_Valvae 8, si inter eas parvum (saepe rudimentale) rostrum et duo parva latera numerentur; incrementi lineis concinne crenatis: pedunculus squamis calcareis parvis vest.i.tus, in verticillis superioribus crenatis; aut calyci basali calcareo aut discorum ordini affixus._
Valves 8, including a small, often rudimentary rostrum and a pair of small latera: lines of growth finely crenated. Peduncle covered with small calcareous scales, those of the upper whorls crenated; attached either to a basal calcareous cup, or to a row of discs.
Body lodged within the peduncle: mandibles with three teeth, the inters.p.a.ces being pectinated; maxillae various: olfactory orifices slightly prominent: caudal appendages multiarticulate.
Lodged in cavities, bored in calcareous rocks, or sh.e.l.ls, or corals; generally within the Tropics.
_Description._--The capitulum is not much compressed, a horizontal section giving an oval figure; it is placed obliquely on the peduncle, the scuta descending lower than the terga and carina. There are eight valves, of which the scuta, terga, and carina are large; the rostrum and a pair of latera are very small and often rudimentary. These three latter valves are essentially distinguished from the scales of the peduncle, the upper ones of which they sometimes hardly exceed in size, by not being moulted at each period of exuviation. The latera overlie the carinal half of the terga; I presume that they are h.o.m.ologous with the carinal latera in Scalpellum. Each successive layer of sh.e.l.l forming the valves is thick, and extends over nearly the whole inner surface; hence the carina and terga, and to a certain extent the scuta, either actually do project freely much beyond the sack, or would have done so, had not their upper ends been removed; for the upper and old layers of sh.e.l.l, in most of the species, either scale off or disintegrate and wear away. A rectangularly projecting rim, serrated by small teeth, is formed at the bottom of each fresh layer of growth, along the external surfaces of each valve (see upper part of fig. 1 _b"_ Pl. VIII.) This structure, as well as that of the crenated scales on the peduncle, is important, for by this means the animal, as we shall presently see, forms and enlarges the cavity in the rock or sh.e.l.l in which it is imbedded.
The scutum overlaps either about one third or even one half of the entire width of the tergum, and abuts against a prominent longitudinal ridge on its exterior surface. In _L. truncata_ and _L. Valentiana_, this ridge on the tergum being folded over towards the scutum, forms a conspicuous furrow, receiving the tergal margin of the latter. In _L.
Valentiana_, there is a second furrow on the carinal side of the tergum, receiving the upper end of the corium-covered or growing surface of the carina. Besides these provisions for holding together the valves, there are, apparently, others for a similar purpose; thus in each scutum, under the rostral angle, there is a roughened k.n.o.b-like tooth, which touches the under side of the little rostrum, and no doubt serves to give attachment to the membrane uniting the three valves together. In some species, the adjoining basal margins of the scuta and terga, where touching each other, are inflected and roughened; again in _L.
Rhodiopus_, the carinal angles of the terga are produced into points, and in _L. truncata_ and _L. Valentiana_ into prominent roughened k.n.o.bs, which touch two corresponding small k.n.o.bs, on the upper part of the growing surface of the carina. Moreover, considerable portions of the inner surfaces of the scuta and terga, are roughened with minute sharp, imbricated points, apparently for the firmer attachment of the corium.
The roughened k.n.o.bs at the rostral angles of the scuta, no doubt are h.o.m.ologous with the teeth in a similar position on one or both scuta in Lepas, and in some fossil species of Pollicipes, as in _P. validus_. The other projections and roughened surfaces are peculiar to Lithotrya. The growth of all the valves is, as in Pollicipes, simply downwards.
The _Scuta_ are triangular, with their umbones or centres of growth at the apex; the tergal margin, as seen from within, is either nearly straight or much hollowed out, accordingly as the scuta simply overlap the terga, or are received in a furrow. In some of the species there is a distinct pit for the adductor muscle, and in others this cannot be distinguished.
_Terga._--These present great differences in shape; but all appear to be modifications, (as seen internally,) of a rhomboidal figure, which seems to be the normal form of the terga in the Lepadidae. Of the lower part of the valve, the whole exterior surface, with the exception of a narrow ridge running from the apex down to the basal angle, is hidden by the overlapping of the scuta, latera, and carina.
The _Carina_, in outline is triangular, with the basal margin in some species extremely protuberant. In the first four species, the internal surface is concave, in _L. truncata_ and _L. Valentiana_ it is convex, with a central raised ridge, and consequently the upper freely-projecting portion of the valve, has a prominent central crest or ridge; in _L. Nicobarica_ and _L. Rhodiopus_ there is only a trace of this ridge. The rostrum, as before stated, is always very small; it, as well as the latera, are most developed in _L. Nicobarica_, and least in _L. truncata_ and _L. Valentiana_; generally only a few zones of growth are preserved, and from their being enlarged at their basal serrated rims, the rostrum sometimes appears like a few beads of a necklace strung together.
The _Latera_ are remarkable from being placed over the carinal half of the terga, in an oblique position, parallel to the lower carinal margin of the terga. A section, parallel to the growth layers, varies in the different species from elliptic to broadly oval, and in _L. Nicobarica_ it is triangular. Only a few layers of growth are ever preserved. In _L.
truncata_, where the latera are represented by mere stiles, (like strings of beads), and are even less in width than the rostrum, they are imperfectly calcified.
_Microscopical Structure of the Valves._--The sh.e.l.ly layers are white, and generally separate easily, so that in _L. dorsalis_ it is rare to find a specimen with the upper part of the valves perfect. The valves are so translucent, that in the thin margins, even the tubuli could be sometimes distinguished. The valves are coated by strong yellow membrane, which, after the sh.e.l.ly matter in _L. dorsalis_ had been dissolved in acid, separated into broad slips, answering to each zone of growth. On the lower margin of each slip, there is a row of closely approximate spines, generally slightly hooked, pointed, 1/650th of an inch in length, and 1/10000th of an inch in diameter; they arise out of a little fold; all are furnished with tubuli of the same diameter with themselves, running through the whole thickness of the sh.e.l.ly layers, and attached, apparently, by their apices, to the underlying corium. As the spines are very numerous, so are the parallel rows of tubuli. After the sh.e.l.ly layers had been dissolved, there was left in _L. dorsalis_ (well seen in the latera), an extraordinary, conferva-like ma.s.s of branching, jointed, excessively thin tubes, sometimes slightly enlarged at the articulations, and appearing to contain brown granular matter: other portions of the valves, instead of this appearance, exhibited membranes or films with similar, branching, articulated tubes or vessels attached to them: I have not seen this appearance in any other cirripede. The yellow exterior enveloping membrane, with its spines, is present in all the species of the genus; in _L. Rhodiopus_ these spines are much larger than in _L. dorsalis_, and on the inner sides of the carina they are trifid and quadrifid, and large enough to be conspicuous with a lens of weak power.
_Peduncle._--The most remarkable fact concerning this part, is that the outer tunic, together with the calcareous scales with which it is covered, is moulted at each successive period of exuviation and growth.
I demonstrated this fact in _L. dorsalis_ and _L. truncata_, by removing the old tunic and finding a new membrane with perfect calcified scales beneath; and as these two species, (I obtained, also, pretty good evidence in _L. Nicobarica_,) are at the opposite extremes of the genus, no doubt this fact is common to the whole genus. I know of no other instance, amongst Cirripedia, in which _calcified_ valves or scales are moulted. I am not certain that the whole skin of the peduncle is thrown off in a single piece; though this almost certainly is the case with the uppermost and lowest portions. The animal"s body is partly lodged within the peduncle, which is generally from one to three times as long as the capitulum, and in the upper part is fully as broad as it. The scales with which it is clothed, extend up in the triangular inters.p.a.ces between the basal margins of the valves. The scales of the upper whorl, or of the two or three upper whorls (Pl. VIII, figs. 1 _b"_ and 3 _d_) are larger than those below; and these latter rapidly decrease in size, so as to become low down on the peduncle, almost or quite invisible to the naked eye. The scales in each whorl, are placed alternately with those in the whorls, above and below. All the upper scales are packed rather closely together; those in the uppermost row are generally nearly quadrilateral; those in the few next succeeding whorls, are triangular, with their basal margins protuberant and arched; the scales, low down on the peduncle, stand some way apart from each other, and generally consist of simple rounded calcareous beads, of which some of the smallest in _L. dorsalis_ were only 1/400th of an inch in diameter. In the lowest part of the peduncle these scales, after each fresh exuviation, are apparently soon worn entirely away by the friction against the sides of the cavity; hence in most specimens this part of the peduncle is quite naked. This same part, however, is furnished with nail- or rather star-headed little projections of hard, yellow, h.o.r.n.y chitine (fig. 3 _e_). The star on the summit seems generally to have about five irregular points; one star which I measured was 7/6000th of an inch in total width, the footstalk being only 2/6000th of an inch in diameter; the whole projected 10/6000ths of an inch above the surface of the peduncle; from the footstalk a fine tubulus runs through the membrane to the underlying corium. These star-headed little points are often much worn down; in one specimen which was on the point of exuviation, there remained, in the lower part, close above the basal calcareous cup, only some hard, smooth, yellow, little discs, on a level with the general surface of the membrane,--these being the intersected or worn down footstalks, with every trace of the calcareous beads gone.
But in this same specimen, under the old peduncular membrane, there was a new one, studded with the usual circular calcareous beads, slightly unequal in size, generally about 1/400th of an inch in diameter, and each furnished with a tubulus; but as yet none of the star-headed points of chitine had been formed. I believe that these latter are developed from the tubuli leading to the calcified beads, and, therefore, are formed directly under them. In _L. cauta_ the lowest scales on the peduncle are a little larger than in _L. dorsalis_, giving a frosted appearance to it, and all of them are serrated (fig. 3 _d_) round their entire margins. Generally only the scales in the uppermost, or in the three or four upper rows are serrated, and this only on their arched and protuberant lower margins. The state of the serrated edge varies extremely in the same species, from elongated conical teeth to mere notches, according to the amount of wear and tear the individual has suffered since the last period of exuviation; so also do the teeth or serrated margins on the valves of the capitulum. Each scale has a fine tubulus pa.s.sing from the corium through the membrane of the peduncle to its bluntly-pointed imbedded fang or base. The membrane is transparent, thin, and tender, to a degree I have not seen equalled in the other Lepadidae, except, perhaps, in Ibla. It is much wrinkled transversely.
_Muscles of the Peduncle._--These consist of the usual interior and longitudinal,--exterior and transverse--and oblique fasciae; the former are unusually strong; downwards they are attached to the basal calcareous cup or disc, and upwards they extend all round to the lower curved margins of the valves. They are, as usual, without transverse striae. Besides these, there are, (at least in _L. dorsalis_ and _L.
Nicobarica_,) two little fans of striae-less muscles, which occur in no other pedunculated cirripede; they are attached on each side of the central line of the carina, near its base; they extend transversely and a little upwards, and each fan converges to a point where the lower margins of the carina and terga touch; of these muscles, the upper fasciae are the longest. Their action, I conceive, must be either to draw slightly together the basal points of the terga, and so serve to open their occludent margins, or to draw inwards the base of the carina: these muscles apparently first shadow forth the posterior or carinal, transversely-striated, opercular muscles of sessile cirripedes.
_Basal Calcareous Cup or Discs._--I have seen this part in all the species, except _L. Valentiana_, and in this it probably occurs, considering its very close alliance with _L. truncata_. The size, form, and conditions of the cup or disc varies infinitely according to the age, size, and position of the individual specimen. We will commence with a full-sized animal, which has ceased to burrow downwards into the rock, in which case the discs usually grow into a cup, and become largely developed. In _L. dorsalis_ alone, I have seen many specimens, so that the following description and remarks, though applicable I believe to all the species, are drawn up from that alone. The cup (Pl.
VIII, fig. 1 _a"_, 1 _c"_) is hardly ever regular in outline, and is either slightly or very deeply concave; I have seen one, half an inch in diameter; it is formed of several thick layers of dirty white, translucent, calcareous matter, with sinuous margins; externally the surface is very irregular, and is coated by yellow membrane presently to be described. The innermost and last-formed layer sometimes covers the whole inside of the cup, and extends a little beyond its margin all round; but more generally it projects beyond only one side, leaving the other sides deserted. I have seen a _single_ new layer extending beyond the underlying old layers, as much as one sixth of an inch; and again I have seen a part of the cup, as much as a quarter of an inch in width, deserted and covered with serpulae. So irregular, however, is the growth, that after a period an old deserted portion will occasionally be again covered by a new layer, though of course without organic adhesion. Again it sometimes happens that the last-formed layer, remaining central, is very much less than the older layers; in one such instance the innermost and last-formed layer (fig. 1 _a"_) had a diameter of only a quarter of that of the whole cup, in the middle of which it was placed; the cup thus tends to become filled up in the middle. The cup, in its fully developed condition, is seated at the very bottom of the cavity in the rock. From the aggregate thickness of the several component layers forming the cup, the old and mature animal rises a little in its burrow; for instance, the bottom of the cup in one specimen which I measured, was 4/10ths of an inch in thickness.
In a younger condition, before the animal has bored down to the full depth, and whilst the cavity is only of moderate diameter, the lower part of the peduncle, instead of being attached to the inside of a cup, adheres to small, irregular, nearly flat, calcareous discs, overlapping each other like tiles (figs. 1, 2 _a"_). They are placed one below the other, generally in a straight line, and are attached firmly to one side of the burrow. The discs are oval, or rounded, or irregular, and are commonly from 1/20th to 1/10th of an inch across: they usually form a quite straight ribbon, widening a little downwards: each little disc overlaps and extends beyond the one last formed, fully half its own diameter. I have seen one row of discs an inch in length, but the upper discs are always worn away by the friction of the calcified serrated scales on the peduncle. It is very important to observe that the lowest disc is not fixed, (as was the case with the cup,) at the very bottom of the burrow, but on one side, just above the bottom, which latter part is occupied by the blunt basal end of the peduncle.
In a valuable paper on _L. Nicobarica_, by Reinhardt, presently to be referred to, the disc is said to be attached on the carinal side (see fig. 2) of the peduncle; and this, I believe, is general. I have seen one instance in which, during the excavation of a new burrow, an old burrow was met with, and the row of discs turned down it, making, with their previous course, nearly a right-angle. In another similar instance, the discs, instead of turning down, became very large and broad, and so fairly formed a bridge across the old burrow (fig.
1),--becoming narrow again as soon as the animal recommenced burrowing into the solid rock. Sometimes, as it appears, the animal, whilst still small, from some unknown cause, stops burrowing downwards, and then a cup is formed at the bottom of the hole. As soon as the animal has got to its full depth, the burrow increases only in diameter, and during this process the linear row of discs is ground away and lost; a cup is then formed. The little discs can be deposited or formed only at each fresh exuviation; and as some of the burrows are above two inches in depth, and as on an average each disc does not extend beyond the underlying disc more than 1/15th of an inch, an animal which has bored two inches in depth, must have moulted at least thirty times. I may here remark that I have reason to believe, from some interesting observations made by Mr. W. Thompson, of Belfast, that some sessile cirripedes moult about every fortnight.
_Internal Structure of the Cup._--When the cup is dissolved in acid, each sh.e.l.ly layer is represented by a rather tough, pale-brown membrane, itself composed of numerous fine laminae, which, under a one-eighth of an inch object gla.s.s, exhibit generally only the appearance of a mezzotinto drawing; but there often were layers of branching vessels, (like moss-agate,) less than the 1/10,000th of an inch in diameter, and of a darkish colour; these vessels are not articulated, but otherwise resemble the same peculiar structure in the valves of the capitulum. The exterior yellow membrane is marked, or rather composed of successive narrow rims, which, in fact, are the lines of termination of the laminae of membrane, which in a calcified state form the cup itself. In most parts, both on the borders and under the centre of the cup, but not everywhere, there are imbedded in the yellow membrane, elongated, irregular, top-shaped ma.s.ses of bright yellow chitine, each furnished with a tubulus, which penetrating the calcareous laminae leads to the corium; the little apertures thus formed, are clearly visible in the layers of membrane, left after exposure to acid. In _L. Nicobarica_, the innermost sh.e.l.ly layer of the cup was punctured, like the surface of the sh.e.l.l in Chthamalus and many other sessile Cirripedes, by the internal orifices of these tubuli. The top-shaped ma.s.ses often have star-shaped summits; and they differ in no essential respects from those on the lower part of the peduncle, excepting that they are quite imbedded in the membrane covering the under surface of the cup, whereas those on the peduncle project freely. I found these top-shaped bodies in the outer membrane of the cups in _L. dorsalis_, _L. cauta_, and _L. Rhodiopus_, which alone I was enabled to dissolve in acid; and I mention this fact, as indicating the probable presence of the more important star-headed projections on the lower parts of the peduncle in these same species.
The basal calcareous cup resembles, in essential structure, the valves of the capitulum; the chief difference being that in the former there is a larger proportion of animal matter or membranous layers.
After the dissolution of the cups, in _L. dorsalis_ and _L. Rhodiopus_, I most distinctly traced the two cement-ducts; they included the usual darker chord of cellular matter; they were of rather small diameter, namely, 2/3000th of an inch. The two (in _L. dorsalis_) ran in a very irregular course, not parallel to each other, making the most abrupt bends. They pa.s.sed through the membranous layers, (as seen after dissolution,) and running for short s.p.a.ces parallel to the component laminae, were attached to them. In their irregular course, these cement-ducts resemble those of _Pollicipes mitella_, but I could not perceive that any cement had been poured out at the abrupt bends. In one specimen of a basal cup, which I was enabled to examine whilst still attached to the rock, I found under the very centre, (and of course outside the yellow membrane,) a very small area of dark brown cement of the usual appearance. In several specimens of full-sized cups, I was not able to perceive any cement on the external surfaces of the upper and later-formed layers; hence I believe that the cup is cemented to the bottom of the hole only during the early stages of its formation; and this, considering its protected situation, would no doubt be sufficient to affix the animal. This probably accounts for the small size of the cement-ducts, and for the facility with which, as it appears, the cups can be removed in an unbroken condition from the rock. In the case, however, of the small, flat, calcareous discs, which are formed whilst the animal is burrowing into the rock, these are attached firmly to the sides of the holes, in the usual manner, by cement. In this cirripede it would be useless to look for the prehensile antennae of the larva under the cup, for the animal, during the formation of the successive discs, must have travelled some distance from the spot on which the larva first attached itself.
The membrane of the peduncle is continuous with the yellow membrane coating the external surface of the cup; and this latter membrane is continuous with those delicate laminae which, in a calcified condition, form the layers of the cup itself. In an exactly similar manner, in this and other cirripedes, the membrane of the peduncle, at the top, is continuous with that coating the valves, and is attached to the lower exterior edge of the last-formed layer of sh.e.l.l. When a new sh.e.l.ly layer is formed, both under the valves of the capitulum and inside the basal calcareous cup, it projects beyond the old layer, and is included within the old, as yet not moulted, membrane of the peduncle. Within the cup of _L. Nicobarica_ I found a lately-formed layer of sh.e.l.l, projecting 1/10th of an inch on one side of the cup, and by its protuberance distinguishable even through the old coat of the peduncle, which was nearly ready to be moulted. In an a.n.a.logous manner, in the capitulum of _L. dorsalis_ and _L. truncata_, I have found a new peduncular membrane bearing the usual, but then sharp, calcified scales, attached to the lower projecting edge of the last-formed sh.e.l.ly layer, lying under the old peduncular membrane, which was attached to the penultimate layer of sh.e.l.l, and with its worn scales was just ready to be moulted.
The final cause of the moulting of the calcified scales, together with the membrane of the peduncle to which they are attached,--a case confined to Lithotrya,--I have scarcely any doubt is the reproduction of a succession of scales, sharply serrated for the purpose of enlarging the cavity in which the animal is lodged. The extreme thinness of the membrane of the peduncle has been noticed; this may be partly related to its protected condition, but partly, I think, to the necessity of its being formed in a very extensible condition; for the new coat, owing to the projection of the new sh.e.l.ly layers under the valves, and within the basal cup, is by so much shorter than the old peduncle, yet after exuviation it has to stretch to a greater length than the old membrane, to allow of the growth of the Cirripede. Owing to the thinness and fragility of this membrane, the basal attachment of the Cirripede is, no doubt, chiefly effected by the unusually strong longitudinal muscles; and the necessity of a surface of attachment for these muscles, stronger than the external membrane of the peduncle, probably is one of the final causes of the basal calcareous disc and cup, and likewise for the unusual manner in which the valves of the capitulum are locked together by folds and small roughened projections. The basal discs and cup, however, apparently serve for several other purposes, namely, for raising the animal a little in its burrow, (which is narrow and pointed at the bottom,) at that period of growth when it has ceased to burrow downwards, but still increases in diameter; also for carrying the animal, as over a bridge, across any pre-existing cavity in the rock; and lastly, perhaps, for removing lower down, in the intervals of exuviation, the point of attachment for the longitudinal peduncular muscles.
_Position of the animal in the rock, and its power of excavation._--A specimen of rock, two or three inches square, in Mr. c.u.ming"s possession, is full of Lithotryas; the cavities extend in every possible direction, and several were parallel, but with the animals in reversed positions; the same thing is apparent in some specimens of Mr.
Stutchbury"s, and it was evident that the positions occupied by the animals were entirely due to chance. In Mr. c.u.ming"s specimen of rock, a considerable portion of the external surface is preserved, and here it can be seen that many of the specimens have their capitulums directed from the external surface directly inwards. These individuals, which were of full size, must have preyed on infusoria inhabiting the cavities of the porous, calcareous rock. On the other hand, I have seen some young specimens of _L. dorsalis_ with their valves not at all rubbed, and others of full size with uninjured Balani and corallines on the tips of the valves, and again a specimen of _L. truncata_ with minute pale-green sea-weed on the summit of the capitulum,--all which appearances induce me to believe that in these cases, the valves had projected freely beyond the cavity in which their peduncles were lodged.
I may here also mention that in Mr. c.u.ming"s specimen, above alluded to, the basal cups of five specimens touched and adhered to each other; I was not able to make out whether there had originally existed separate burrows, as I think is most probable, and that the walls had been wholly worn away, or whether the five specimens had fixed themselves on one side of a large pre-existing, common cavity. Young specimens seem to burrow to the full depth, before nearly acquiring the diameter which they ultimately attain. I measured one burrow, 1.2 of an inch in depth, which, at its mouth or widest part, was only .17 in diameter.
The several species occur imbedded in soft calcareous rocks, in ma.s.sive corals, and in the sh.e.l.ls of mollusca and of cirripedes. It has been doubted by several naturalists, whether the basal calcareous cup at all belongs to the Lithotrya, but after the foregoing microscopical observations on its structure, it is useless to discuss this point. So again it has been doubted whether the cavity is formed by the cirripede itself; but there is so obvious a relation between the diameters of specimens of various sizes, and the holes occupied by them, that I can entertain no doubt on this head. The holes, moreover, are not quite cylindrical, but broadly oval, like the section of the animal. The simple fact, that in this genus alone each fresh sh.e.l.ly layer round the bases of the valves, and therefore at the widest part of the capitulum, are sharply toothed; and secondly, that in this genus alone a succession of sharply serrated scales, on the upper and widest part of the peduncle, are periodically formed at each exuviation; and that consequently the teeth on the valves and scales are sharp, and fit for wearing soft stone, at that very period when the animal has to increase in size, would alone render the view probable that the Lithotrya makes or at least enlarges the cavities in which it is imbedded.
Although it may be admitted that Lithotrya has the power of enlarging its cavity, how does it first bore down into the rock? It is quite certain that the basal cup is absolutely fixed, and that neither in form nor state of surface it is at all fitted for boring.[66] I was quite unable to answer the foregoing question, until seeing the admirable figures by Reinhardt[67], (Pl. VIII, figs. 2 and 2 _a"_) of _L.
Nicobarica_, still attached in its cavity. Subsequently I obtained from Mr. Stutchbury several pieces of rock completely drilled with holes, many of small diameter, by _L. dorsalis_, and in these I found numerous instances of the linear rows of little discs, like those of _L.
Nicobarica_, showing in the plainest manner, that each time a new disc is formed, that is, at each exuviation, the animal moves a short step downwards; and as the lowest of these little discs _in none of the burrows_ was placed at the very bottom, we see that the lowest point of the peduncle must be the wearing agent. In the peduncle of an individual of _L. dorsalis_, nearly ready to moult, I found, it may be remembered, beneath and round the basal disc, under the old membrane of the peduncle, a new membrane studded with calcified beads, but with the h.o.r.n.y star-headed spines not yet developed, whilst on the old outer coat these latter had been worn down quite smooth, and the calcified beads worn entirely away. Here, then, we have an excellent rasping surface.
With respect to the power of movement necessary for the boring action, the peduncle is amply furnished with transverse, oblique, and longitudinal striae-less muscles,--the latter attached to the basal disc.
In all the pedunculata, I have reason to believe that these muscles are in constant slight involuntary action. This being the case, I conceive that the small, blunt, spur-like portion of the peduncle, descending beneath the basal rim of the lowest disc, would inevitably partake slightly of the movements of the whole distended animal. As soon as the Lithotrya has reached that depth, which its instinct points out as most suitable to its habits, the discs are converted into an irregularly growing cup, and the animal then only increases in diameter, enlarging its cavity by the action of the serrated scales on the peduncle, and of the serrated lower edges of the valves of the capitulum. With respect to those reversed individuals attached with their capitulums downwards, I suppose that the larvae had crept into some deep cavity, perhaps made originally by a Lithotrya, of which the rock in the specimen in question was quite full, and had there attached themselves. Finally, it appears that in Lithotrya the burrowing is simply a mechanical action; it is effected by each layer of sh.e.l.l in the basal attached discs overlapping, in a straight line, the last-formed layer,--by the membrane of the peduncle and the valves of the capitulum having excellent and often renewed rasping surfaces,--and, lastly, by the end of the peduncle (that is h.o.m.ologically the front of the head) thus roughened, extending beyond the surface of attachment, and possessing the power of slight movement.
[66] Mr. Hanc.o.c.k, in his admirable account of his burrowing Cirripede, _Alcippe lampas_, ("Annals of Nat. Hist.," Nov. 1849, p. 313,) came to this conclusion regarding the cup of Lithotrya, and hence was led to think that this genus did not form its own burrows, but inhabited pre-existing cavities. I am much indebted to this gentleman, who has been so eminently successful in his researches on the boring powers of marine animals, for giving me his opinion on several points connected with the present discussion.
[67] I owe to the great kindness of Prof. Steenstrup the sight of this Plate, published in the "Scientific Communications from the Union of Natural History," Copenhagen, January 30, 1850, No. I.
Since this sheet has been set up in type, I have received from Prof. Steenstrup the memoir, in Danish, belonging to the figures in question; and the greater part of this has been translated to me by the kindness of a friend. My account of the means of burrowing is essentially the same as that published by Reinhardt; but the moulting of the scales on the peduncle, the presence of scales and of points of a different nature, the method of attachment by cement, the conversion of the discs into a cup, &c., seem not to have been known to this naturalist. Reinhardt states that the points on the peduncle will scratch Iceland spar, and that, apparently, they are formed of phosphate of lime: in the case of the closely-allied _L. dorsalis_, I must believe that the scales or beads on the peduncle are formed of carbonate of lime, for they were quickly dissolved with effervescence in acetic acid; and the star-headed points, which are subsequently developed under the calcareous scales, appeared to me, under the compound microscope, to be formed of a horn or chitine substance.
Reinhardt states that the basal point of the peduncle is arched a little under the lowest disc, and there forms for itself a slight furrow (as represented in the lateral view, Pl. VIII, fig. 2); but in the burrows examined by me, this furrow or depression did not really exist, the appearance resulting from the basal margin of the lowest disc, projecting beyond the wall of the cavity by the amount of its own slight thickness.
We will now proceed with our generic description.--
_Animal"s Body._--This, as already stated, is partially lodged within the peduncle. The prosoma is rather largely developed.
The _Mouth_ is placed at a moderate distance from the adductor muscle.
The _Labrum_ is moderately bullate, with a row of blunt bead-like teeth, mingled with fine bristles, on the crest, which in the middle part is generally somewhat flattened.
The _Palpi_ are blunt, and even squarely truncated at their ends; they are of large size, so that, if they had been half as large again, or even less, their tips would have met.
_Mandibles_ (Pl. X, fig. 2), with three nearly equal large teeth, and the inferior angle produced, broad, and strongly pectinated: in the inters.p.a.ces between these teeth there are, in all the species, some very fine teeth or pectinations, which are seated a little on one side of the medial line. The mandibles are somewhat singular from the size of the transparent flexible apodemes (_a_ _a_) to which the muscles are attached; these are oval and constricted at their origins: in _L.
dorsalis_ they are roughened with little points; in _L. cauta_ and _L.
truncata_ they are large, of the same shape, but smooth.
_Maxillae._--These are larger, compared to the mandibles, than is usual with pedunculated Cirripedes; they differ in shape in the different species, being either nearly straight on their edge, and notched or not (fig. 10), or notched with the inferior part forming a double prominence (fig. 12); the spines on the inferior angle, which is sometimes slightly produced, are always crowded together into a brush, and are finer than those on the upper parts. The apodemes are less straight than is usual, and at their origin take, in all the species, a rather abrupt bend; their extremity is enlarged into a little disc, which in _L. dorsalis_ is covered with strong points, but in the other species is, as usual, smooth.
_Outer Maxillae._--The inner margin is slightly concave, and in _L.
truncata_ alone, the bristles are hardly continuous, being interrupted in the middle part. The olfactory orifices are only very slightly prominent. The spines on all the trophi are more or less doubly serrated.
_Cirri._--The three posterior pair are elongated, with their anterior surfaces not at all protuberant. The segments bear from three to five pair of spines, with a row of three or four small intermediate spines; there are, as usual, some little lateral upper rim spines; the dorsal tufts contain some thick and thin spines mingled. _First_ cirrus is short, and placed not quite close to the second pair; the basal segments are broad and thickly paved with bristles. The _second_ pair is rather short compared with the _third_ pair; a varying number of the basal segments in both rami of both these cirri are protuberant, and are thickly paved with bristles; such segments are more numerous and are broader on the anterior rami than on the posterior rami. In _L. cauta_ alone, none of the basal segments in the posterior rami of the second and third cirri are thickly paved with bristles. The pedicels of the first three pair are irregularly covered with spines; those of the three posterior pair have the spines arranged in a regular double line. Most of the spines are doubly serrated.
_Caudal Appendages_ (Pl. X, fig. 23 and 24), multiarticulate, with thin elongated segments fringed with short spines; in length generally exceeding the pedicel of the sixth cirrus, and in _L. Nicobarica_ equalling half the entire length of this cirrus.
_Stomach_, dest.i.tute of caeca; oesophagus somewhat curled.
_Filamentary Appendages_, none.
_Ovaria_ filling up the peduncle and surrounding the sack, but not extending up to the bases of the scuta and terga; I saw the ova only in _L. truncata_; they were here oval and large, being nearly 9/400ths of an inch in length.
_p.e.n.i.s_, elongated; vesiculae seminales extending into the prosoma. I noticed the ovigerous fraena only in _L. truncata_; here they were large, with an almost bilobed outline; the margin and whole lateral surface being covered with elongated cylinders, finely pointed, but not enlarged at their extremities, as are the glands observed in most of the other genera.