Chapter X: Introduction (6)

In the Southern Atlantic I took some specimens of the pupa of _Lepas australis_, not yet attached, and therefore with the muscles of the antennæ, not having suffered any of that absorption, which they undergo, as soon as the pupa is permanently cemented to some floating object. In my former volume I noticed a pair of strong muscles, attached to the tips of the middle forks (Pl. 30, fig. 7) of the apodemes, and I now find two pairs attached to the bases of the two outer forks, and directed dorso-anteriorly; and two other pairs, also attached to their bases, but directed dorso-posteriorly, so that altogether there are five pairs of muscles attached to the apodemes; their chief function, I should think, was to draw the antennæ posteriorly and upwards within the carapace; but as the apodemes cannot be moved without the great compound eyes being likewise moved, the muscles probably serve a double purpose. When the pupæ were alive, I noticed that their eyes were constantly kept in a state of vibratory movement. Flexor and extensor muscles are attached at one end to the posterior margin of the basal segment, and at the other end to the second or main segment; other powerful muscles attached to this latter segment, are prolonged by ligaments into the disc. In Cryptophialus I observed that the disc-segment had a movement almost like that of the wrist. Whether any muscles enter the small terminal segment, I know not.

The drawing in Pl. 30, fig. 8, of part of the second segment, of the third or disc segment, and of the fourth or ultimate segment, in _Lepas australis_, is, I think, very accurate. The second segment articulates on the upper or dorsal surface of the disc, and has the articulation on one side constricted and formed of thin and flexible membrane; the little terminal segment, which is turned outwards at right angles, also, articulates on the disc. That the disc forms a true segment is shown clearly in Cryptophialus (Pl. 24, fig. 18), where the articulation with the second segment is not in such close contact. The disc is either circular, as in Lepas, or hoof-shaped, as in Ibla: in _B. balanoides_ the disc is rather hollowed out on the inner side. It has the power of adhering even to so smooth a substance as glass, placed vertically. It is surrounded by a rim of transparent membrane. On the hinder margin some spines arise from the central and more opaque part: in _Lepas australis_, there are no less than seven of these spines (fig. 8): in _Conchoderma virgata_ there are only four, in _Scalpellum_ and _Ibla_ only one. When the disc is placed on the surface of attachment, these spines lie parallel to it. The middle part of the disc is, almost always, nearly opaque; and in tracing the cement-ducts from within the body of the pupa, or of the young Cirripede, I in many cases traced them as far as this point, but here lost them. From this same obscure central part of the disc, in most, if not in all species of the Lepadidæ, spokes radiate, which sometimes are branched, and are not regular, not always even resembling each other on the opposite sides of the same individual. Round the proper membranous border of the disc, a second one may be observed (fig. 8, _p_), which differs in shape and extent in different specimens: under favorable circumstances, and very high powers, it may be seen to have a reticulated structure, and to be of a very pale brown colour; towards the exterior margin, the reticulations become finer, and are blended together and lost; on the inner margin, the substance forming this membrane may be seen to come out of the spokes. This substance is the cement, which has the power of adhering to whatever substance it grows against; and thus the disc of each antenna becomes cemented down, and soon both the antennæ are surrounded by a common border of cement, which gradually increases, after the metamorphosis, in extent. Occasionally the cement forms little projections, like short spines, on the edges opposite to the orifices of the spokes.

The small terminal segment usually bears on its truncated extremity six spines, some of which are occasionally hooked; in Scalpellum, two spines, rather longer than the others, are borne on a step some way down on the inner side of this segment; but in Lepas, two spines (fig. 8), very much longer than the others, arise from the outer corner of the extremity. These two are very different from the other four borne by this segment, or indeed any other spines on the body; for they are quite flexible, and are furnished with a double row of very long, straight, excessively fine hairs, which seem to be articulated on them--the whole presenting a very beautiful appearance. These spines are of considerable length, and in _Conchoderma virgata_ they even equal in length the whole antenna. I can hardly doubt that these beautiful, plumose, flexible spines, into the thick bases of which the coloured corium could sometimes be seen to enter, serve as feelers. Owing to the facts immediately to be mentioned, I erroneously stated, in my former volume, that there were three long spines.

In three species of Lepas, in _Dichelaspis Warwickii_, and in _Scalpellum Peronii_, after having torn the lately-cemented antennæ from the surface of attachment, I observed proceeding from the end of the terminal segment, from between the above two groups of spines, what appeared to be a long narrow ribbon with its end torn off; and which, in the case of _Lepas australis_, I fancied was one of the plumose, long, flexible spines ripped open. But now that I have examined some of the pupæ of this species before their attachment, I find (as represented in Pl. 30, fig. 8, _v′_) a flattened tube, ending in a blunt point, and having a peculiar ringed structure. I have noticed similar appendages to the antennæ of specimens just attached of _Lepas anatifera_. In the Dichelaspis and the Scalpellum, the tube was very long, and seemed, from its torn appearance, to have been firmly attached to the supporting surface. In both these cases, the tube came out from within another slightly larger tube, which had been broken off close to the extremity of the terminal segment of the antenna. In the case of the _Lepas anatifera_, the tube expanded a little after leaving the antenna. In the Dichelaspis, it had exactly the same diameter as the cement-duct, which could be clearly distinguished within the two lower segments. From these several facts, and from the peculiar appearance of the tube itself, I believe it to be a tube of cement-tissue which thus, sometimes even before the pupa is attached, independently grows outwards. That the cement-tissue can grow outwards and assume definite forms, we know from the singular case of _Lepas fascicularis_, in which the cement proceeding from several apertures, forms a vesicular float round the peduncle of not only a single individual, but often of a group of specimens: we shall presently find a somewhat analogous fact in the case of Coronula. It is possible that this tube, proceeding from the extremity of the antenna, may be the channel through which cement continues to be poured forth during the continued growth of the above Cirripedes; and the manner in which this is effected, considering how firmly the end of the peduncle is cemented down, has always appeared to me a difficulty. In those pupæ of _Lepas australis_, which I caught swimming about unattached, it is surprising that the disc should have been edged with cement, and that a tube, similarly formed, should have grown out of the ultimate segment: it shows, I presume, that the cement-tissue will grow out, whether or no the pupa has succeeded in finding a proper object for attachment. Lastly, I have felt some surprise, in two or three instances in observing some dark purple pigment-cells, like those in the corium, within the terminal tube of cement; and likewise within the spokes of cement in the disc: this is the only fact which causes me the least doubt, whether I have rightly determined the nature of the terminal tube, as wholly formed of cement tissue; or whether it may not be covered by an outer integument, itself lined by true corium, coloured purple.

Finally, I may add, that, excepting in small details, the prehensile antennæ present no difference throughout the Order: I have minutely examined them in several genera of the Lepadidæ; and in the Balanidæ, I have seen them in Coronula, and in several species of Balanus. In _B. balanoides_ I have examined them carefully; they are smaller and thicker than in Lepas, with the second or main segment bowed outwards, carrying its usual single spine; with the disc excised on its inner margin and apparently without the spoke-like vessels for the cement; and with the ultimate segment proportionably longer, and carrying, I believe, six spines, of which two appeared to be longer and more flexible than the other four shorter and somewhat hooked spines. In _Coronula balænaris_, also, the terminal segment is, proportionably to the others, of large size. Not only throughout the order, but throughout the whole Class, the antennæ are singularly uniform in structure, as will be seen, when the last two orders are described.

_Eyes._--These present no difference, except in size, throughout the class; and have been sufficiently described in my former volume. The true basal segments of the antennæ (incorrectly designated formerly as sternal plates or segments) are separated from each other by a deep fold; and are separated from the edges of the carapace on each side by a crest and slight fold (Pl. 30, fig. 7, _c_; and 4); these folds and crests die out posteriorly, and disappear. The hinder, rounded margins of the basal segments are inflected inwards, and their corners are produced far up into the body, thus forming the curious UU-like apodemes. These apodemes exist throughout the whole class; and the outer arms always carry the great compound eyes. I noticed, in _Lepas pectinata_, that the two middle arms are proportionably longer than in _L. australis_. Owing to the presence of these apodemes, and to certain coloured marks on the adjoining corium, the eyes, though enclosed fairly within the carapace, yet deceptively appear pedunculated, so that even J. Vaughan Thompson was thus deceived. I have already described the several muscles attached to these apodemes, and the constant vibratory movement of the eyes. Whilst the pupa remains a freely swimming animal, the eyes are included, not only within the shell or carapace, but (as would naturally happen) within the corium or true skin lining the carapace; but after the pupa has become attached, preparatory to its final metamorphosis (in the state represented at Pl. 30, fig. 2.), not only are the muscles, as before remarked, which are attached to the apodemes, absorbed, but so is the corium investing the apodemes and the immediately adjoining parts of the carapace. Hence it comes that the new corium of the young Cirripede within, is formed in a deep transverse fold across the whole lower half of the animal, and the apodemes with the eyes are thus, as it were, rejected from within the corium, though still remaining within the carapace. Consequently in this final stage, the eyes and apodemes are very conspicuous from the outside, being seen only through the transparent carapace. I presume that the eyes at this period have become functionless, with the optic nerve divided and absorbed. The eyes, apodemes, and carapace soon afterwards are all moulted together.

The eyes of Cirripedes certainly undergo a remarkable series of changes: in the larvæ in the first stage, there is a single eye, perhaps formed by the confluence of two eyes, occupying the normal position in the front of the head: in the second stage, according to Burmeister, the eye has become double, but the two are as yet simple; they are now situated posteriorly to the second pair of antennæ: in the third or pupal stage, they remain in the same situation, but have become compound, of great size, and are attached to the apodemes of the antennæ: in the mature and fourth stage, they have moved someway posteriorly, and again have become simple, of minute size, and are either confluent, as in the Lepadidæ, or tolerably far apart, as in the Balanidæ. It must not be supposed that the eye of the mature Cirripede is metamorphosed from the eye of the pupa, for such is not the case; the new eyes and old eyes being formed someway apart, and frequently both can be seen within the pupa (as in Alcippe, Pl. 23, fig. 16) at the same time. It is scarcely possible that the eye of the larva in the first stage, can be changed into the double eyes of the second stage; though these latter may possibly be multiplied into the eyes of the pupa, as both continue to occupy nearly the same position.[65]

[65] Zenker, in his 'Physiological Remarks on the Daphnidæ,'
('Journal of the Microscopical Society,' 1853, p. 274), speaks of
a "tripartite azygous eye" as common amongst Crustacea, and as
occurring "in conjunction with the aggregated eyes in Artemia,
Argulus, &c.; but as appearing regularly in all the Branchiopoda
and Siphonostomata as the _earliest_ visual organ." Hence I
conclude that this azygous eye is the homologue of that single eye
which appears in the earliest larval stage of Cirripedes; and that
the compound eyes of the cirripedial pupa, answer to the aggregated
eyes of Artemia and Argulus, &c., with the difference, that in
these latter genera the single eye is retained. See, also, Von
Siebold, 'Anatomie Comparée,' tom. i, p. 435.

_Mouth_, _thorax_, _limbs_, _abdomen_.--I have nothing to add regarding the mouth, except to confirm my former account; viz., that it is functionless, consisting merely of crests, which project inwardly between the gnathites of the young Cirripede, and of a shrivelled closed tube representing the œsophagus. In fact the mouth is a model of the outside of the mouth of the young Cirripede. I may remark that some little way beneath the membrane answering to the labrum, a pair of ligamentous apodemes, the use of which I do not know, slightly penetrate the body. The degree of prominence of the mouth varies, but it is far less than in the mature animal. On the limbs I have nothing particular to add: the drawing of the first pair of legs (Pl. 30, fig. 5) is, I think, very accurate: I observed all the spines here figured, on the corresponding leg of the pupa of _Balanus Hameri_. The five posterior pairs of legs differ only in the outer ramus having five plumose spines, instead of four, and one short simple spine at the exterior angle, making six altogether. The legs, in their natural position (fig. 2), have only the terminal segments of their two rami directed posteriorly; and as a consequence the spine (close to _i_ in fig. 5), borne on the penultimate segment of the outer ramus, is directed in the same line with that segment and with the pedicel, namely, anteriorly, and at right angles to the natatory plumose spines. This short spine acts, I imagine, as a defensive weapon; it has been omitted in fig. 2. Of the thorax I need not give, from my notes, any more details. The abdomen (fig. 6) is similarly constructed, as far as I have seen, throughout the order, with the exception of Alcippe (Pl. 23, fig. 17), in which it is composed of only one segment instead of three. In this genus the caudal appendages likewise consist of only one segment, with very short spines. In the pupa of _Balanus balanoides_, the three spines borne on each caudal appendage are very much more unequal in size than in the pupa of _Lepas australis_, although in the latter (fig. 6) the inner spine is considerably thicker than the two outer. Whether the three segments of which the abdomen is composed, are the three anterior or three posterior, of the normal seven segments, I know not: on the view that they are the three posterior segments, I presume, according to analogy, that the caudal appendages are borne on the penultimate segment, and that the ultimate segment is here quite aborted.

On the _internal viscera_ I have nothing to add. The cement-duct is represented in Pl. 30, fig. 2, _t′_, on the near side, running into the antennæ; and I repeatedly traced it, for the duct is very strong, as far as the disc segment; at the other end it joins the cement-gland (_t_) on the same side of the body. This cement-gland is proved, by the clearest series of facts, to be converted into the incipient ovaria and ovarian cæca. The cement-glands in the older pupæ could be traced as far as the cæca of the stomach, exactly where the ovaria lie in the mature animal; but in some young pupæ, they extended further posteriorly, past the mouth, between the outer and inner membranes of the overlapping carapace. I have faintly shown the course of the stomach, with its two cæca at the upper end; the anus lies between the caudal appendages, at the extremity (above _b′_) of the abdomen. At this age there is no trace of the vesiculæ seminales, so conspicuous in the mature Cirripede.

_Young Cirripede, whilst within the pupa._--I repeatedly succeeded in dissecting the young _Lepas australis_ out of the pupa; and by the previous action of boiling potash, and by a strong light, I was enabled to make a camera sketch (Pl. 30, fig. 2) of the relative positions of their several parts. The young Cirripede is drawn very faintly, and is best seen by holding the plate in the same position with the mature animal, of which a section is given in my volume on the Lepadidæ, Pl. 9, fig. 4. I may just notice how complicated are the membranes in a longitudinal section taken at this period: we have, 1st, beginning at the back, the horny tissue of the carapace or bivalve shell of the pupa; 2d, the primordial valve (_z_, in fig. 3) of the young Cirripede; 3d and 4th, two folds of corium; 5th, the membrane of the sack of the Cirripede; 6th, the membrane of the sack of the pupa; 7th, the outer tunic of the thorax of the pupa; 8th, the outer tunic of the thorax of the young Cirripede; 9th, the corium lining the latter membrane; and these nine membranes would be repeated on the opposite side of the section, if it were taken through either side of the shell or carapace, bordering the orifice.

After the exuviation of the outer membranes of the pupa, certain pre-existing coloured marks in the corium, such as those round the eyes and round the acoustic orifices, along the ridge of the back and on the borders of the orifice, &c., are still retained by the young Cirripede, either temporarily or permanently; so that the correspondence of part with part of the external surface admits of no doubt. Moreover, the three terminal segments of the antennæ are invariably retained by the young Cirripede, though in a functionless condition, and into them the outer membrane of the body, and an important organ, viz., the cement-ducts are still prolonged; hence these prolongations must be considered as aborted antennæ. Again, we have seen that the mouth of the young Cirripede is formed under the rudimentary mouth of the pupa, with the new œsophagus, round the old œsophagus, leading into the same alimentary canal. The twenty-four extreme tips, likewise, of the six pairs of biramous cirri of the Cirripede are formed within the twenty-four extremities of the six pairs of biramous, natatory legs of the pupa. Consequently, in the Cirripede and pupa, thus far, part corresponds with part, notwithstanding that new eyes are formed posteriorly to the old eyes, and new acoustic organs in a quite different position from the old ones; but now we come to a most important diversity in the metamorphosis, or rather to follow Professor Owen,[66] in the metagenesis, of the young Cirripede. Although, as just stated, the extremities of the cirri are formed within the legs of the pupa, yet, from the great length of the cirri, they occupy more than the whole of the thorax of the pupa; so that the thorax of the young Cirripede is not formed within the pre-existing thorax of the pupa, but within that part of the pupa, (homologically a portion of the first three cephalic segments), which lies anteriorly to the thorax. As a consequence of this, the pedicels and lower portions of the cirri, the segments of the thorax and its dorsal surface, all come to occupy a position at nearly right angles to that of the corresponding parts in the pupa: this is shown in Pl. 30, fig. 2. And as a further consequence, (and this is the more important point), the sack, which both in the young Cirripede and pupa is formed by the overhanging and produced portion of the carapace, and which is _internally lined by a reduplication of the membrane of the thorax_, is necessarily, owing to the changed position of the thorax, altered in extent and carried much further; namely, from extending merely parallel to the longitudinal axis of the pupa (from _b_ to _b′_), it is now in the young Cirripede, in addition, carried (to _s′_) almost quite across the inside of the animal. Hence it comes that the young Cirripede is, as I have said in my former volume, internally almost intersected; and its body remains attached only by a small space, (see the broken line, round _a_ and _b_ in Pl. 25, fig. 1, of a Balanus with the shell, &c., removed from one side), to the sternal or ventral, inner surface of the carapace,--the carapace being modified either into the capitulum and peduncle, or into the shell with its operculum and basis. As a still further consequence of this change of position of the body of the young Cirripede within the body of the pupa, the alimentary canal becomes shortened to fully half its former length. At the same time, the interspace between the mouth and first pair of legs of the pupa, (consisting of the seventh and eighth segments of the archetype), is quite lost in the Cirripede by coalescence. The final cause of the thorax of the young Cirripede not being developed within the thorax of the pupa, probably is, that the cirri may be formed of considerable length, so as to be immediately enabled to seize prey; and that the thorax, which supports the cirri (and this probably is even more important) should be as free as possible within the sack, so as to aid the captorial action of the cirri.

[66] 'Parthenogenesis,' pp. 13 and 26.

After these remarks, more especially with regard to the formation of the sack, if any one will look at the sectional drawing of a pedunculated Cirripede in my former volume, or of a sessile Cirripede (Pl. 25, fig. 1) in this present volume, in which latter the shell adds to the complexity, he will perceive the cause of the extreme difficulty in understanding the relative position of the parts throughout the whole class. Even after I had discovered that the prehensile antennæ of the pupa might always be found in the centre of the basis or surface of attachment, and which fact, it might have been thought, should have convinced me that this was the anterior end of the whole animal, yet still I fancied that the prominent mouth represented the entire head, and that the shell was something quite distinct. It is clear that others have been equally perplexed; for that which is the anterior end in the eyes of one naturalist, is the posterior end in the eyes of another; so with the dorsal and ventral surfaces: one naturalist considers the peduncle of the Lepas as the abdomen; another considers it as a pair of metamorphosed, thoracic limbs, &c.! The probable position of the segments of the body of a mature Cirripede, in relation to the three anterior cephalic segments, or carapace, is shown in the diagram (Pl. 25, fig. 6) of the supposed position of the mature Proteolepas within its pupal envelopes. Here, in the diagram, the two segments immediately succeeding the mouth (_c_), which are the seventh and eighth of the archetype, (for the mouth consists of three segments, and all in front of the mouth of three other segments), have come to adhere by their dorsal surfaces to the internal surface of the carapace,--that is, of the first three segments, which ought of course to have stood quite in advance of these two segments, and these two segments again ought to have stood in advance of the mouth. The mouth is directed posteriorly, instead of from the body; and the three segments of which it is formed (closed at their anterior end by the labrum), and are very small compared to the relatively monstrously great, three anterior cephalic segments, composing the carapace. To place the segments of the body of Proteolepas in proper sequence, in respect to those of the carapace, and in accordance with the sequence of the archetype Crustacean, it would be necessary, by seizing the extremity of the abdomen (_a_), to tear the two segments succeeding the mouth from their dorsal attachment, as far back as the basal margin of the labrum; and then pull them till they stood posteriorly to (or in the diagram, above) the mouth; which latter part would, by the same movement, be made to project out at right angles to the ventral surface, and would then be preceded only by the first three, great, confluent segments of the head, which being produced backwards, form the carapace. All that has just been said on the position, in Proteolepas, of the segments of the body in relation to those forming the carapace, I believe to be applicable to all ordinary Cirripedes, with this difference, that in the latter, after the metamorphosis, the two segments succeeding the mouth quite disappear on the ventral surface, and dorsally are either aborted or have coalesced with the adjoining segments.

_Act of Metamorphosis._

When the due time for the act of metamorphosis has arrived, the pupal carapace splits along the dorsal ridge, and is cast off, together with the acoustic sacks, the basal segments of the two antennæ, and the great, black, compound eyes, hanging to the UU-like apodemes. The three terminal segments of the antennæ invariably remain cemented to the surface of attachment. The exuviæ usually continue for a time united to the cemented antennæ, but are finally washed away. Besides the split along the dorsal ridge, the carapace separates, all round the orifice, from the delicate tunic lining the sack and investing the thorax and natatory legs of the pupa; for these membranes are not moulted for some considerable time afterwards. Hence all these inner parts retain for a period the appearance and structure of the natatory pupa, whilst the exterior resembles, in every respect, a fixed and perfect Cirripede.

In my former volume, I have insisted on the important and curious results which ensue from the eye-apodemes penetrating so deeply into the body (see Pl. 30, fig. 7, in which the proportions are more correct than in fig. 2), with the eyes attached exteriorly to their outer arms; for as these apodemes have to be ejected, the external membrane of the young Cirripede (Pl. 30, fig. 2) has to be formed in a deep fold or arch over them, and consequently the membrane on the sternal surface is formed considerably longer than on the dorsal surface. From this it follows, when all the membranes are free and are stretched fully out after the moult, that the whole animal, posteriorly to the cemented-down surface, turns vertically up, and assumes its normal position at right angles to the surface of attachment, and to that which it held in its pupal condition; for the pupa always adheres with its sternal surface parallel to the surface of attachment. A young Lepas, which has just moulted its pupal carapace, and has assumed its proper vertical position, with the cemented antennæ and the surface of attachment remaining as before, is shown at fig. 3, but is drawn on a smaller scale than the pupa fig. 2, out of which it may be supposed to have been excluded. In this fig. 3, it may be observed that the natatory legs and caudal appendages of the pupa have not as yet been moulted. The fact of the stretching out, in the young Cirripede, of the fold of membrane, which in pupa, just before the metamorphosis passes over the apodemes and eyes, is well shown by three darkly-coloured bands in the corium, which in the pupa are curled, but after the moult, are stretched straight out on the peduncle of the young Lepas.

The pupa, and consequently the young Cirripede, from being attached at first by the antennæ, does not adhere by the actual anterior extremity, but by the sternal surface near it; the anterior extremity, however, soon becomes cemented down, and afterwards, in ordinary cases, ceases to grow. In Cryptophialus, however, and in certain genera of the Lepadidæ, as Alcippe, Lithotrya, and Anelasma, the anterior or basal extremity does continue to grow, and is not cemented down, and therefore comes to be prolonged beyond the original point of attachment; in order to allow of this, the surface to which the Cirripede is attached has to yield, apparently simply to the pressure exerted in the case of Anelasma, but in the three other genera, to the rasping action of the roughened surface of the extremity of the peduncle.

When after a period the pupal membranes of the sack, thorax, and natatory legs are moulted, the cirri of the young Cirripede are curled up, and its thorax is raised towards the orifice, and we have the animal in its ordinary position, and perfect with the exception of a few parts to be further developed or modified. For, instead of calcareous valves, we have at this period only the so-called primordial valves, composed of chitine; and in the case of _Lepas australis_, some minute spines and some coloured marks on the peduncle, which soon disappear. The muscles, which enter the three terminal segments of the antennæ in the pupa, have to be absorbed and converted into ligamentous threads. In Lepas, the labrum has to become bullate; and the cæca have to increase in number round the upper end of the stomach, and their dark colour and that of the whole alimentary canal has to disappear or be much weakened. The filamentary appendages at the bases of the cirri, which generally contain some of the testes, have to be developed. The probosciformed penis, which at first equals only the pedicels of the posterior cirri in length, and is apparently imperforate, has to increase greatly in length. The testes and vesiculæ seminales have to be formed. And lastly, and this is a more important point, the two gut-formed cement-glands (or incipient ovaria, _t_, fig. 2, Pl. 30) which, at the period of the moulting of the carapace and eye-apodemes, and when the whole animal was upturned, came to occupy, together with the cement-ducts (_t′_), their normal position, _i. e._ nearly parallel to the sternal surface, now undergo further changes. Their upper and posterior ends lying near the cæca of the stomach, increase in size, but retain nearly the same character, and thus form the two true ovaria; their middle parts become emptied of their cellular contents, and are converted into the two simple ovarian tubes; and their lower ends branch out, inosculate, and form the inextricable mass of ovarian tubes and cæca. The points of junction on each side between the two cement-ducts and the newly branched ovarian tubes, become now developed into the two cement-glands. The cement-ducts, which continue throughout life growing, either still enter the old antennæ and there pour out the cement-tissue, or they pour it out through special orifices formed for this purpose in the lower part of the peduncle. The changes, supervening during the metamorphosis, in the ovaria and in the cementing apparatus, here described, I have no doubt are general throughout the Order.

I have above alluded to the _primordial valves_; these are beautiful objects when viewed under a high power: they are composed of chitine without a trace of calcareous matter, but prefigure in shape, size, and direction of growth, the shelly valves soon to be formed under and round them. They are composed of an outer membrane, with its margins separated by yellow thickened rims from the membrane uniting the several primordial valves together; and this outer membrane is underlaid by a single layer of generally hexagonal, thickish cells (Pl. 30, fig. 3 _a_), varying from 1 to 2/6000th of an inch in diameter. These cells seem to contain a nucleus; and they are at first separated from each other by clear interspaces. If a specimen be taken, only a little before the formation of the calcareous valves, one or more layers of membrane, marked by an hexagonal reticulation, can be separated from the lower surface of the main hexagonal network. It is a singular fact, that in those genera in which there are several valves, the primordial valves occur only on five, namely, on the two scuta, two terga, and the carina; and these are the most persistent valves in the several genera. The other valves are prefigured only by brownish membrane, without the hexagonal tissue. In the mature _Lepas_, the membrane connecting the several shelly valves is not moulted, but disintegrates; in the primordial valves, however, which stand far separate from each other, this membrane is moulted; and immediately after the first moult, the first layer of shell appears under and a little way beyond each primordial valve; shelly matter likewise appears, at least in some cases, between the cells of the hexagonal tissue. The young shelly valves are connected together, at each successive moult, by narrower strips of membrane, till, in the case of Lepas, the valves when mature come to touch each other (Lepadidæ, Pl. 1, fig. 5). The primordial valves are often preserved for a long time on the umbones, or centres of growth of the five valves, on which they occur, in the same manner as the larva-shell is sometimes preserved on the apex of certain spiral molluscs. Had not Cirripedes gone through so many and such complicated metamorphoses, this last state, when furnished only with primordial valves and with several internal organs only partially or not at all developed, would have deserved to have ranked as a special stage, and not as merely subordinate to the last or pupal condition.

In the Balanidæ, or sessile Cirripedes, the young animal, immediately after the metamorphosis, or still better if dissected out of the pupal carapace, as I succeeded in doing with _Balanus balanoides_, may be said to be pedunculated; for it is attached by a little disc of cement closely surrounding the antennæ, the rest of the membranous basis forming an almost semi-globular, flexible peduncle. The valves, at this the earliest period, are all membranous, and do not overlap each other. In the Balaninæ they do not present the peculiar structure of the primordial valves of the Lepadidæ; but in the Chthamalinæ, in Chthamalus, I saw traces of this structure. Calcareous valves are soon formed under the membranous valves. The opercular valves, at this early period, are much larger than the valves or compartments of the shell, which are only four in number, for the carino-lateral compartments are not yet formed. The compartments from the first are surprisingly strong, and have their alæ already formed and overlapped by the adjoining compartments; but of the radii there is as yet no trace. The four compartments form a narrow but nearly circular hoop, which, from its relatively large diameter, tends to draw down the upper or posterior end of the animal, now forming the opercular valves; and as the basis soon becomes throughout cemented to the surface of attachment, the young Cirripede is much depressed. Soon the opercular valves are drawn a little way down within the shell, becoming attached to the sheath, instead of, as at first, to the very summits of the compartments. In regard to the changes which take place in the shell, in the number of the segments in the cirri, and in the number of spines borne on these segments, &c., during the continued growth of the animal, as they are chiefly important for the identification of the species, I will here refer to a discussion on this subject under the head of the Genus Balanus.

_On the Homologies of the Carapace and Shelly Valves._

In the pupa, the carapace is produced, not only posteriorly, but anteriorly, so as to cover the entire animal, with the exception of a narrow sternal surface (Pl. 30, fig. 4): in front it is notched, where the sternal surface terminates, and from this notch a faint line runs along the dorsal surface, separating its tergal elements. In the young Cirripede, after the metamorphosis, there is no trace of this medial dorsal suture, or of the wider sternal surface. Looking at the several genera of the Lepadidæ, the external covering of the whole peduncle and capitulum is so continuous and of so uniform a nature, that I think we must consider the whole as a carapace, of which the sternal borders have become completely confluent; formerly I was inclined to look at the capitulum alone as formed by the carapace, and at the peduncle as being composed of the two or three anterior cephalic segments, cased only by their own integuments. As far as can be discerned, the carapace in the pupa, and consequently in the Cirripede, consists only of the tergal elements of the segments; and this seems likewise to be the case with the carapace of the Podophthalmia. Until lately,[67] Prof. Milne Edwards doubted whether the carapace in the higher Crustaceans (to which I believe the carapace of Cirripedes must be compared) was formed by the backward production of the third segment, which bears the second pair of antennæ, or of the fourth, _i. e._ the mandibular segment; but from the distribution of the nerves, he now argues that it must mainly belong to the third segment. In Cirripedes, the course of the nerves leads to the same conclusion; for the whole shell, sack, and peduncle are supplied with nerves proceeding from the compounded ganglion, which belongs to the second and third cephalic segments.[68]

[67] Compare 'Histoire Naturelle des Crustacés,' tom. i, p. 27,
with 'Annales des Sciences Nat.,' 3d series, tom. xvi, 1851, p. 233.

[68] This conclusion is supported by the structure of Proteolepas:
in this Cirripede there is not a vestige of a carapace, and as
the whole of the animal in front of the mouth is almost utterly
aborted, being reduced to a mere covering to the two cement-ducts,
and as, on the other hand, the mouth with the mandibles, though
peculiarly modified, is not at all aborted, there is a strong
probability, that the abortion of the carapace is connected with
the aborted state of the three anterior cephalic segments; and that
the carapace in its origin is not any way related to the fourth or
mandibular segment.

The whole of the head in front of the mouth, together with the carapace, is, as we know, formed of three segments; and each of these segments, homologically, ought to consist of eight elements; I recall to mind these facts, inasmuch as the transverse separation between the peduncle and capitulum in the Lepadidæ, and between the basis, the shell, and the opercular valves in the Balanidæ, might be thought to be connected with the separation of the three cephalic segments. So again, as in the Balanidæ the shell normally consists of eight compartments, these might be thought to be related to the eight elements of one or other of the three segments. But I see no reason for admitting this view; and in the case of the carina and rostrum, two of the most persistent and important of the compartments, they exactly cover the sutures which ought to separate the two tergal and two sternal elements of the segment. The valves, moreover, often form many more whorls than three, or the number of the true cephalic segments in front of the mouth; and in each whorl the valves tend to stand in tile-like or alternate order, with respect to those in the whorls both above and below, which would not be the case, if they were the eight elements of the segments.

For the true homologies of the sclerodermic plates, or Shelly valves, with which the external covering of Cirripedes is so generally strengthened, we must, I believe, look to the carapace of the Podophthalmia. In these latter, we find the carapace composed of sclerodermic plates, which, though closely joined and only occasionally separated by sutures, yet in their origin are distinct;[69] they tend, also, to be arranged in alternate or tile-like order. As the animal grows, the old sclerodermic plates, all joined together, are moulted, and new ones, also all joined together, of a larger size, are formed beneath. Now let us imagine the growth to be more gradual but yet periodical, and the new and larger sclerodermic plates, when formed under the old ones, to adhere firmly to them; the older plates would thus be prevented from becoming confluent, and instead of being all moulted together, as is now the case, they would be almost continually separated from each other, owing to the almost continuous increase in size of the new underlying plates. Consequently lines of splitting would run between the several plates, however numerous they might be, instead of there being, as now, a single line of splitting extending down the back. In fact, we should have the identical manner of growth of the shell or carapace, which occurs in Cirripedes. It is on this ground, and from the several points of homological resemblance incidentally mentioned in the last few paragraphs, that, in the early part of this Introduction (p. 13), when discussing the whole class, I stated that I believed that the carapace of Cirripedes presented more real resemblance with the carapaces of the Podophthalmia, or higher Crustacea, than with those of the lower Crustacea, though in mere shape they more nearly resembled the latter.

[69] 'Annales des Sciences Naturelles,' 3d series, tom. xvi, pp.
233, 236, 237.

_Cementing Apparatus._ (Plate 28.)

I have already (p. 128) given an account of the manner in which, in the pupa of Lepas, the cement-tissue escapes from the prehensile antennæ, and of the structure of the cement-ducts, and of the cement-glands or incipient ovaria; and likewise of the changes by which these organs assume their ultimate form in the mature Cirripede. In my former volume, on the Lepadidæ, I described the cement-glands and the cement-tissue in several genera, and I have there shown (singular as the fact is) that the two cement-glands, with their contents, actually consist of ovarian tubes with their contents (for there seemed to be a relation in the state of fulness in both) in a modified condition. In the Balanidæ, I am not able, from the difficulty of the dissection, to confirm these conclusions, excepting in so far that the tubes on which the cement-glands are formed, run into the mass of ovarian cæca; but, I may add, that in the abnormal Proteolepas, belonging to another Order (see the section, Pl. 24, fig. 1), nothing could be plainer than that the membrane of the ovarian sack (_b_) formed the cement-ducts, and that their cellular contents, which within the sack (_a_) were in process of conversion into ova, within the ducts were converted into the cement-tissue. This cement, by some unknown power, travels down the ducts, and debouches at the antennæ.

In the Lepadidæ, there are only two cement-glands, which are situated high up in the midst of the ovarian cæca, with one cement-duct proceeding from each: both the glands and ducts increase in size with the age of the animal:[70] the cement issues either permanently from the prehensile antenna, or, after a short period, through apertures in the peduncle, arranged irregularly or in straight lines,--the last formed apertures being furthest from the central and basal point of the peduncle. In the Balaninæ, on the other hand, at each period of growth, a pair of new cement-glands is developed, larger than those last formed, and making, with the older glands, a chain, connected together by what I have called the cement-trunk. The cement-trunk consists of a tube, which generally becomes enlarged just before entering each gland. The glands, the cement-trunk, and cement-ducts, all adhere to the basal membrane or basal shelly plate. Each gland gives rise to two cement-ducts, these often bifurcate, and sometimes repeatedly bifurcate and inosculate before pouring out their contents round the circumference of the basis; and sometimes they all first enter a circumferential cement-duct. The probable cause of the greater complexity of the cementing apparatus and of the greater number of the excretory orifices in the Balanidæ, compared with the Lepadidæ, is that the entire surface of the broad basis, which answers to the whole peduncle in the Lepadidæ, is firmly cemented down to the supporting object, instead of merely the basal end of the peduncle. The cement issues either in a cellular condition, or more commonly as a fine network, which, at a short distance from the orifices (Pl. 28, fig. 4 _a_, _z_), becomes so fine as to form a sheet or layer: I may here recall the fact, that in the cement proceeding from the disc of the antennæ, in some species of Lepas, a similar structure was observed. The cement itself presents the same transparent, brown, laminated, structureless appearance, and the same chemical reaction, as described in my former volume. The cement has the capacity of occupying and filling up all inequalities in the supporting surface; I have seen it, when spread over an encrusting Flustra, present an exact model of every cell; in the case of Coronula, it seems, as we shall immediately see, to have the power of penetrating into, and even almost blending with the epidermis of the supporting Cetacean. The last-formed cement-glands and cement-ducts present a delicate and transparent appearance, and contain cellular matter; whereas the old cement-glands, and sometimes the old cement-ducts, are filled with brownish cement, not acted on by boiling potash. The foregoing remarks are confined to the sub-family Balaninæ, for I have not been able to examine thoroughly the Chthamalinæ, and can only affirm, that in Chthamalus and Pachylasma the cement-ducts repeatedly bifurcate and inosculate, in the same manner as in the Balaninæ. I will now proceed to describe, in some detail, the cementing apparatus in the several following genera.

[70] I had some slight reason to suspect in Pollicipes that new
cement-glands were successively formed: this is more probable in
this genus than in the others, inasmuch as it is the most nearly
related to the Balanidæ.

_Coronula._--The cementing apparatus is here more simple than in any other genus of the Balaninæ, and I have studied it more carefully. The basal membrane of _Coronula balænaris_ is figured in Pl. 28, fig. 1 _a_, and must first be described; its relation to the shell will hardly be understood without looking at the outline of the folded walls of this species, in Pl. 16, fig. 5. The basal membrane closes the central circular hollow, and is continuous with rays (not represented in Pl. 28) extending under the doubled walls and terminal transverse loops. It has eighteen concave sides, corresponding with the inner ends of the folded walls, for each of the six compartments is trebly folded. The membrane consists of successive, conformable slips (_c′_, _c′_), bordered exteriorly by thickened yellowish rims, and internally overlapping (when viewed from the inner side) the few last-formed slips, and then thinning out. The membrane forming each slip is itself laminated. The middle portion, about 1/50th of an inch in diameter, is rather opaque, owing to the slips being so close together. Beyond this central part, the slips suddenly increase in size, but yet have a different shape from the 18-sided outline, which they ultimately assume: this difference is owing to the great changes in shape, as explained under the genus Coronula, which the shell undergoes, when the walls at first assume their folded structure. The walls are invested by longitudinally striated membrane (_p_, _p_, _p_, fig. 1 _a_), which turns in under their basal edges; and this membrane is united with the basal membrane, by what I shall call the circumferential slip (_b_), and which is shaded in fig. 1 _a_, simply for the sake of catching the eye. It is the circumferential slip of membrane which sends rays under the spoke-like folded walls: thin as it is, this slip is yet laminated, but is not bordered by thickened edges. The membrane investing the walls is, like the basal membrane, formed of successive slips with thickened edges, which overlapping (viewed from the inside) the last-formed slips, project beyond them, and so face the edges of the slips in the basal membrane; they are only obscurely indicated in fig. 1 _a_. The circumferential slip (_b_) lies over (as viewed from within) both the basal and wall membrane. This whole structure will, perhaps, be best understood by the sectional diagram (fig. 1 _b_), in which the letters (_c′_, _c′_) show the slips of basal membrane; (_p_) the parietal membrane, coating the outside surface of the walls of the shell, not here represented; (_b_) the circumferential slip overlying both; and (_z_, _z_) the layers of cement, which may for the present be disregarded. In order to allow, of the growth of the shell, the circumferential slip (_b_) periodically splits in the middle, all round, in a line exactly conformable to the edge of the last-formed slip of basal membrane; and likewise in straight, medial lines under the spoke-like (cut off in fig. 1 _a_) doubled walls. I have seen, under a high power, the line of splitting, very shortly after its formation, with the two edges ragged and near together, with an extremely narrow, new circumferential slip just formed, between and over the edges of the previously formed slip. What causes the circumferential slip to split so symmetrically, I cannot say: the opposed edges, after a time, become thickened, apparently from adhering to the underlying layer of cement, as will presently be described. The circumferential slip continues increasing in breadth till the period of its splitting arrives, by which time it has become much broader than the last-formed slip of basal membrane; and after the splitting takes place, the interior half towards the basal membrane, forms a new basal slip all round the basis, and the exterior half adds a new slip to the membrane investing the walls. This latter membrane being inflected under the basal edges of the walls, is, during the growth of their edges, drawn straight down, the newly-formed portion taking the inflected position.

In the sectional diagram, (1 _b_) the circumferential slip is not yet broad enough to split; when it has become so, it will split under the letter (_b_). The slips of basal membrane are, as may be seen in fig. 1 _a_, narrower towards the circumference; but the two or three last-formed slips, are out of proportion narrower than the others; and it is certain, from the comparison of the basal membranes of specimens of different ages, that these will afterwards increase in width.[71] I have seen no other instance, in Cirripedes, of growth in membranes, except at their extreme margins: I suspect that these last-formed slips are pulled, during the downward and outward growth of the shell, a little from over the last-formed slips, new and larger laminæ being all the time thrown down, so as to prevent any fissure being formed. I also suspect that the gradual increase in width of the circumferential slip itself, is due to the opposed edges of the underlying and last-formed circumferential slip being dragged further apart from each other, new and wider laminæ of membrane being continually thrown down; the new circumferential slip being thus, also, all the time thickened, as well as rendered broader.

[71] In the case of one _young_ shell, I found that the
previously-formed circumferential slip must have split, long before
it had assumed its proper and ordinary width; for the last-formed
slip of basal membrane was of extreme narrowness, and would have
to be considerably added to in width, whilst the new and narrow
circumferential slip was likewise being added to in width. This
slip of basal membrane, though so extremely narrow, had its own
cement-ducts and glands.