Supra-oesophageal ganglion, } transverse or longest axis } 126/6000 of both together }
Supra-oesophageal ganglion, } longitudinal axis of } 45/6000
Infra-oesophageal ganglion, } transverse axis of } 120/6000
Infra-oesophageal ganglion, } longitudinal axis of } 114/6000
[16] Proceedings of the Academy of Natural Sciences, Philadelphia. No. i, vol. iv, Jan. 1848.
In _Conchoderma aurita_ the ophthalmic ganglia are much smaller, and nearer to the supra-oesophageal ganglion, than in _L. fascicularis_. In _Alepas cornuta_ the ophthalmic chords run towards each other from the two distant and separate supra-oesophageal ganglia; and the ophthalmic ganglia, (instead of being quite separate, as in _L. fascicularis_,) are united by their front ends, and the two eyes instead of standing some way in front, with nerves running to them, are embedded on the double ophthalmic ganglion; the pigment-capsules here, also, have the shape of mere saucers, and are joined back to back, with the two lenses projecting far out of them. In neither s.e.x of Ibla could I perceive that the eye was double. In _Pollicipes spinosus_ the ophthalmic ganglion stands in front of the single supra-oesophageal ganglion, and shows no signs of being formed of a lateral pair; the eyes themselves, however, differently from, in all the foregoing cases, are, though approximate, quite distinct. In _Pollicipes mitella_ I did not see the eyes; but the ophthalmic ganglion consists, as I believe, of a single globular one, placed exactly between the two globular, supra-oesophageal ganglia, all three being of nearly equal size. Professor Leidy does not mention the ophthalmic ganglia; hence I infer that in Bala.n.u.s, which is a more highly organised Cirripede, they are fused into the supra-oesophageal ganglion.
In all the genera, the double eye is seated deep within the body; it is attached by fibrous tissue to the radiating muscles of the lowest part of the oesophagus, and lies actually on the upper part of the stomach; consequently, a ray of light, to reach the eye, has to pa.s.s through the exterior membrane and underlying corium connecting the two scuta, and to penetrate deeply into the body. In living sessile Cirripedes, vision seems confined to the perception of the shadow of an object pa.s.sing between them and the light; they instantly perceived a hand pa.s.sed quickly at the distance of several feet between a candle and the basin in which they were placed.
As the infra-oesophageal ganglion sends nerves to the trophi and to the first pair of cirri, it must correspond to the segments, from the fourth to the ninth inclusive, of the archetype crustacean. The state of the supra-oesophageal and ophthalmic ganglia appears to me very interesting: I do not believe that in any _mature_ ordinary crustacean, the first or ophthalmic ganglion can be shown to be distinct from the two succeeding ganglia, or to be itself composed of a pair laterally distinct. The ganglia, corresponding with the second and third segments of the body, which should normally support two pair of antennae, are in the Lepadidae united together; but laterally they are generally distinct in outline, and are actually separate in Alepas: the supra-oesophageal ganglion shows also its double nature, by giving rise to a pair of large double nerves, evidently corresponding with the two pair of antennular nerves in ordinary crustaceans. The embryonic condition of the whole supra-oesophageal portion of the nervous system in the Lepadidae, corresponds with the rudimentary state of the only organ of sense supplied by it, namely, the eye, which in size and general appearance has retrograded to the state in which it was in, during the first stage of development of the larva;--I have used the term embryonic, because, in the embryos of ordinary crustacea, all the ganglia are at first longitudinally distinct, and laterally quite separate. The conclusion at which we before arrived from studying the metamorphoses, namely, that the whole peduncle and capitulum consisted of the first three segments of the head, is beautifully supported by the structure of the nervous system, in which these parts are seen to be supplied with nerves exclusively from the supra-oesophageal ganglion: now in ordinary crustacea the supra-oesophageal ganglion sends nerves to the eyes and the two pair of antennae corresponding, as is known by embryological dissections, to the first three segments of the body. Moreover, it is a.s.serted that the carapace which covers the thorax in crustacea, is not formed by the development of the first segment; and this, likewise, may be inferred to be the case with the peduncle and capitulum in the Lepadidae, as the nerves of the ophthalmic ganglia go exclusively to the eyes. Finally, I may remark that in Pollicipes, looking to the whole nervous system, the state of concentration nearly equals that in certain macrourous decapod crustaceans, for instance the _Astacus marinus_, of which a figure is given by Milne Edwards.
_Olfactory Organs._--In the outer maxillae, at their bases where united together, but above the basal fold separating the mouth from the body, there are, in all the genera, a pair of orifices (Pl. X, fig. 16); these are sometimes seated on a slight prominence, as in Lithotrya, or on the summit of flattened tubes (Pl. X, fig. 17), projecting upwards and towards each other, as in Ibla, Scalpellum, and Pollicipes. In Ibla these tubular projections rise from almost between the outer and inner maxillae. It is impossible to behold these organs, and doubt that they are of high functional importance to the animal. The orifice leads into a deep sack lined by pulpy corium, and closed at the bottom. The outer integument is inflected inwards, (hence periodically moulted,) and becoming of excessive tenuity, runs to near the bottom of the sack, where it ends in an open tube: so excessively thin is this inflected membrane, that, until examining Anelasma, I was not quite certain that I was right in believing that the outer integument did not extend over the whole bottom. I several times saw a nerve of considerable size entering and blending into a pulpy layer at the bottom of the sack of corium; but I failed in tracing to which of the three pair of nerves, springing from the front end of the infra-oesophageal ganglion, it joined. I can hardly avoid concluding, that this _closed_ sack, with its naked bottom, is an organ of sense; and, considering that the outer maxillae serve to carry the prey entangled by the cirri towards the maxillae and mandibles, the position seems so admirably adapted for an olfactory organ, whereby the animal could at once perceive the nature of any floating object thus caught, that I have ventured provisionally to designate the two orifices and sacks as olfactory.
_Acoustic_ (?) _Organs._--A little way beneath the basal articulation of the first cirrus (Pl. IX, fig. 4 _d_, and Pl. IV, fig. 2 _e_), on each side, there may be seen a slight swelling, and on the under side of this, a transverse slit-like orifice, 1/20th of an inch in length in Conchoderma, but often only half that size. In Ibla this orifice is seated lower down (Pl. IV, fig. 8 _a"_, _e_), between the bases of the first and second cirri, which are here far apart: in _Alepas cornuta_ it is placed rather nearer to the adductor scutorum muscle, namely, beneath the mandibles. The orifice leads into a rather deep and wide meatus; the external integument is turned in for a short distance, widening a little, and then ends abruptly. The meatus, enlarging upwards, is lined by thick pulpy corium, and is closed at the upper end; from its summit is suspended a flattened sack of singular and different shapes in the different genera. This, the so-called acoustic sack of _Conchoderma virgata_, is figured Pl. IX, fig. 6. The deep and wide notch faces towards the posterior end of the animal; the inferior lobe, thus almost cut off, is flattened in a different plane from the upper part; the lobe is lodged in a little pouch of corresponding form, leading from the open meatus in which the upper part is included. In _Conchoderma aurita_, the top of the acoustic sack is narrower and more constricted, the whole more rounded, and the lobe more turned down. In _Lepas fascicularis_ the notch is not so deep or wide, and the lobe larger. In _Ibla c.u.mingii_ the sack is of the shape of a vase, with one corner folded over. In _Scalpellum vulgare_ it is small, oval, with the lower end much pushed in, and furnished with a little crest. Lastly, in _Pollicipes mitella_ it is simply oval. In all cases the sack is empty, or contains only a little pulpy matter: it consists of brownish, thick, and remarkably elastic tissue, formed, apparently, of transverse little pillars, becoming fibrous on the outside, and with their inner ends appearing like hyaline points. The mouth of the acoustic sack (removed in the drawing) is closed by a tender diaphragm, through which I saw what I believe was a moderately-sized nerve enter; I have not yet succeeded in tracing this nerve. The first pair of cirri seem, to a certain extent, to serve as antennae, and therefore the position of an acoustic organ at their bases, is a.n.a.logous to what takes place in crustacea; but there are not here any otolites, or the siliceous particles and hairs, as described by Dr. Farre, in that cla.s.s. Nevertheless, the sack is so highly elastic, and its suspension in a meatus freely open to the water, seems so well adapted for an acoustic organ, that I have provisionally thus called it. In the larva, as I have shown, a pouch, certainly serving for some sense, I believe for hearing, is seated in quite a different position at the anterior end of the carapace. I may mention that I found sessile Cirripedes very sensitive of vibrations in objects adjoining them, though not, apparently, of noises in the air or water.
In a group of specimens, I could not touch one even most delicately with a needle, without all the adjoining ones instantly withdrawing their cirri; it made no difference if the one touched had its operculum already closed and motionless.
_Reproductive System_,--_Male Organs._--All the Cirripedia which I have hitherto examined, with the exception of certain species of Ibla and Scalpellum, are hermaphrodite or bis.e.xual.[17] I shall so fully describe the s.e.xual relations of the several species of these two genera, under their respective headings, and at the end of the genus of Scalpellum, that I will not here give even an abstract of the grounds on which my firm belief is based, that the masculine power of certain hermaphrodite species of Ibla and Scalpellum, is rendered more efficient by certain parasitic males, which, from their not pairing, as in all hitherto known cases, with females, but with hermaphrodites, I have designated _Complemental Males_.
[17] I am compelled to differ greatly from the account given by Prof. Steenstrup of the reproductive system in the Cirripedia, in his "Untersuchungen uber das Vorkommen des Hermaphroditismus, ch.
v, 1846;--a translation of which I have seen, owing to the great kindness of Mr. Busk. Mr. Goodsir has described ("Edin. New Phil.
Journal," July 1843,) what he considers the male of Bala.n.u.s; but I have seen this same parasitic creature charged with ova, including larvae! From the resemblance of the larvae to the little crustacean described by Mr. Goodsir, in the same paper, as a distinct parasite, I believe the latter to be the male of his so-called male Bala.n.u.s, and that all belong to the same species, allied to Bopyrus. This genus, as is well known, is parasitic on other crustacea; and it is a rather interesting fact thus to find, that this new parasite which is allied to Bopyrus, in structure, is likewise allied to it in habits, living attached to Cirripedia, a sub-cla.s.s of the crustacea.
The male organs have been well described by M. Martin St. Ange, whose observations have since been confirmed by R. Wagner.[18] The testes are small, often leaden-coloured, either pear or finger-shaped, or branched like club-moss,--these several forms sometimes occurring in the same individual; they coat the stomach, enter the pedicels, and even the basal segments of the rami of the cirri, and in some genera occupy certain swellings on the thorax and prosoma, and in others the filamentary appendages: the testes seen in the apex in one of these appendages in Conchoderma, is represented in Pl. IX, fig. 5. The two vesiculae seminales are very large; they lie along the abdominal surface of the thorax, and generally (but not in some species of Scalpellum) enter the prosoma, where their broad ends are often reflexed; here the branched vessels leading from the testes enter. The membrane of the vesiculae seminales is formed of circular fibres; and is, I presume, contractile, for I have seen the spermatozoa expelled with force from the cut end of a living specimen. The two ca.n.a.ls leading from the vesiculae generally unite in a single duct at the base of the p.e.n.i.s; but in _Conchoderma aurita_, half-way up it. The probosciformed p.e.n.i.s, except in certain species of Scalpellum, is very long; it is capable of the most varied movements; it is generally hairy, especially at the end; it is supported on a straight unarticulated basis, which in _Ibla quadrivalvis_ alone (Pl. IV, fig. 9 _a_), is of considerable length; in this species, the upper part is seen to be as plainly articulated as one of the cirri; in Alepas, the articulations are somewhat less plain, and in the other genera, the organ can be said only to be finely ringed, but these rings no doubt are in fact obscure articulations. In the females of _Ibla c.u.mingii_ and _Scalpellum ornatum_, there is, of course, no p.e.n.i.s.
[18] In "Muller"s Archiv," 1834, p. 467. I have already several times referred to M. Martin St. Ange"s excellent Memoir, read before the Academy of Sciences, and subsequently, in 1835, published separately.
_Female Organs._--M. Martin St. Ange has described how the peduncle[19]
is gorged with an inextricable ma.s.s of branching ovarian tubes, filled with granular matter and immature ova. In Conchoderma and Alepas, the ovarian tubes run up in a single plane (Pl. IX, fig. 3,) between the two folds of corium round the sack. Here the development of the ova can be well followed: a minute point first branches out from one of the tubes; its head then enlarges, like the bud of a tulip on a footstalk; becomes globular; shows traces of dividing, and at last splits into three, four, or five egg-shaped b.a.l.l.s, which finally separate as perfect ova. Within the peduncle, the ovarian tubes branch out in all directions, and within the footstalks of the branches (differently from what takes place round the sack), ova are developed, as well as at their ends. Close together, along the rostral (_i. e._, ventral) edge of the peduncle, two nearly straight, main ovarian tubes or ducts may be detected, which do not give out any branches till about half way down the peduncle, where they subdivide into branches, which inosculate together, and give rise to the ma.s.s filling the peduncle, and sometimes, as we have just seen, sending up branches round the sack. These two main unbranched ovarian ducts, followed up the peduncle, are seen to enter the body of the Cirripede (close along side the great double peduncular nerves), and then separating, they sweep in a large curve along each flank of the prosoma, under the superficial muscles, towards the bases of the first pair of cirri; and then rising up, they run into two glandular ma.s.ses. These latter rest on the upper edge of the stomach, and touch the caeca where such exist; they were thought by Cuvier to be salivary glands. They are of an orange colour, and form two, parallel, gut-formed ma.s.ses, having, in Conchoderma, a great flexure, and generally dividing at the end near the mouth into a few blunt branches. I was not able to ascertain whether the two main ducts, coming from the peduncle, expanded to envelope them, or what the precise connection was. The state of these two ma.s.ses varied much; sometimes they were hollow, with only their walls spotted with a few cellular little ma.s.ses; at other times they contained or rather were formed of, more or less globular or finger-shaped aggregations of pulpy matter; and lastly, the whole consisted of separate pointed little b.a.l.l.s, each with a large inner cell, and this again with two or three included granules. These so closely resembled, in general appearance and size, the ovigerms with their germinal vesicles and spots, which I have often seen at the first commencement of the formation of the ova in the ovarian tubes in the peduncle, that I cannot doubt that such is their nature. Hence I conclude, that these two gut-formed ma.s.ses are the true ovaria. I may add, that several times I have seen in the two long, unbranched ducts, connecting the true ovaria and the ovarian tubes in the peduncle, pellets of orange-coloured cellular matter (_i. e._, ovigerms) forming at short intervals little enlargements in the ducts, and apparently travelling into the peduncle.
[19] I may here mention, that in all sessile Cirripedes, the ovarian branching tubes lie between the calcareous or membranous basis and the inner basal lining of the sack, and to a certain height upwards round the sack: the true ovaria and the two ducts occupy the same position as in the Lepadidae.
The structure here described is quite conformable with that which we have seen in the larva; in the latter, two gut-formed ma.s.ses of equal thickness extended from the caeca of the stomach to within the future peduncle, where the cement-ducts entered them, and where, after a short period, they were seen to expand into a ma.s.s of ovarian tubes. In the mature Cirripede, the cement-ducts can still be found united to the ovarian tubes in the middle of peduncle; and the cause of the wide separation of the true ovaria and ovarian tubes, can be simply accounted for by the internal, almost complete intersection of the animal, which takes place during the last metamorphosis.
The ova, when excluded, remain in the sack of the animal until the larvae are hatched; they are very numerous, and generally form two concave, nearly circular, leaves, which I have called after Steenstrup and other authors, the _ovigerous lamellae_ (Pl. IV, fig. 2 _b_). These lamellae lie low down on each side of the sack: in _Conchoderma virgata_, however, there is often only a single lamella, forming a deeply concave cup: in _C. aurita_ there are generally on each side four lamellae, one under the other. The ova lie in a layer from two to four deep; and all are held together by a most delicate transparent membrane, which separately enfolds each ovum: this membrane is often thicker and stronger round the margins of the lamellae, where they are united, in a peculiar manner, presently to be described, to a fold of skin, on each side of the sack: these two folds, I have called the _ovigerous fraena_ (Pl. IV, fig. 2 _f_).
M. Martin St. Ange, describes an orifice under the carina, by which he supposes the ova to enter the sack; this, after repeated and most careful examinations, I venture to affirm does not exist; on the contrary, I have every reason to believe that the ova enter the sack in the following curious manner. Immediately before one of the periods of exuviation, the ova burst forth from the the ovarian tubes in the peduncle and round the sack, and, carried along the open circulatory channels, are collected (by means unknown to me) beneath the chitine-tunic of the sack, in the corium, which is at this period remarkably spongy and full of cavities. The corium then forms or rather (as I believe) resolves itself into the very delicate membrane separately enveloping each ovum, and uniting them together into two lamellae; the corium having thus far retreated, then forms under the lamellae the chitine-tunic of the sack, which will of course be of larger size than the last-formed one, now immediately to be moulted with the other integuments of the body. As soon as this exuviation is effected, the tender ova, united into two lamellae, and adhering, as yet, to the bottom of the sack, are exposed: as the membranes harden, the lamellae become detached from the bottom of the sack, and are attached to the ovigerous fraena. To demonstrate this view, an individual should have been found, with both the old and new chitine tunic of the sack, and with the lamellae lying between them; this, I believe, I have seen, but it was before I understood the full importance of the fact: a great number of specimens would have to be examined in order to succeed again, for the changes connected with exuviation supervene very quickly. I have, however, several times found the ova so loose under the sack, as to be detached with a touch from the ovarian tubes; and I have twice carefully examined specimens, which had just moulted, as shown by even the mandibles being flexible, in which the lamellae had not become united to the fraena, but still adhered to the newly-formed chitine tunic of the sack; in these, the ova were so tender, that they broke into pieces rather than be separated from the membrane of the lamella, itself hardly perfectly developed, for pulpy cellular matter adhered outside some of the ova. These and other facts are quite inexplicable on any other view than that advanced.
As the lamellae are formed without organic union with the parent, they would be liable to be washed out of the widely open sack of the Lepadidae, if they had not been specially attached to the _fraena_. These fraena consist of a pair of more or less semicircular folds of skin, depending inside the sack, on each side of the point of attachment of the body. The fraena are often of considerable size, but in Ibla, they are very minute; they are formed of chitine tunic with underlying corium, like the rest of the sack; on their crests, there is a row, or a set of circular groups, or a broad surface, covered, either with minute, pointed, bead-like bodies mounted on long hair-like footstalks, or with staff-formed bodies on very short footstalks. I measured some of the bead-like bodies, in _Lepas anserifera_, and they were 1/2000th of an inch in diameter, and the footstalks three or four times as long as the elongated heads. These heads, of whatever shape they may be, have an opaque, and, I believe, glandular centre; I could not make out with certainty an aperture at their ends, but, I believe, such exists, and they seem to secrete a substance, which hardens into a strong membrane, serving to unite the crest of the fraenum to the edges of the lamellae. In one case, this bit of membrane seemed formed of a woven ma.s.s of threads.
These little glandular bodies, with the membrane formed by them, are cast off at each exuviation, and new glands formed on the crest of the fraenum underneath. In some species of Pollicipes, (viz., _P. cornucopia_ and _elegans_,) the fraena, though present and large, are functionless and dest.i.tute of the glands: I believe, they exist in this same functionless condition, and in rather a different position in the sessile Cirripedes, and that in this family they serve as Branchiae.
The above-described method by which Cirripedia lay their eggs, namely, united together in a common membrane, placed between their old outer and new inner integuments, and the manner in which the lamellae, when thus formed, are retained for a time fastened to the fraena, and are then cast off, appears to me very curious. In some of the lower Crustacea, it is known, that the ova escape by rupturing the ovisacs formed by the protruded ovarian tubes, and this is the nearest a.n.a.logy with which I am acquainted. The ova are impregnated (as I infer from the state of the vesiculae seminales), when first brought into the sack, and whilst the membrane of the lamellae is very tender: the long probosciformed p.e.n.i.s seems well adapted for this end. In the male of _Ibla c.u.mingii_, which has not a probosciformed p.e.n.i.s, the whole flexible body, probably, performs the function of the p.e.n.i.s: in _Scalpellum ornatum_, however, the spermatozoa must be brought in by the action of the cirri, or of the currents produced by them. That cross impregnation may and sometimes does take place, I infer from the singular case of an individual, in a group of Balani, in which the p.e.n.i.s had been cut off, and had healed without any perforation; notwithstanding which fact, larvae were included in the ova.
_Exuviation; Rate of Growth; Size._--I have had occasion repeatedly to allude to the exuviation of the Lepadidae: with the exception of the genus Lithotrya,[20] in which the calcareous scales on the peduncle, together with the membrane connecting them, is cast off, neither the valves nor the membrane uniting them, nor that forming the peduncle with its scales and styles, are moulted; but the surface gradually disintegrates and is removed, perhaps sometimes in flakes, whilst new and larger layers are formed beneath. In Scalpellum, I ascertained that the new membrane, connecting together the newly-formed calcified rims under the valves of the capitulum, was formed as a fold, with the articulated spines which it bears, all adpressed in certain definite directions. This fold of new membrane, when the old membrane splits and yields, of course expands, and thus the size of the capitulum is increased. In the peduncle, lines of splitting can seldom be perceived, except, indeed, in the sub-globular, embedded, downward-growing peduncle of Anelasma, as described under that genus. I do not understand what determines the complicated lines of splitting of the old membrane between the several valves of the capitulum,--without it be simply, that along these lines alone, the old membrane is not strengthened by the new membrane being closely applied under it, the new being formed, as we have just said, in a fold, in order to allow of increase in size.
Although, as I believe, there is strictly no exuviation in the outer membranes of mature Lepadidae, it seems that narrow strips of membrane are cast off from between the valves, for the few first moults, after the final metamorphosis of the larva. I may here remark that, in most sessile Cirripedes, the outside membrane connecting the operculum and sh.e.l.l, is regularly moulted.
[20] The external integuments being moulted in Crustacea, but not in the Cirripedia, may appear, at first, an important difference: but we here see that non-exuviation is not universal amongst the Lepadidae, and, on the other hand, according to M. Joly, ("Annales des Sciences Naturelles," 2d series, Zoolog.), there is one true crustacean, the _Isaura cycladoides_, which has a persistent bivalve sh.e.l.l.
The delicate tunic lining the sack, (a mere duplicature of that thick one, forming the outside of the capitulum, and generally transformed into valves,) and the integuments of the whole body, are regularly moulted. With these integuments, the membrane lining the oesophagus, the r.e.c.t.u.m, and the deep olfactory pouches, and the h.o.r.n.y apodemes of the maxillae, are all cast together. I have seen a specimen of Lepas, in which, from some morbid adhesion, the old membrane lining one of the olfactory pouches had not been moulted, but remained projecting from the orifice as a brown shrivelled scroll. The new spines on the cirri (and on the maxillae) are formed within the old ones; but as they have to be a little longer than the latter, and as they cannot enter these up to their very points, their basal portions are not thus included, but are formed, running obliquely across the segments of the cirri; and what is curious, these same basal portions are turned inside out, like the fingers of a glove when hastily drawn off. After the exuviation of the old spines, the new spines have their inverted basal portions drawn out from within the segments, and turned outside in, so as to a.s.sume their proper positions.
All Cirripedia grow rapidly: the yawl of H. M. S. _Beagle_ was lowered into the water, at the Galapagos Archipelago, on the 15th of September, and, after an interval of exactly thirty-three days, was hauled in: I found on her bottom, a specimen of _Conchoderma virgata_ with the capitulum and peduncle, each half an inch in length, and the former 7/20ths in width: this is half the size of the largest specimen I have seen of this species: several other individuals, not half the size of the above, contained numerous ova in their lamellae, ready to burst forth. Supposing the larva of the largest specimen became attached the first day the boat was put into the water, we have the metamorphosis, an increase of length from about .05, the size of the larva, to an whole inch, and the laying of probably several sets of eggs, all effected in thirty-three days. From this rapid growth, repeated exuviations must be requisite. Mr. W. Thompson, of Belfast, kept twenty specimens of _Bala.n.u.s balanoides_, a form of much slower growth, alive, and on the twelfth day he found the twenty-first integument, showing that all had moulted once, and one individual twice within this period. I may here add, that the pedunculated Cirripedes never attain so large a bulk as the sessile; _Lepas anatifera_ is sometimes sixteen inches in length, but of this, the far greater portion consists of the peduncle.
_Pollicipes mitella_ is the most ma.s.sive kind; I have seen a specimen with a capitulum 2.3 of an inch in width.
_Affinities._--Considering the close affinity between the several genera, there are, I conceive, no grounds for dividing the Lepadidae into sub-families, as has been proposed by some authors, who have trusted exclusively to external characters. In establishing the eleven genera in the Lepadidae, no one part or set of organs affords sufficient diagnostic characters: the number of the valves is the most obvious, and one of the most useful characters, but it fails when the valves are nearly rudimentary, and when they are numerous: the direction of their lines of growth is more important, and fails to be characteristic only in Scalpellum: with the same exception, the presence or abscence of calcified or h.o.r.n.y scales on the peduncle is a good generic character.
For this same end, the shape of the scuta and carina, but not of the other valves, comes into play. In three genera, the presence of filamentary appendages on the animal"s body is generic; in Pollicipes, however, they are found only on three out of the six species. The number of teeth in the mandibles, and the shape of the maxillae, often prove serviceable for this end; as does more generally the presence of caudal appendages, and whether they be naked or spinose, uniarticulate or multiarticulate; in Pollicipes alone this part is variable, being uni-and multi-articulate; and in one species of Scalpellum they are absent, though present in all the others. The shape of the body, the absence or presence of teeth on the labrum, the inner edge of the outer maxillae being notched or straight, the prominence of the olfactory orifices, the arrangement of the spines on the cirri, and the number and form of their segments, are only of specific value.
Comparing the pedunculated and sessile Cirripedes, it is, I think, impossible to a.s.sign them a higher rank than that of Families. The chief difference between them consists, in the Lepadidae, in the presence of three layers of striae-less muscles, longitudinal, transverse and oblique, continuously surrounding the peduncle, but not specially attached to the scuta and terga; and on the other hand, in the Balanidae, of five longitudinal bundles of voluntary muscles, with transverse striae, fixed to the scuta and terga, and giving them powers of independent movement. In the Lepadidae, the lower valves, or when such are absent, the membranous walls of the capitulum, move with the scuta and terga when opened or shut; and the lower part of the capitulum is separated by a moveable peduncle from the surface of attachment; in the sessile Cirripedes, the lower valves are firmly united together into an immovable ring, fixed immovably on the surface of attachment. I will not compare the softer parts, such as the cirri and trophi, of the Lepadidae with those of the Balanidae, as my examination of this latter family is not fully completed: I will only remark, that there is a very close general resemblance, more especially with the sub-family Chthamalinae.
_Geographical Range; Habitats._--The Pedunculated Cirripedes extend over the whole world; and most of the individual species have large ranges, more especially, as might have been expected, those attached to floating objects; excepting these latter, the greater number inhabit the warmer temperate, and tropical seas. Of those attached to fixed objects, or to littoral animals, it is rare to find more than three or four species in the same locality. On the sh.o.r.es of Europe I know of only three, viz., a Scalpellum, Pollicipes, and Alepas. At Madeira (owing to the admirable researches of the Rev. R. T. Lowe), two Paecilasmas, a Dichelaspis, and an Oxynaspis are known. In New Zealand, there are two Pollicipes and an Alepas, and, perhaps, a fourth form. From the Philippine Archipelago, in the great collection made by Mr. c.u.ming, there are a Paecilasma, an Ibla, a Scalpellum, Pollicipes, and Lithotrya. Of all the Lepadidae, nearly half are attached to floating objects, or to animals which are able to change their positions; the other half are generally attached to fixed organic or inorganic bodies, and more frequently to the former than to the latter. Most of the species of Scalpellum are inhabitants of deep water; on the other hand, most of Pollicipes,[21] of Ibla, and Lithotrya are littoral forms. The species of Lithotrya have the power of excavating burrows in calcareous rocks, sh.e.l.ls, and corals; and the singular manner in which this is effected, is described under that genus. Anelasma has its sub-globular peduncle deeply embedded in the flesh of Northern Sharks; and I have seen instances of the basal end of the peduncle of _Conchoderma aurita_, being sunk into the skin of Cetacea; in the same way the point of the peduncle in the male of Ibla, is generally deeply embedded in the sack of the female. I believe in all these cases, the cementing substance affects and injures the corium or true skin of the animal on which the creature is parasitic, whilst the surrounding parts, being not injured, continue to grow upwards, thus causing the partial embedment of the Cirripede. In the case of Anelasma, we have growth at the end of the peduncle, and consequently downward pressure, and this may possibly cause absorption to take place in the skin of the shark at the spot pressed on.
[21] I am informed by Mr. L. Reeve that _Pollicipes mitella_ is eaten on the coast of China; and Ellis states ("Phil. Trans.,"
1758) that this is the case with _P. cornucopia_ on the sh.o.r.es of Brittany. It is well known that the gigantic _Bala.n.u.s psittacus_ on the Chilian coast, is sought after as a delicacy; and I am a.s.sured, by Mr. c.u.ming, that it deserves its reputation.
_Geological History._--Having treated this subject at length, in the volume of the Palaeontographical Society for 1851, I will not here enter on it: I will only remark, that the Lepadidae or Pedunculated Cirripedes are much more ancient, according to our present state of knowledge, than the Balanidae. The former seem to have been at their culminant point during the Cretaceous Period, when many species of Scalpellum and Pollicipes, and a singular new genus, Loricula, existed; Pollicipes is the oldest genus, having been found in the Lower Oolite, and, perhaps, even in the Lias. The fossil species do not appear to have differed widely from existing forms.
_Genus_--LEPAS. Plate I.
LEPAS. _Linnaeus._[22] Systema Naturae, 1767.
ANATIFA. _Brugiere._[23] Encyclop. Method. (des Vers), 1789.
ANATIFERA. (_Lister_) et plerumque Auctorum Anglicorum.
PENTALASMIS. (_Hill._) _Leach._ Journal de Physique, July, 1817.
PENTALEPAS. _De Blainville._ Dict. des Sci. Nat., 1824.
DOSIMA. _J. E. Gray._ Annals of Philosophy, vol. x, 1825.
[22] Linnaeus, as is well known, included under this genus both the pedunculated and sessile Cirripedes. According to the rules of the British a.s.sociation, the name Lepas must be retained for part of the genus; and as the sessile division was named Bala.n.u.s, by Lister and Hill, even before the invention of the binomial system, and subsequently, in 1778, by Da Costa, and again, in 1789, by Brugiere, there can be no question that Lepas must be applied to the pedunculated section of the genus. In this instance it is particularly desirable to recur to the Linnean name, as no other name has been _generally_ adopted. Had not Lister and Sir J. Hill published before the binomial system, their names of Anatifera and Pentalasmis would have had prior claims to Lepas.
[23] The date of this publication is almost universally given as 1792, apparently caused by an error in the t.i.tle-page of the First Part, which has consequently been cancelled. The First Part contains Anatifa and Bala.n.u.s, and was published in 1789. The Second Part was published in 1792, and has a corrected t.i.tle-page for the whole _volume_.
_Valvae 5, approximatae: carina sursum inter terga extensa, deorsum aut furca infossa aut disco externo terminata: scuta subtriangula, umbonibus ad angulum rostralem positis._
Valves 5, approximate: carina extending up between the terga, terminating downwards in an embedded fork, or in an external disc: scuta sub-triangular, with their umbones at the rostral angle.
Filaments seated beneath the basal articulation of the first cirri; mandibles with five teeth; maxillae step-formed; caudal appendages uniarticulate, smooth.
_Distribution._--Mundane; attached to floating objects.
_Description._--Capitulum flattened, sub-triangular, composed of five approximate valves. The valves are either moderately thick and translucent, or very thin and transparent; and hence, though themselves colourless, they are often coloured by the underlying corium. Their surfaces are either smooth and polished, or striated, or furrowed, and sometimes pectinated. They are not subject to disintegration; they are generally naked, except on the borders, where they are coated, and held together by membrane; in _L. fascicularis_, however, the valves are covered with thin membrane, bearing very minute spines. The manner of growth of the valves will be best described under each. All the valves, even in the same species, are subject to considerable variation in shape, more especially the terga.
_Scuta._--These valves are sub-triangular in outline, with the basal margin straight and rather short; and with occludent and tergo-carinal margins more or less protuberant; in _L. fascicularis_, however, the basal (Pl. I, fig. 6), and occludent margins are slightly reflexed and prominent. A ridge, generally runs from the umbo to the upper point.
Internally, there is no conspicuous pit for the adductor muscle; under the umbones, there is generally either on both valves, or only on the right-hand side (Pl. I, fig. 1 _c_), a small calcareous projection or tooth, of variable size and shape, even in the same species; it is generally largest on the right-hand valve; these teeth at first sight appear to form a hinge, uniting the opposite scuta at their umbones, but this is not really the case, and their use appears to be only to give attachment to the membrane uniting the valves together, and to the peduncle. The basal margin is internally strengthened by a calcified rim, more or less developed. The umbones (and primordial valves when distinguishable,) are seated at the rostral angles; during growth the basal margin is not added to, and the occludent margin only to small extent; hence the main growth of the valve is at the upper end, and along the carina-tergal margin. In _L. fascicularis_, however, the basal reflexed margin is slightly added to beneath the umbo.