A Monograph on the Sub-class Cirripedia · Charles Darwin
TABLE OF CONTENTS.
_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]
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.
Ideas discussed in this chapter
Text from Project Gutenberg, public domain in the United States.