Genus—VERRUCA. Pl. 21.

The shell in this genus is extremely unsymmetrical, not two of the six pieces of which it is composed quite resembling each other. At first it appeared hopelessly difficult to identify, in a homological sense, these six valves, with those of ordinary cirripedes, but the difficulty soon quite vanished. The operculum consists of two moveable valves on one side, namely, a scutum and tergum, but without any moveable valves on the opposed side: the scutum, though remarkable from being much smaller than the tergum, can be easily recognised by giving attachment to the animal’s body and to the adductor scutorum muscle. The four other pieces are articulated together, and form the shell surrounding the sack, in which the animal’s body is enclosed: of these, the two against which the moveable scutum and tergum shut, are smaller, differ greatly in shape, and are articulated together in a different manner from the remaining two pieces; from these facts alone there would be a strong presumption that they were of a different nature. The fixed valve, against which the scutum shuts, is either furnished with a remarkably prominent plate (a in fig. 1 c, s′; compare this with s′ in the reversed shell in fig. 1 e), or is hollowed out, as in V. nexa, for the attachment of the adductor scutorum muscle. Thus it is rendered probable that this fixed valve is a modified scutum; but a surface of attachment for one end of the adductor muscle might, perhaps, have been developed on any other valve, or a scutum might have become fused with a lateral valve of the shell; the shell on this latter view being rendered in idea more symmetrical. But when a very young specimen is carefully examined, it is found that the moveable and fixed scutum, the moveable tergum and its opposed valve or fixed tergum, at the first period of calcification, resemble each other quite closely; but that, as each zone of shell is added, the differences become rapidly greater and greater: hence, it may be considered as directly proved, that the two fixed valves (S′ and T′ in all the figures in Pl. 21), which are opposed to the moveable valves of the operculum (S and T), consist of an extraordinarily modified scutum and tergum. It has been shown (p. 129), that at the period of the metamorphosis, the two scuta, the two terga, and the carina of the Lepadidæ, commence their growth, under the form of the so-called “primordial valves,” and so differ from all the other valves when such occur: now, in two species of Verruca, I have found closely analogous primordial valves on the apices of both the moveable and fixed scutum and tergum (thus affording strong additional evidence that their nature has been rightly interpreted), and on one of the two remaining valves, namely, that at the posterior or carinal end of the shell. Hence, we may safely infer, that this latter valve, which, though very much more developed on one than on the other side, is so far medial as to curl round and cover the line of opening between the moveable and fixed tergum, is really a carina. The sixth valve differs only very slightly in shape from the carina, and is directly opposed to it; therefore, in accordance with all analogy, it must be the rostrum. Consequently, the shell in Verruca consists of a moveable scutum and tergum, a fixed scutum and tergum, a carina and rostrum, and, as we shall immediately see, a membranous basis—the basis being, as in all sessile cirripedes, the homologue of the peduncle in the Lepadidæ.

The moveable scutum and tergum stand at about right angles with the fixed pair; and as these latter form a part of the wall of the shell, which is always steep on this side, the moveable pair, which close the orifice, are nearly horizontal or parallel to the basis and surface of attachment. Hence, the animal’s body, which is attached between the two scuta, but nearest to the moveable scutum, also, lies nearly parallel to the surface of attachment; and I was consequently at first led to suspect that the basal membrane was one side of the shell in a modified condition; but the presence of the prehensile antennæ of the pupa in nearly the middle of this membrane, and the sheet of cement-tissue on its under side, demonstrate that this membrane, though lying on one side of the animal, is the true basis. To make all the parts in Verruca hold the same position as in other cirripedes, relatively to the surface of attachment, we must develope the carina and rostrum equally on both sides of the true longitudinal axis of the shell, and insert the newly-developed portion between the basis and the fixed scutum and tergum, reducing the latter in size, and tilting a little up the moveable scutum and tergum; and by this means the animal’s body would be turned, so that its dorso-ventral longitudinal plane would stand at right angles to the basal membrane.

Extraordinarily great as is the difference between the right and left sides of the whole shell, yet in all the species it seems to be entirely a matter of chance whether it be the right scutum and tergum with the right side of the rostrum and carina, or the left scutum and tergum with the left side of the rostrum and carina, which become abnormally developed. Nor does there seem to be any relation between the side of the operculum to be attached, whether right or left, and the nature of the surface of attachment; for I have seen many specimens adhering to perfectly level surfaces, and to quite cylindrical branches of Laminariæ; and in these cases, however the larva might attach itself, there could be nothing to favour the development of one side more than the other. Although the attached scuta and terga are larger than the moveable pair, yet, owing to the small development of the carina and rostrum on the attached side, the upper or unattached side must be considered as the most developed. In this respect, and in the circumstance of either right or left side being modified, we are reminded of the structure of Pœcilasma Kæmpferi (described in my former volume on the Lepadidæ), in which the valves on the side of the capitulum, nearest to the crab’s body, to which the specimens were attached, were somewhat less developed than those on the opposite side. I may add, that in ordinary Crustaceans, as I am informed by Professor Bell, the unequal development of the thoracic limbs seems quite capriciously to affect either the left or right side of the body.

General Appearance of the Shell.—The shell is in most cases much depressed and irregularly circular; the side formed by the fixed scutum and tergum is always steeper than the other side: the colour is white or pale brownish, and in V. nexa pale red. The surface is naked. The size is small, rarely exceeding a quarter of an inch in diameter, and the whole shell often appears like a mere scale on the surface of attachment. The most remarkable feature in the external aspect is due to the suture between the rostrum and carina, which is formed by oblique, interlocking plates or folds; as all these plates continue to be added to at their extremities during growth, the upper plates become longer than the lower ones; and the plates on both sides of the suture together form a triangular area, with the broad end uppermost, somewhat like the radius of a sessile cirripede: they act, also, like a radius, for their growth serves to separate these two valves, and so adds to the diameter of the shell. The suture between the rostrum and fixed scutum and that between the carina and fixed tergum are nearly of the same nature, but the former is more conspicuous than the latter; neither are so conspicuous as that between the carina and rostrum: accordingly as the right or left scutum and tergum are moveable, so the suture, second in plainness, (see Pl. 21, fig. 1 a, and 1 d,) is placed to the left or right hand. The fourth suture, between the fixed scutum and tergum, as viewed externally, is straight, and so very obscure that it has been overlooked by some authors, and the shell described as consisting of only three nearly equal pieces, for the fixed scutum and tergum together are about equal in size to the carina or rostrum. The orifice approaches more nearly to an unequal-sided triangle, with the apex broadly truncated, than to any other figure. The operculum fits with remarkable closeness, and is surrounded by a slight rim, formed by the edges of the four other valves.

Moveable Scutum and Tergum.—The scutum (S in 1 b and 5) is narrow and very small, barely equalling half the size of the tergum, and therefore proportionally much smaller than in any other cirripede; in a very young shell, however, (of V. Strömia) less than a pin’s head in size, the scutum equalled the tergum in size. The valve is remarkably thick; it is generally depressed down the middle; but in V. nexa this part is longitudinally ribbed. The occludent margin is curved. On the tergal margin there are two articular ridges (with a deepish furrow between them), of which the upper one (′ in S, in fig. 1 b, and 5) extends from the apex about half-way down the valve; and the other, or lower articular ridge (′′ in S), generally runs down nearly to the basal margin: an angle, running from the apex to the basi-tergal corner of the valve, appears like a third articular ridge, but cannot properly be considered such. The above two articular ridges interfold with analogous ones on the scutal margin of the tergum, and so lock the valves together. On the under side (fig. 1 f), the surface is bounded along the occludent margin by a slight rim: there is generally a very slight depression for the adductor muscle; but in V. Spengleri there is a straight, short, sharp (Pl. 21, fig. 2), prominent adductor ridge.

The moveable tergum is broad and rhomboidal. Externally a prominent axial ridge (′′′ in T, in fig. 1 b, &c.), which widens downwards, runs from the apex of the valve to the basal point, and there projecting slightly, causes the scutum to be indented; this indentation on the scutum appears like a third articular ridge, lying beneath (′′), S, in fig. 1 b, &c. Above the lower and axial ridge, on the scutal margin of the tergum, there is a middle articular ridge, which locks in, between the lower (′′) and upper articular ridges (′) of the scutum (S). Again above the middle ridge there is an upper and third articular ridge (′), which is either quite distinct, as in fig. 5, T, or more commonly is formed by the occludent margin of the valve, as in T, fig. 1 b. The broad extremity of this upper articular ridge is often produced into a slight projection, or shoulder, and this always underlies the scutum, of which the under and upper surface is indented or furrowed (see fig. 1 f), in order to receive this shoulder. The upper articular ridge of the scutum (′, S, 1 b) locks in between the upper articular ridge or occludent margin (′, T), and the middle ridge (′′, T) of the tergum.

Hence, altogether, there are three articular ridges on the scutal margin of the tergum, the occludent margin being generally counted as one; whereas, on the tergal margin of the scutum, there are only two ridges, though, as before noticed, an outer indentation, which is developed as a ridge in V. nexa (fig. 5, S), might almost be counted as a third articular ridge.

I may here just remark, that the furrow between the two ridges on the tergal margin of the scutum, resembles the articular furrow in the scutum of the Balanidæ; but it may be doubted whether the resemblance be more than superficial, as this furrow, in the case of Balanidæ, receives the edge itself of the tergum, whereas here it receives only a ridge, proceeding from the apex of the tergum, to a nearly middle point on its scutal margin. Finally, I may add, that the tergum in this genus, in general shape, in growth (presently to be referred to), in the manner in which the upper scutal shoulder is overlapped by the scutum, and in the presence of the axial ridge, presents a very striking resemblance to certain old fossil species of Pollicipes, and to a limited extent to the living species of Lithotrya.

The scutum and tergum being interlocked, move together; they can be firmly shut by the contraction of the long adductor scutorum muscle. Their opening appears partly due to the elasticity of the membranous hinge (representing the opercular membrane), by which they are attached transversely, just beneath the summit of the carina and rostrum. No doubt the protrusion of the cirri effectively aids the act of opening. These valves are not capable (nor, of course, the other valves) of any other movement; for there are no muscles for such movements.

Fixed Scutum and Tergum.—The fixed scutum is larger than the fixed tergum, and therefore has the same proportions as the homologous valves in ordinary cirripedia, but reversed proportions compared with the moveable scutum and tergum. The shape of neither valve can hardly be described. The fixed Scutum (S′ in all the figs.), externally, seems at first to consist of two portions, namely, a curved occludent rim (a), closely resembling the opposed occludent margin (a) of the moveable scutum (S), having in fact undergone very little modification, (as may be best seen in Pl. 21, fig. 1 b); and secondly, of a much modified portion (marked ′′), which resembles in outline and state of surface the rest of the walls of the shell, and may be called the parietal portion. The rostrum (A) curls round the end of the occludent portion, under an edge (b), evidently answering to the basal margin (b) of the moveable scutum, and is simply united to this portion by membrane, but beyond this part, it is articulated to the parietal portion (′′) of the fixed scutum, by oblique interlocking ridges, like those forming the suture between the rostrum and carina. In V. nexa, however, (fig. 5) the rostrum (A) does not curl round any part of the interlocking fixed scutum (S′), but articulates with it by a straight suture. Internally, the fixed scutum (S′ in figs. 1 e, 1 c, of reversed shells) has a surprisingly large, thin adductor plate (m in fig. 1 c, 1 b), with a rounded outline, projecting nearly parallel to the basis or surface of attachment; the adductor scutorum muscle is attached to its upper surface, and consequently the animal’s body lies between this plate and the moveable scutum. In the fixed scutum of V. nexa, however, there is a deep pit, instead of a plate, for this muscle.

The fixed Tergum, likewise, consists of two portions—a middle and lower, or parietal portion, and a rim or upper portion; the rim consists of two unequal arms, answering to the two upper margins (not merely the edges) of the rhomboidal moveable tergum; the longer rim (x in T′, see fig. 5) answers to the carinal margin (x) of the moveable tergum, and may be called the carinal rim; and the shorter rim (o and ′ in fig. 5) answers to the occludent margin (o and ′) of the moveable valve, and may be called the occludent rim. The carina curls round the end of the carinal rim, under an edge, z (much foreshortened in T′ in fig. 1 b, and best seen in fig. 5), answering to at least a large part of the basal margin (z) of the moveable tergum, and interlocks, by a serrated suture, with the edge of the parietal portion of the valve. Internally (fig. 1 e, less plain in 1 c) there is a transverse ledge, notched in the middle, and sometimes deeply hollow beneath, running across the valve in about the line of the adductor plate of the fixed scutum: this ledge, in fact, marks and is partly caused by, the line of separation between the central or parietal, much modified, and the scarcely modified, upper or rim portion of the valve. The use of this ledge is apparently to give attachment, as does the under side of the adductor plate of the fixed scutum, to ligamentous fibres, presently to be mentioned, by which the shell is attached to the basal membrane: the carina and rostrum being so much more gently inclined, do not stand in need of a ledge for their attachment.

By comparing the moveable scutum and tergum with the corresponding fixed valves, in all the species, the modification of the latter may be clearly made out to have been effected as follows; and the case appears to me a striking and interesting one. The moveable scutum and tergum lie in the same plane, and are articulated by the means of three ridges on the tergum (including the occludent margin), and by two on the scutum. The fixed scutum and tergum have to be curved, and to be greatly increased in size; and this is brought about, as we shall see, by the large development of a certain small portion of each valve. Comparing first the moveable tergum (T) with the fixed tergum (T′), the umbo of growth matches the umbo of the four margins of the moveable valve, the carinal (x), basal (z), and occludent (o in fig. 5), margins can be identified with certainty in the fixed valve, from their close similarity in shape, their absolute apposition, or correspondence in position. There remains only the scutal or articular margin, with its three articular ridges; of these, the uppermost (′), inasmuch as in most of the species it is hardly distinct from the occludent margin, can, as we have just seen, be clearly identified, and is overlapped, as it normally should be, by the upper tergal corner of the fixed scutum: the second or middle articular ridge, though not so distinct as in the moveable valves, can be plainly recognised (′′), T′, in fig. 1 b, and 5; and it serves its normal function of articulating the two valves together. But when we look in the fixed valve for the third or axial ridge (′′′), we find in its exact place, namely, extending from the umbo to the extreme opposite end of the valve, between the second articular ridge (′′) and the basal margin (z, see fig. 5), only that portion of the valve which I have called the parietal portion; consequently, I do not doubt that this really is the axial ridge largely expanded. So again in comparing the moveable scutum (S) with the fixed scutum (S′); two of the three margins of the former, namely, the occludent (a, see fig. 1 b) and basal (b), can be identified without a doubt in the fixed valve: the third and tergal margin remains; this should have two articular ridges; of these the upper one, still serving its normal function, can be detected in all the species (′ in fig. 1 b), and can be seen pretty plainly (′ fig. 5) in V. nexa: but of the lower and other articular ridge there is no sign,—excepting indeed the whole parietal portion of the valve, which, from holding an exactly homologous position with the lower articular ridge of the moveable valve, I cannot doubt in this ridge expanded and curiously metamorphosed. Hence, in both fixed scutum and tergum, it is the outermost or lowest of the articular ridges which has been modified and expanded, so as to rest on and be fixed to the surface of attachment. It would appear as if it had resulted from one ridge in each of these valves having been thus used up by expansion (so to express myself), that the suture between the fixed scutum and tergum is more simple than any other suture in the whole shell; and it is owing probably to this straightness, and consequent tendency to weakness, that the valves do not grow along this line, and so do not become separated from each other during growth, as on the three other lines of suture. As it actually is, owing to this suture never being separated, it is even stronger than the others; its edges on the inside (fig. 1 c), I may add, are a little inflected or prominent.

Rostrum and Carina: these valves differ from each other, only in the former (A) being rather the largest, and in being more plainly articulated with the fixed scutum, than is the carina (B) with the fixed tergum. Their umbones stand in their normal places, at the two ends of the orifice leading into the sack, that is, facing the dorso-ventral longitudinal plane of the animal; but they are very unequally developed on the two sides, and hence they rise very obliquely from the surface of attachment. Their summits are nearly square, which is caused by the continued growth on both sides of the oblique plates or ridges, by which they are articulated with the adjoining valves. These plates strikingly resemble, as already stated, the radii in certain species of Chthamalus. Without these articulating plates, the outline of the rostrum and carina would have been triangular, with the apex upwards. In V. nexa, in which the walls of the shell are almost perpendicular, the rostrum (A, fig. 5) is very peculiar and patelliformed, with the umbo sub-central: this results from the development of a border at the upper end of the valve. In this same species, the basal edges of the rostrum, carina, fixed scutum and tergum, are rectangularly inflected, so as to form a ledge round the basis, as in the case of some few species of Chthamalus,—the ledge appearing like part of the real basis. During the growth of the shell, the upper internal ends of the carina and rostrum are either rendered solid, or a ledge is formed on the inside across their summits, hollow beneath, like the sheath of the Balanidæ, to which solid or hollow ledge the basal margins of the moveable scutum and tergum are attached by a rim of membrane, forming a hinge.

Direction of Growth: Minute Structure of Valves.—The shell grows downwards all round its basal margin. As far as the diametric growth of its upper part is concerned, there may be said to be only three valves, for the fixed scutum and tergum never become, as already stated, separated; on the three other lines of suture, the valves are added to on both sides; and thus the whole upper part of the shell, and the orifice, increases in diameter. The moveable scutum and tergum grow along their basal margins, and along the margins by which they are articulated together; but the scutum in this latter respect, less than the tergum. The summits of the moveable scutum and tergum, during continued growth, become either worn away, or they project freely; in this latter case, an internal ledge is added round the upper end of the fixed scutum and tergum, so as to keep the orifice accurately closed. In V. nexa the rostrum, with its sub-central umbo, is anomalous, as already stated, owing to a broad upper internal border growing in a direction almost directly opposed to the basal growth of the moveable opercular valves.

In young specimens, on the apices of both scuta and both terga, and on the carina, but not on the rostrum, primordial valves may be distinguished, resembling the valves, so called, which first appear (p. 129, Introduction) after the metamorphosis in, the Lepadidæ. In the Verrucidæ, however, they are calcareous; and the minute transverse cylinders, of which they appear to be composed, stand further apart, causing the surface of the primordial valve to be marked with little separate circles, instead of by hexagons.

The shelly matter of which the valves are composed is translucent: it is remarkably destitute of any investing membrane. The under surface is marked with rows of minute approximate pores, parallel to the lines of growth, into which the corium enters: after a portion of shell has been dissolved in acid, these threads of corium are seen to change, a short distance within, into cylinders of yellow chitine, running obliquely through the substance of the valve. These cylinders are about 1/2000th of an inch in diameter, but in parts they are spindle-shaped and twice as thick: they vary in length, about 1/100th of an inch being the average length: these cylinders at their upper ends suddenly contract into a point, more or less long, or are produced into a very fine tortuous tubulus of chitine, imbedded in the shell: I have seen in no other Cirripedes tubuli of this structure. There are other ordinary tubuli, such as occur in the valves of most Cirripedes, about 1/6000th of an inch in diameter, and which sometimes alternate with the above-described thicker cylinders. There are no external spines. From the number and length of the tubuli of both kinds, the tissue left after the action of acid is singularly complicated.

Basis.—The basal membrane is thin, and is divided, but not very plainly, into concentric slips, marking the successive increments of growth. In the middle of it, in two young specimens, I found with great difficulty the pupal prehensile antennæ: they were of small size, measuring from the extreme edge of the main or second segment to the end of the disc, only 27/6000ths of an inch: the disc appeared narrow (as in Pollicipes and Scalpellum), with a single spine at the proximate end: the ultimate segment, placed as usual at about right angles to the disc, bore two groups of shorter and longer spines, but I could not count how many. The antennæ were enveloped in a mass of cement of a yellow colour, resembling in all its characters the cement of other Cirripedes. In only one case, I believe I saw bifurcating cement-ducts, of extreme tenuity, viz. 1/15,000th of an inch in diameter. The sheet of cement on the whole under side of the basal membrane, not rarely shows a very irregular reticulated structure. For convenience sake, it will be best to defer the discussion on the very anomalous, though slight, powers of excavation which this genus possesses, and which I must attribute to the effects of some substance secreted probably by the cement-organs. I will here only mention, that the specimens which have excavated a depression, are less firmly attached than those, which have not acted on their support; and that, in the former case, the basal membrane, for a considerable space in the middle, becomes quite detached.

Animal’s Body.—The body is much flattened and, owing to the little development of one side of the shell, lies parallel to the surface of attachment. The prosoma is but little protuberant. The articulations of the thorax are unusually straight and transverse. The Mouth is also much flattened: it is placed rather distantly from the adductor scutorum muscle, owing to the production of the lower margin of the labrum. The Labrum is not notched, or even hollowed out in the middle, or (excepting in V. nexa) bullate; its crest is surmounted by about eight (more numerous in V. nexa) little teeth, or by some fine bristles. The Palpi are of moderate size, with their tips nearly meeting; they are slightly curved, and have bristles only on their outer sides and extremities: they are apparently capable of being lifted up and down by a muscle attached to them, just outside the rounded swelling on each side of the labrum to which they are articulated: in V. nexa, however, the palpi are very small and narrow, and their tips do not nearly meet. In this genus, therefore, we find the swollen state of the labrum and the size of the palpi—characters generally invariable and of high classificatory importance—variable. The mandibles have three upper main teeth, with two or three minute lower teeth, or, in V. nexa, with the lower part pectinated with small spines: in V. Strömia, I have seen traces of the second tooth being laterally double—a character of some importance. The Maxillæ have a notch under the upper pair of large spines, with the lower part bearing, as usual, a double row of bristles, and forming a large step-formed projection: these organs are furnished with the usual apodeme and muscles. The Outer Maxillæ are prominent, and deeply lobed on their inner surfaces, the two lobes being clothed with bristles.

Cirri.—The first pair are attached, as usual, on each side of the mouth, and stand some way apart from the five posterior pairs. The second and third pairs differ considerably in structure from the three posterior pairs, which are much elongated. The first pair (excepting in V. nexa) is short, with the two rami slightly unequal in length, and with the segments thickly clothed, as usual, with spines. The second pair is remarkable from the posterior ramus being more than twice as long, and containing thrice as many segments, as the anterior ramus, which is barely as long as the shorter ramus of the first pair: the segments in the anterior ramus of the second pair (only five in number in a full-sized specimen) are broader and more protuberant in front, and more thickly clothed with spines (the terminal spines being doubly pectinated), than are the segments on the posterior ramus; on the latter, the uppermost segments have their bristles arranged in front in simple pairs, with the dorsal spines long, the lower segments being more thickly clothed with bristles, owing to the development of lateral rows. The third pair resembles in every respect the second pair, except in being a little longer, and in the bristles on the posterior ramus being less crowded, more resembling the arrangement of those on the posterior cirri. In V. nexa, however, there is not so great an inequality in length or dissimilarity in structure in the two rami of the second cirrus, and only a very slight difference of any kind in the two rami of the third pair. Fourth, fifth, and sixth pairs have numerous elongated segments, bearing four or three pairs of long slender spines in front, with a single minute bristle between each pair, and with two or three slender spines in the dorsal tuft.

There is a considerable amount of variation in the proportional length, and in the number of the segments, of the several cirri in V. Strömia; in some specimens the two rami of the fourth pair were unequal in length; in some, nearly all the cirri on the lower or attached side were shorter than those on the upper side.

Caudal Appendages.—These are of most unusual length, sometimes even exceeding those of Ibla quadrivalvis, which surpasses, in this respect, all other cirripedes. They arise on each side and over the anus. They consist of numerous (sometimes as many as twenty-four), unequal, cylindrical, thin segments, bearing, at their upper ends, a circle of long and very slender spines. They sometimes equal two thirds or even four fifths of the length of the sixth cirrus; but their length, and the number of their segments, (sometimes imperfectly divided), varies much in different specimens of the same species, and sometimes even on opposite sides of the same individual. In some very young shells, as big as a pin’s head, the caudal appendages were proportionally extremely short, and consisted of only two or three segments. No muscles enter these organs; and when the animal is taken out of its sack, they project straight out behind, instead of being curled in, like the cirri.

Anatomical Structure.—The animal’s body is attached to the two scuta by the adductor scutorum, and by the other usual muscles running towards the mouth, and surrounding the prosoma. The whole external covering or shell has no other muscles; Verruca thus differing from the Balanidæ and Lepadidæ; but the shell is attached all round, near its circumference, to the basal membrane, by a band of very short fibres, appearing like muscles, but really ligamentous, as determined for me by Professor Quekett. Branchiæ are entirely absent. The alimentary canal presents all the usual characters, but in the prosoma is rather abruptly bent back on itself. The orifices of the two olfactory pouches are not at all prominent; they are placed directly under the outer maxillæ, (homologically in their middle segment), just above a small, medial, tongue-like apodeme. The orifices of the acoustic sacks appeared to be in their usual position beneath the basal articulations of the first pair of cirri. The vesiculæ seminales occupy their usual position in the prosoma; they are not much convoluted; they unite before entering the penis. The probosciformed penis is imperfectly ringed; it is thick and short, and tapers much more abruptly than is usual; it supports a few very thin hairs. The ovarian cæca are spread over the basal membrane, at the bottom of the sack; hence they in fact lie almost on one side of the animal: they consist of two main trunks, proceeding out of the animal’s body at the rostral end of the sack, which then branch and inosculate. In specimens of V. Strömia collected by Mr. Peach for me, in Cornwall, during the first week of April, there were included two ovigerous lamellæ, placed transversely across the rostral and the carinal end of the sack: the lamellæ were .11 of an inch in length; they appeared loose and not attached, as in the Lepadidæ, to any ovigerous fræna. The ova, in their earliest age, have one end much pointed, and are 8/1000ths of an inch in length; they become blunter and increase a little in size before the larvæ burst forth. The larvæ, both during their earliest stage and after the first moult, have been excellently figured and described[137] by Mr. C. Spence Bate: they present no particular characters distinct from the larvæ of other Cirripedes. I will only further add, that the structure of the prehensile antennæ still adherent to the basal membrane, indicates that the larva in its last stage,—that is the locomotive pupa,—has a normal character.

Affinities.—These have been sufficiently discussed under the family; I need here only remark that all the species, with the exception of V. nexa, are intimately allied together.

Range—Habits—Geological History.—The genus Verruca ranges, being represented by four species, from Iceland to Cape Horn. The species that is found in Tierra del Fuego extends up the west coast to Peru. Our northern form, V. Strömia, (if I may trust a specimen in the British Museum, apparently ticketed in an authentic manner), occurs also in the Red Sea; and this is the only locality in the eastern hemisphere whence I have seen this genus. The species seem generally to live in rather deep water: I procured V. lævigata from nineteen fathoms, on the east coast of Patagonia: V. Strömia is found, according to information given me by Professor Forbes, on the British shores, between five and fifty fathoms, and on the steep shore off Mull, in ninety fathoms; but Mr. Thompson assures me that he once saw it adhering to tidal rocks and likewise to some floating bark. Generally the species are attached to living organic bodies, especially shells of Mollusca and of Cirripedes, to Gorgoniæ, and Laminariæ; less frequently to rocks. We shall immediately see that it has slight powers of excavation. This genus is geologically older than any true sessile cirripede or member of the Balanidæ: V. Strömia is found in the Glacial Deposits and in the Red and Coralline Crag of England: another species (in a state not to be identified) occurs in the ancient Tertiary formations of Patagonia; and another in the Chalk of England and Belgium. The fact of this Family ascending to a Secondary epoch accords, in an interesting manner, with its affinities; inasmuch as though in appearance a sessile cirripede, it is almost equally related to the Lepadidæ and Balanidæ, and is more nearly related to the Lepadidæ than to the Balaninæ, or typical members of the Balanidæ: of the latter, none have hitherto been found in any Secondary deposit, whereas the Lepadidæ culminated during the Cretacean period.

Powers of Excavation.

My attention was called to this subject by Mr. Hancock, whose excellent researches on the boring of Mollusca are well known. Verruca Strömia, when attached to shells destitute of an epidermis, excavates, as he informed me, a slight depression, deepest in the middle; but when the epidermis is present no effect whatever is produced. We shall presently see that the central depression is in some degree distinct from that of the circumference. I have since found Mr. Hancock’s observations strictly applicable to V. lævigata, V. Spengleri, and to an ancient tertiary species from Patagonia. From having found that the following cirripedes, viz., Lithotrya, Alcippe, and Cryptophialus, all form their deep excavations by mechanical means, and from having read the above-mentioned memoirs by Mr. Hancock on the boring of mollusca, I was strongly impressed with the idea that the action in Verruca would likewise prove mechanical: but from the following facts I have come to the conclusion that the excavation must be due to a solvent, probably poured out from the cement-ducts, which debouch on the under side of the basal membrane.

In the first place, an epidermis, as just stated, perfectly preserves the shells of the various species of mollusca and certain cirripedes, to which I have seen Verruca attached: this is well shown by comparing the effect produced on the same shell in parts covered by the epidermis and in parts whence it has been abraded; or where the shell of the Verruca had fixed itself, whilst very young, within a crack in the epidermis, and had subsequently, by its growth, turned up the edges, and had then acted on the underlying shell; whereas the specimens attached to the sound epidermis had not produced the smallest effect. Again, I have seen an epidermis-covered mussel-shell encrusted by a hard nullipora, on which V. lævigata was attached; and here the calcareous nullipora, under the middle of the basal membrane, was entirely corroded away, whilst the underlying epidermis and the shell beneath it, were not in the least affected. The protection afforded by the epidermis is still more strikingly shown by contrasting shells with very sharp prominent ridges, when thus invested and when naked, to which Verrucæ have been attached: I have given a figure (Pl. 21, fig. 6) of a piece of an invested Venus, from the surface of which a V. Spengleri had been just removed; on the other hand, I have seen a Peruvian Discina in which even sharper ridges, covered by epidermis, were left absolutely untouched, although projecting deeply into the shell of an attached V. lævigata. I have seen several specimens of this latter Verruca (which has the power of corroding naked shell as deeply as its congeners), attached to the membrane-covered variety of Balanus lævis, the shell of which was thus perfectly preserved: now this membrane is little more than the 1/2000th of an inch in thickness; it is not hard, and so brittle that it generally separates with the Verruca, leaving the underlying shell of the B. lævis with its lines of growth glossy and perfect: it appears to me impossible that a membrane so thin and brittle could resist an action, if mechanical, which has worn away from twenty to forty times as great a thickness of hard shell; but the thinnest film of any matter on which acid does not act, as of grease in certain forms of printing, will perfectly preserve the underlying substance, and as I have ascertained by putting on a drop of acid, is the case with this membrane. I have removed several scores of shells of V. Strömia from the stems of Laminariæ, and when the latter were washed and slightly dried, generally not the least effect could be seen, except that the spots where the shell had adhered were glossy from the still adherent basal membrane: yet the stems of Laminariæ are far from hard. In some cases, however, the attachment of the Verruca seemed to have produced a very slight depression on the Laminaria, but this, I think, may be safely attributed to the growth of the surrounding surface; for I have seen exactly the same effect produced by the attachment of the discs of the antennæ of a Lepas, and these discs, with their long spines, could not possibly produce any excavation; nor is Lepas or its pupa in any case a burrowing animal. Again, I have seen a few specimens of Verruca attached to Gorgoniæ, and they had not acted in the least on the horny axis. I have examined numerous specimens of V. Strömia attached to three pieces of slate-rock, and to one piece of red sandstone, all from different localities, and no effect whatever had been produced; yet the slate-rock, especially in one instance, was soft. Mr. Bate, to whom I am indebted for some of these specimens, also informs me that he could discover no impressions on the slate-rocks, whence specimens of the Verruca had been removed. On the other hand, I have had two specimens of limestone, with attached Verrucæ, one coarse and very impure, and the other hard and marble-like; and in both cases there was a distinct central slight cavity, including loose gritty matter. The loose particles evidently resulted from the unequal action either of a solvent or of some mechanical power on the rock, for it is improbable in the highest degree that the shells should have fixed themselves exactly over small collections of loose particles, even if such could possibly have remained on projecting surfaces of sea-washed rocks.

The above facts seem to indicate pretty plainly that the excavation of the support does not depend on its hardness, but on its containing calcareous matter, liable to be acted on by some solvent: but as this view, considering what we know of Lithotrya and of the two other burrowing genera of cirripedes to be hereafter described, appears improbable, I will add a few additional observations. I most carefully examined the shell and basal membrane of Verruca, and likewise the tissues left after the dissolution of the shell in acid, and could detect no structure at all fitted for boring; and what appears more important, there was no apparent difference in the state of the specimens which had and had not excavated a hollow; and this, I think, would certainly have been the case (as in Lithotrya) if the action had been mechanical. It is not easy to ascertain, owing to the small effect at any time produced, at how early an age Verruca begins to act on its support; but I found two sets of specimens only 1/20th of an inch in basal diameter, which had certainly commenced. The ribbed shell, (Pl. 21, fig. 6), especially the middle rib, shows, in a somewhat exaggerated degree, the typical form of the excavation; it may be here seen that the excavation is of the same depth for some little distance from the circumference towards the centre, but that in the middle it suddenly becomes deeper. I have seen several specimens with a central hollow, without any, or with scarcely any, marginal depression, and likewise the reversed case. These several facts show that the central excavation cannot be due to an equable action, prolonged during the whole growth of the shell, having thus affected the middle more than the circumferential parts, for in this case the excavations would have sloped into each other. In specimens which have not at all acted on their support, the whole basal membrane is firmly attached, as in all ordinary cirripedes, to the supporting surface; but in those which have acted, the middle portion of the basal membrane is quite unattached, and the circumferential portion is, I think, less firmly attached than is usual; but between these two portions, there is a circular zone strongly cemented to the supporting surface, and which alone keeps the shell in its place. Now, on the mechanical theory, to account for the circumferential hollow, the basal edges of the shell together with the circumference of the basal membrane must be subjected to movement, but the shell is united to the basal membrane by corium and by transparent structureless chitine (both of which may be left out of question) and by a circle of short fibres, which adhere at their lower ends to the firmly cemented circular zone, and by their upper ends to the shell; and these fibres have been very carefully examined by Professor Quekett, and pronounced to be not muscular, but exclusively ligamentous, and therefore incapable of moving the edge of the shell. The basal membrane over the central hollow is, as stated, quite loose: its lower surface, formed by a reticulated layer of horny cement-tissue, shows no signs of abrasion, and the membrane is so brittle and tender, that in specimens which have been once dried and then well soaked, it almost invariably cracks when the shell is removed, owing to its mere adhesion to the delicate inner tunics of the sack; yet on the mechanical theory, the wearing of the central hollow must have been caused by the action of this middle portion of the basal membrane,[138] which, it may be repeated, is destitute of muscles. From the presence of the prehensile pupal antennæ, enveloped in cement, nearly in the centre of the basal membrane, it is certain that this spot was originally attached to the supporting surface, and has since been detached from it; as, moreover, the central hollow goes on increasing in diameter with the growth of the shell, it is certain that the inner edge of the firmly attached circular zone of basal membrane must likewise continually go on becoming detached: it may, then, be asked by what force can the basal membrane, seeing that it is united to its own shell above only by fibres of ligament near the circumference, be continually torn away from the underlying support, to which it is strongly cemented? On the other hand, on the theory of a solvent slowly poured out from the cement-ducts, its separation from its support is simply explained. It might be supposed that the calcareous matter, when dissolved, would not be able to escape from the central hollow, owing to the basal membrane being so firmly cemented all round it; but the attachment is by a reticulated layer of cement; and I infer that it must be permeated by open passages, from the fact of the hollow being often filled, in dried specimens, by a bubble of air, instead of the basal membrane being pressed closely down into the hollow, as would have been the case had the hollow been hermetically sealed up. I have seen a few instances in which the bottom of the central hollow was occupied, (as was remarked to me by Mr. Hancock), by a little chalky and gritty matter; and in the case of one of the specimens of calcareous rock, before alluded to, by coarse grains and oxide of iron; this seems quite compatible with a solvent acting more readily on certain parts of the rock or shell than on other and less soluble parts or particles.

The greatest depth of the central hollow, in any specimen seen by me, even measuring from the top of a rib in the case of a ribbed shell, to the deepest point, was only 1/50th of an inch; but considering how much depressed the shell of Verruca is, I have no doubt that this small gain of space is of service to the animal: we must suppose the loose middle portion of the basal membrane is stretched slightly, or splits and is repaired, so as to fit the hollow. With respect to the even much slighter circumferential excavation, it barely equals in depth the thickness of the extreme edges of the walls; it must, I presume, give strength to the shell when laterally pushed; but it certainly appeared to me that the individuals which had excavated a depression for themselves, could be pried vertically up much more easily than those which had not acted on their support. Finally, we must suppose that the hypothetical solvent is poured out of the cement-ducts at the extreme circumference of the basal membrane, which is almost loose and destitute of cement, so as to slightly corrode outwards and downwards the calcareous support; the action here then stops, and this rim of basal membrane becomes, after a new rim has been formed and as the shell grows outwards, firmly cemented down to the now slightly excavated surface of attachment; but during all the time the solvent goes on acting in the middle, and continues, during the whole growth of the shell, to encroach on and dissolve the supporting surface from under the inner edges of the previously cemented down, circular zone of basal membrane. I have discussed this subject at considerable length, as it appears to me an interesting one. In this case we have the action of ciliæ[139] and of respiratory currents, to which in the case of Mollusca so much has been attributed, entirely eliminated. It is, also, an interesting fact, that within the same Order we should have some Cirripedes boring by simply mechanical means, and others by a chemical solvent.[140]