirripede differs, I believe, from that of Octomeris, only in the fusion together or abortion of some of the eight normal compartments: in one genus, however, Catophragmus, outer whorls of small compartments, arranged like the lower valves in the capitulum of Pollicipes, are superadded. The genus Chthamalus (fig. 5) differs from Octomeris only in the carino-lateral compartments being aborted, (as will presently be discussed), and hence has six compartments. Chamæsipho (fig. 6) differs from Chthamalus only in the rostro-lateral and lateral compartments being fused together; and hence has only four compartments. In Balanus (fig. 7) and the whole sub-family of the Balaninæ, the rostrum is compounded of the true rostrum, as seen in the type Octomeris, and of the two rostro-lateral compartments; hence this compounded rostrum has radii instead of alæ on both sides, and there are only six compartments. Tetraclita (fig. 8) and Elminius differ from Balanus only in having the carino-lateral compartments absent, and probably aborted; hence there are only four compartments. Lastly, in Pyrgoma, all the compartments are blended together into a single piece.

In Pollicipes, the old type-form of the whole order, and in Scalpellum, we have four valves, (answering to the operculum), surrounding the aperture leading into the sack, and the valves below are arranged in successive whorls, with a strong tendency to alternation. For, the rostrum alternates with, that is faces the interval between, the two scuta; the carina alternates with the two terga; and the upper lateral valves alternate with the scutum and tergum on each side. These four valves, namely, the carina and rostrum, which resemble each other in structure, and the pair of upper latera, which are larger than the other lateral valves, together form the uppermost whorl, or that beneath the scuta and terga. In the next whorl we have the rostro- and carino-lateral valves, alternating with those above them; and beneath them there are generally other valves, which decrease in size and display the same tendency to alternation. The valves here just specified, namely, the rostrum, carina, and three pairs of lateral valves, in the Lepadidæ, are so much larger, and are so much more commonly present, than the other valves of the capitulum, that to them alone I affixed special names. Now if amongst sessile Cirripedes we look to that genus, viz., Catophragmus, which comes in its whole structure the nearest to Pollicipes, one of the Lepadidæ, we find (as in fig. 4), firstly, a rostrum and carina resembling each other, and a pair of lateral compartments, larger than the other lateral pairs; these four valves alternating with the opercular valves: and, secondly, we find, but forming part of the same whorl, a pair of rostro-lateral and a pair of carino-lateral compartments, which, just as in Pollicipes, are larger than the exterior and lower valves. These lower little valves, I may remark, decrease in size in the successive whorls, and tend to alternate in position, just as in Pollicipes. Observing these several striking points of correspondence in the valves, (and indeed in the whole structure), of Catophragmus and Pollicipes, one is strongly inclined to suspect that in Catophragmus, and therefore in Octomeris and other sessile Cirripedes, although the rostro- and carino-lateral compartments appear to lie in the same whorl with the rostrum, carina, and large lateral compartments, yet that they really belong, as in Pollicipes and Scalpellum, to a lower whorl. Now if a very young shell of Balanus, immediately after the metamorphosis, be examined, the carino-lateral compartments will be found not to have been developed; they first appear after two or three zones of growth have been added to the other compartments; bearing in mind that in Pollicipes and in Catophragmus the lower whorls are added successively during growth, we find in this fact strong confirmation of the view that the carino-lateral compartments normally belong to a whorl beneath that including the rostrum, carina, and lateral compartments. Whether the rostro-lateral, like the carino-lateral compartments, are developed subsequently to the others, I have had no opportunity of ascertaining, and therefore cannot confirm the above analogical conclusion, namely, that they, also, belong to a lower whorl.

In the sub-family Balaninæ, which includes Balanus (woodcut 7), and Tetraclita (woodcut 8), the shell is characterised by not having rostro-lateral compartments, and by the rostrum having radii: now in _Pachylasma giganteum_, which undoubtedly belongs to the sub-family Chthamalinæ, at a very early age the rostro-lateral compartments become blended with the true rostrum, making a compound rostrum, exactly like the rostrum in the Balaninæ; distinct evidence of a similar fusion is retained throughout life (Pl. 15, fig. 1) in all three species of Chelonobia, which is undoubtedly a member of the Balaninæ. Hence, I think, we may conclude that in all the genera of the Balaninæ the rostro-lateral compartments are probably not aborted, but are blended with a normal rostrum (resembling that in woodcuts 4, 5, 6), making together a compound rostrum furnished with radii: it must, however, be observed that I could not detect any actual evidence of this fusion in Balanus, even immediately after the metamorphosis. In Chamæsipho (woodcut 6), either the rostro-lateral compartments attain a most unusual breadth, or, as is more probable, they have become confluent with the lateral compartments, which in the Lepadidæ seem to be the most persistent of all the lateral valves. In such genera as Tetraclita and Chthamalus, in which the carino-lateral compartments are absent, they may be fused with the lateral compartments or with the carina; but seeing that they are normally developed later than the other valves, it appears to be the simplest theory to assume, until the contrary be proved, that they are aborted. Finally, the somewhat unexpected conclusion that the shell (not including the operculum) of sessile Cirripedes normally consists of eight valves, four belonging to an upper whorl, and four to a lower whorl, all forced into a single ring, and often more or less fused together, though not strictly proved, is rendered highly probable. I will only further add, that the Basis perhaps represents several whorls of the small valves or scales on the peduncle of Pollicipes, fused together; the comparison of the basis with the calcareous cup, forming the lowest portion of the peduncle in Lithotrya, which has been made by some authors, I do not think is very accurate, as the cup in Lithotrya seems to have a special relation to the boring habits of that genus.

Basal edge of wall of compartment in _Balanus tintinnabulum_; _a_, _a_, outer lamina; _b_, _b_, inner lamina; _c_, _c_, longitudinal septa uniting the inner and outer laminæ with their ends denticulated.]

_Structure of the Individual Compartments._

If the basal margin of a compartment, for instance, of _Balanus tintinnabulum_, be examined, it appears sufficiently complicated, being composed of an outer and inner lamina, separated by longitudinal septa, which are denticulated at their bases; and the tubes formed by these longitudinal septa are crossed by transverse septa. On the other hand, in some cases, as in the genera Chthamalus and Elminius, each compartment consists of a simple shelly layer. These two extreme states graduate into each other: we have, firstly, on the internal surface, quite irregular points and ridges; these become regular, causing the internal surface to be longitudinally ribbed; then these ribs themselves become finely furrowed on their sides and at their lower ends, producing sharp, minute ridges, the ends of which I have called the denticuli; and, lastly, some of the denticuli on the adjoining longitudinal septa become united into a solid layer, forming the internal lamina of the wall. These denticuli do not generally cover the whole surface of the longitudinal ribs, but leave a portion near the outer lamina of the compartment smooth. The denticulated ends of the longitudinal septa project beyond the basal edge of both the outer and inner laminæ, and enter the mouths of the tubes (where such occur) in the basis, and thus strengthen the shell. The whole of the internal lamina generally is more or less striated longitudinally, thus displaying its origin from the union of the inner edges of the longitudinal septa. I need only further remark that on the internal surface of the outer lamina, between the main longitudinal septa, there are generally (as in the woodcut) smaller longitudinal ridges, which do not reach the inner lamina, and on this account alone are not called septa.

Tubes are formed by the longitudinal septa, between the outer and inner laminæ. These tubes are almost square, and are occupied by threads of corium, which enter at pores left open between the edge of the compartment and that of the basis on which it rests. The tubes extend high up the compartments; but in the uppermost part they are generally cut off by thin, transverse, calcareous septa, deposited by the ends of the threads of corium; a cancellated structure being thus produced. Or the uppermost part of each tube becomes filled up solidly with compact shelly layers, which are always first thrown down on the side of the tube facing the outside, and thus greatly strengthen the shell: in several instances, as in _Balanus perforatus_ and _Tetraclita porosa_, in which the disintegration of the upper part of the shell is a necessary element in its growth for the enlargement of the orifice, these filled up tubes become exposed. In Coronula and Tubicinella, the tubes in their upper parts are, I believe, crossed only by transverse membranous septa.

_Anomalies and exceptions._--In Tetraclita (Pl. 10, fig. 1 _g_, 1 _h_) from the branching of the longitudinal septa, several irregular rows of tubes are formed. In certain varieties of _Balanus balanoides_ (Pl. 7, fig. 2 _b_), and in _B. cariosus_ (Pl. 7, fig. 3 _b_), slight branching ridges on the internal surface of the walls, seem to answer to the longitudinal septa, and produce, during the downward growth of the shell, extremely irregular cells, and short tubes. In _Balanus vinaceus_ (Pl. 2, fig. 7 _d_), the internal lamina, instead of being solid, as in every other species, is left cancellated, and thus betrays, much more plainly than usual, its origin in the united denticuli of the adjoining longitudinal septa. In _Balanus porcatus_, between the main longitudinal septa, there are (Pl. 6, fig. 4 _e_) what may be called rudimentary and disconnected longitudinal septa. In Coronula and its allies (Pl. 16, fig. 6, and Pl. 17, fig. 4 _c_) it is the outer lamina of the compartment which is anomalous; for in the two or three lower zones of growth, it forms only a ledge on each side of the longitudinal septa; which ledges, higher up, become confluent, and so form an ordinary outer lamina. In Coronula, also, the wall of each compartment (see transverse section, Pl. 16, figs. 5, 7) is very remarkable from being deeply folded, the folds being on their internal faces firmly calcified together, and on their external faces closely pressed together (often with a neatly serrated suture), so that the whole nature of the shell might be, as has happened, easily quite misunderstood; and the walls be considered as very thick, instead of being, as is really the case, very thin. In Chelonobia (Pl. 15, fig. 1), however, the walls are truly of such great thickness, that the nature of the relative parts might likewise be misunderstood; in this genus the ovarian tubes enter the walls, extending up between the longitudinal septa, or, as they may here be more naturally called, the radiating septa. I will specify a few more peculiarities worthy of remark:--in some species of the sub-genus Acasta, clefts are left, covered only by membrane, on the lines of suture (Pl. 9, figs. 7 _a_, 8 _a_), between the compartments, just above the basis; and in other species the basis is perforated by numerous membrane-covered, minute orifices. In Platylepas, each compartment has one deep inward fold (Pl. 17, fig. 1), somewhat analogous to the three folds in Coronula; this fold is produced into an internal midrib, supporting and rendering convex the membranous basis; in this genus, also, the rostrum, owing to its midrib, is generally thrust a little on one side, and the shell thus rendered asymmetrical. In _Chamæsipho scutelliformis_ the shell is symmetrically perforated (Pl. 19, fig. 4 _a_) by four apertures. Lastly, in _Chthamalus Hembeli_ and _intertextus_, after a certain age, the basal edges of the walls become inflected, and continue to grow inwards till they entirely take the place of the true membranous basis.

_Structure of the Radii._

_Radius._--This term, as we have seen, is applied to that side of the compartment, the growth of which is modified, by abutting against and overlapping the adjoining compartment. Hence the structure of the radius is essentially the same with that of the parietal portion of the compartment. When best developed, as in _Balanus tintinnabulum_, the radius consists of an outer and inner lamina, separated by denticulated septa, extending in horizontal lines parallel to the basis, and is consequently perforated by minute tubes or pores. The tubes become filled up solidly much more commonly than do the parietal tubes; and the inner lamina, in such cases, is hardly distinct from the outer lamina. The denticuli often fail, or are present only on the lower sides of the septa; and very frequently the edge of the radius can only be said to be crenated. Notwithstanding these frequent anomalies, if a series of species and genera be taken, it is certain that there is, as might have been expected, a close relationship in internal structure, between the radii and the parietes. The edge of the radius is received in a slight furrow (generally marked like a seal, with the impression of the denticulated septa) in the opposed compartment: sometimes the outer edge or lid of the recipient furrow, is so broad as to give the false appearance of a radius having been developed, at least in the lower part of the shell, on both sides of the suture. A crest of corium runs into each suture between the edge of the radius and the furrow in the opposed compartment; and when the radius is permeated by pores (as in woodcut 10), threads of corium branch off this crest, and enter the pores. In the lower part of the shell, these crests of corium project from the corium forming and surrounding the sack; but in the upper part of the shell, above the opercular membrane, and therefore above the sack, the corium is produced up each line of suture as a separate ribbon. In proportion as these ribbons extend more or less near to the summit of the shell, so do the edges of the radii continue to be added to, to a greater or less height from the basis; and consequently their summits become less or more oblique.

Edge of the radius of _Balanus tintinnabulum_. _a_, outer lamina; _b_, inner lamina; _c_, denticulated septa, uniting the two laminæ.]

_Peculiarities in the Structure of the Radii._--In some of the species of Tetraclita, in which genus the walls consist of several rows of tubes, the radii are likewise perforated by several rows; and in some of the other species (Pl. 10, fig. 1 _h_), the edge, or disarticulated surface of the radius, is marked by irregularly branching ridges; and these evidently correspond with the branching septa or ridges of the wall. In Chelonobia, the outer lamina of the radius, as well as of its recipient furrow, is of extraordinary thickness; and this lamina, in _C. testudinaria_ (Pl. 14, fig. 1 _a_, 5, _b_, and Pl. 15, fig. 1, _f_), is modelled into sharp transverse ridges and valleys. In the Chthamalinæ, the radii, like the parietes, are simply solid; and apparently in consequence, for the sake of strengthening the sutures, the edges of the radii, and of the recipient furrow in Octomeris (Pl. 20, fig. 3 _a_) and in _Chthamalus dentatus_ and _Hembeli_ (Pl. 18, fig. 3 _b_, 5 _a_), are neatly dentated. In some other species of Chthamalus (Pl. 19, fig. 1 _a_), the radii present a slight modification of this structure, the sutures being formed by oblique interfolding laminæ. In the radii of Coronula and Tubicinella, there is a peculiarity, in apparent connection with the fact, that in these genera the parietal tubes are not crossed by transverse calcareous septa, namely, that the pores by which the radii are permeated keep unclosed throughout their length, and open into a special longitudinal tube (Pl. 16, fig. 7, _d′_), which runs along that margin of the wall, whence the radius arises. In Coronula the wall is of extreme thinness, and in conformity so is the true radius, but that the shell might not thus be rendered very weak, complementary or pseudo-radii are developed on their inner sides (Pl. 16, fig. 7, adjoining the true radii A _d_, C _d_, and shaded by distant convex lines). Even in the allied genus Xenobalanus, in which the whole shell tends to become rudimentary, traces of these pseudo-radii (Pl. 17, fig. 4 _b_, _d_) can be detected. In Coronula, though the radii (Pl. 16, fig. 7, A _d_, C _d_) are, by the above special means, rendered thick, and though the alæ also are thick (C _a′_, D _a′_), yet together they do not equal in thickness the folded walls; and consequently, there is left between the radii and alæ square chambers (_v_), occupied by the branching ovarian tubes.

_Structure of the Alæ._

These project, generally abruptly, from the sides of the upper part of the compartments; they appear from the first growth of the shell; they are overlapped by the radius and by part of the wall of the adjoining compartment; they are thinner, and have, owing apparently to being overlapped, a very different aspect from the parietal portion; but they do not differ from it in essential nature. They are solid, that is, they are never permeated by pores; but their edges are generally crenated, and there is, in some cases, as in Chelonobia, sufficient evidence that these crenations answer to the horizontal septa on the edges of the radii (also often reduced to mere crenations), and consequently, likewise, to the longitudinal septa of the parietes. In Coronula the edge of each ala consists of a medial ridge, sending off denticulated septa on _both_ sides, and is therefore anomalous as compared with the alæ in other genera, but corresponds in structure with the similarly anomalous radius of Coronula. In order to allow of the growth of the edge of the ala, a fine thread of corium runs up the narrow furrow in which the edge is lodged, proceeding from the corium of the sack. In proportion as this thread runs up higher or lower, so are the summits of the alæ rendered, during growth, less or more oblique.

_Structure of the Sheath._