Chapter XXXI: Introduction (27)

The fissure leading into the cavity is required to be broad at the posterior end, in order that the cirri may be there freely exserted out of the sack; and narrow in other parts, to prevent, as it would appear, anything injurious getting in between the animal's body and the cavity in the shell of the mollusc. As the fissure is increased in length by attrition at the broad posterior end, which end during growth becomes broader and broader, the lower part of the fissure has to be narrowed, and this is effected in a very singular manner, namely, by advantage being taken of the strong tendency, which triturated shell with animal matter, has to set into a solid shelly mass, although constantly agitated.[147] Mr. Hancock noticed this edging of hard shelly matter, and naturally thought it was a secretion. Lines of deposition (Pl. 22, fig. 4, _b_), parallel to the edges of the furrow can often be perceived in it: its thickness and extension vary much: I have seen it on one side alone of the orifice: it is, of course, never found at the broad end where the process of enlargement goes on. The peculiar worn surface with which it irregularly thins away downwards, on the sides of the cavity, made me (together with the apparent impossibility of such a secretion proceeding from an animal wholly invested by a chitine membrane) suspect it to be inorganic; and this view is certainly correct, for when a fragment is dissolved in acid, a considerable residuum is left of bits of membrane, rubbish, and, in one instance, even of the remnants of a foreign animal, apparently an annelid. We have here all the circumstances favorable for inorganic deposits of this nature, namely, finely triturated shell and chitine or animal matter, produced by the excavation of the chamber, sea-water, and movement.

[147] I have given some remarkable cases in my volume on 'Volcanic
Islands,' (p. 49), in which limestone, having almost the hardness
and specific gravity of marble, has been thus deposited. Almost
every coral-reef offers similar examples. The curious substance
described by Mr. Horner and Sir David Brewster, ('Philosoph.
Transact.,' 1836, p. 65), which is formed during the manufactory of
cloth, offers another example of the strong tendency which lime and
animal matter have to unite. Lately, Dr. Horsford, in 'Silliman's
North American Journal,' Jan. 1853, has discussed the chemical
theory in an analogous case on the coast of Florida; he attributes
the aggregation to the formation of a hydrate of lime through
the action of the animal matter. Mr. G. B. Sowerby, Junr., has
described a case very analogous to that of Alcippe, ('Proceedings
of Zoolog. Soc., Mollusca,' Pl. 5, fig. 4, p. 162, 1850), namely,
that of _Pholas calva_, in which a tube is formed of _inorganic_
calcareous matter, serving to narrow the entrance.

From the manner of growth of the animal, the fissure leading into the cavity in the shell becomes much longer than the orifice leading into the sack, and to prevent the body being unnecessarily exposed, the upward projection of the disc, already described, is formed under the narrow and disused end of the fissure; moreover, the two rims of the inorganic calcareous deposit sometimes here approach so closely, as almost or actually to touch each other; and between them, as remarked by Mr. Hancock, there is usually a little accumulation of grains of sand. This narrow end of the fissure is generally curled either to the right or left hand; and I can only account for this fact by supposing that, whilst the cirripede is young, and has not a large horny disc attached to the cavity, it cannot keep its body straight during the long-continued boring process.

The animal is attached by its horny disc to the thin shelly roof over the peduncle, and likewise to the under side of the narrow end of the fissure, but is elsewhere quite free. I carefully examined the disc in many specimens, but could not see any cement-ducts: I believe I saw layers of cement at the upper end of the disc, but it is not easy to discriminate between this substance and the yellowish, somewhat disintegrated, layers of the horny disc. The pupa certainly becomes attached by ordinary cement, so that the attachment in early life, at least, is normal. In some full-grown specimens, I found the lower parts of the horny disc attached, along the edges of the layers, to the roof of shell; and as I looked here in vain with the highest powers for cement-ducts, or for cement, it appears to me probable that the rough edges of these layers were united to the roof by a thin layer of the inorganic calcareous deposit. The animal, from its very protected situation, certainly requires to be less firmly cemented than other cirripedes; and even in Lithotrya, which is less deeply imbedded than Alcippe, the cementing apparatus was feebly developed. From the length of the pupal antennæ, cemented by their terminal segments, the position of the young cirripede (Pl. 22, fig. 12) can be changed to a considerable extent, like a ship swinging at her moorings, but in order to assume its final position, the animal must, I think, travel like Lithotrya, but to a much less extent, by a short succession of overlapping horny discs,--the old discs being partially deserted, each new one extending beyond the last-formed one: even in the case of the mature animal, we have seen that, under certain circumstances, it changes, to a certain extent, its position; portions of the old disc being deserted and attached to the roof of a deserted portion of the cavity.

_Affinities._--In the preliminary remarks under the Family, I have discussed this subject almost sufficiently: I will here only remark, that the genus, though so abnormal, yet stands naturally between Ibla and Anelasma, having clear affinities, on the one side, through and beyond Anelasma to Alepas; and on the other side, beyond Ibla to Scalpellum, and so to Lithotrya. Moreover, it is very distinctly related to Cryptophialus in the succeeding Order.

MALE. Pl. 23.

On every specimen of the female Alcippe, which I carefully examined, I found some minute parasites (or epizoons) attached to the lateral edges of the upper part of the horny disc, and therefore lying within the narrow end of the fissure leading into the chamber excavated in the shell of the Buccinum. Although having had some experience in the very anomalous forms which male cirripedes assume, yet when I first casually inspected these parasites under a weak lens, from their transparency, their elongated and lobed body, including an internal folded up organ, I actually threw them away, thinking that they were probably Bryozoa. Subsequently, a more careful inspection immediately showed the cemented prehensile antennæ, and their cirripedial nature was demonstrated. I soon found specimens with the perfect still adherent exuviæ of the locomotive pupa, undistinguishable from the pupa already described as probably belonging to the female Alcippe. But as this latter fact, may perhaps be doubted, I must show that there is other evidence sufficient to prove that these cirripedial parasites are the males of the female Alcippe. Of the females, I inspected many specimens, and all certainly were without external male organs; and in the four or five specimens which I rigidly examined, there were no testes or vesiculæ seminales, the latter being in all hermaphrodite and male cirripedes so conspicuous. On the other hand, I examined at least thirty specimens of the parasite, and they were all exclusively males, for all had a probosciformed penis, and the greater number had their vesiculæ seminales filled with spermatozoa, and hence were ready to perform the act of impregnation, but undoubtedly they contained no ova. It would, then, be very strange, if these two cirripedes of opposite sexes, thus attached together, were not sexually related. Wonderfully different as the parasite is from the female Alcippe, yet, in one very important character it is related to Alcippe, and to no other member of the Family, namely, in the sack extending down to the extreme lower point of the peduncle; the male organs, I may add, occupying an analogous position with the peculiar position of the female organs in Alcippe. The lateral lobes of the peduncle in the parasite seem to represent the sides of the broad depressed peduncle in Alcippe; and in both the peduncle grows at its lower end--a very rare circumstance--observed only in two genera in this Family, namely, in Anelasma, and in a slight degree in Lithotrya. Besides these points of resemblance between Alcippe and its parasite, which are striking, considering their external utter dissemblance, the affinities of both point, judging from certain small characters, in the same direction, namely, towards Ibla and Alepas. Finally, then, I think, we may confidently admit that this parasite or epizoon is the male of the female Alcippe: indeed, considering the facts given in my former volume, on Ibla and Scalpellum, I have, perhaps, here discussed the question at unnecessary length.

The males are generally attached, as already stated, to the two hollowed out sides of the upward prolongation of the horny disc; they adhere by means of little patches of cement, proceeding from the terminal segments of their antennæ, to the overlapping edges of the few later-formed zones of the disc; hence, they lie protected, within the narrow end and a little under the edges of the fissure leading into the cavity in which the female is lodged. In some specimens, however, the males are attached rather lower down on the disc, and are not confined exclusively to its upper margin, so that they live fairly under the roof of shell which covers the main part of the disc: but they are never attached very low down, so as to lie far from the lower end of the orifice leading into the sack of the female. I have two or three times seen as many as three males on each side, but sometimes there is only one on each side, or none on one side. A large distorted specimen actually had twelve males, and two pupæ on the point of undergoing their final metamorphosis, all fourteen attached on one side, and all evidently must have been alive together! Another specimen had nearly the same number, a few on one side, and the rest on the other side.

The male immediately, after the exuviation of the pupal carapace, 25/1000th of an inch in length, is only 23/1000th of an inch long, but ultimately it becomes, chiefly from the growth of the lower end of the peduncle, nearly twice this length; for the largest specimen which I have seen, that figured, was 45/1000th of an inch long (_i. e._ under 1/20th of an inch), and 1/100th of an inch in breadth across the peduncle, beneath the lateral lobes. The whole external membrane of the animal (as well as the internal membrane of the sack), is very thin, quite structureless, and as transparent as glass; hence, even the spermatozoa, within the vesicula seminalis, can be seen from the outside. The whole structure of the animal is very simple. The ventral surface can be at once recognised by the attachment of the antennæ (fig. 19, _a_), and these organs mark the point which was the anterior end of the male, just at the period of its metamorphosis, and before the lower end of the peduncle had grown. These antennæ have already been fully described; they are conspicuous from being composed of membrane, rather thicker than that investing the body of the male, and which external membrane can be traced entering these organs, and appearing like cement-ducts; but within these tubular prolongations of the outer membrane, I could obscurely see the real cement-ducts.

The part answering to the capitulum is much flattened and elongated; it widens but little from the upper to the lower end, where it blends with the carinal or dorsal surface (the under surface in fig. 19) of the lobed peduncle. At the upper end there is a small orifice, and close to this, on the ventral or rostral side, there is a thin, apparently double projection (_i_, fig. 19) or flap of membrane, one flap lying exactly over the other. The whole length of this capitulum probably corresponds with that small portion of the capitulum in the female, between the upward prolongation of the horny disc and the lower end of the orifice; and the two broad flattened projections in the male, probably answer to the two sharp narrow points (_a_, fig. 1, Pl. 22) in the female. The peduncle has two lateral lobes (_h_, _g_, fig. 19), and, whilst young, what may be called a third and medial lobe, but this soon increases largely by growth, and forms the main part of the peduncle. The lateral lobes are intimately connected with the ventral surface; they tend to lie in a plane, at right angles to the compressed capitulum, but owing to the excessive thinness and flexibility of the whole external membrane, it is difficult to ascertain the relative position of the different parts. Moreover, owing to the pupa being so much flattened, these lobes are necessarily formed folded up; and, I believe, it depends on the position, with respect to surrounding objects, which the male ultimately holds, whether the lobes ever assume, their apparently normal position, in a plane at right angles to the sides of the pupa; owing, also, to the form of the pupa, the two lobes seem generally to be actually formed of unequal sizes, that formed in the dorsal region of the pupa being the largest. I believe that these lobes correspond with the lateral margins of the upper end of the peduncle of the female, which margins project laterally beyond the sides of the capitulum. The lower lobe, or end of the peduncle, is depressed in the same plane with the lobes; it is of variable length; when first formed it hardly extends beyond the basal articulation of the prehensile antennæ. Commonly it does not lie quite in a straight line of the capitulum; and I have seen specimens in which it stood at nearly right angles to the capitulum and to what was the ventral surface of the pupa; this irregularity in the relative position and sizes of the different parts of the peduncle, no doubt, to a considerable extent, depends on the form of surface to which the male becomes attached, just in the same way as we have seen that the peduncle of the female becomes altered in shape during the excavation of the chamber in which it is lodged.

I feel some difficulty on one point: in the pupa the single eye of the future male can be clearly distinguished, and it lies some way from the anterior end of the body; but in two males, which certainly had just moulted, and in which none of the internal organs were as yet developed, the eye lay close to the anterior end, directly over the basal articulation of the antennæ. I suspect this is somehow caused by the great change of form which supervenes, during the metamorphosis, at this anterior end of the body; the extremely compressed body of the pupa having to become depressed and lobed in the young male. I have given a figure of a young male, just as it appeared (Pl. 23, fig. 18), somewhat distorted from lying on a flat surface; _c_, being the eye.

The sack extends, in a very remarkable manner, down to the lower end of the peduncle, the whole inside of the animal being thus freely open to the water. In the upper part, the sack forms a mere narrow tube; it does not appear to have been formed in the same manner as in all other cirripedes, namely, surrounding the thorax and natatory legs of the pupa, but in an abnormal position, along the dorsal surface, above the sack and thorax of the pupa: a transparent line, where the new narrow sack is in process of formation, is the first indication of the coming metamorphosis. The sack in the capitulum of the male is not central, but lies near the dorsal surface; the ventral interspace, between the outside and the sack, is occupied by oblique fibres (_l_, fig. 19), which may be striæ-less muscles, but I suspect are ligamentous fibres, giving support to the whole projecting capitulum. These fibres enter a little way within the lobed peduncle; they are probably homologous with the strong muscles, which run from beneath the upper end of the horny disc of the female to the lower end of the orifice leading into the sack. Round the lobed peduncle,[148] there are two bands (_e_, _f_) of thin muscular fasciæ, slightly oblique to each other, and attached at the ends to the outer membrane; they are evidently homologous with the external transverse muscles, which are best developed round the same part in the female. Some of these muscles present a singular chain-like appearance, from being strangled at intervals: they act probably in aiding the long probosciformed penis to protrude itself out of the sack. I could not detect any longitudinal muscles, and the lower part of the peduncle seems destitute of muscles of any kind.

[148] I believe I saw in one specimen, most delicate transverse
muscular fibres round the lower part of the elongated capitulum.

The internal structure of the animal is very simple. Within the lower end of the peduncle there is a dark purple eye (_c_), under the 1/1000th of an inch in diameter, a testis (_d_) and a (_b_) vesicula seminalis. These organs falsely appear as if suspended in the middle of the peduncle, but they are really attached, I believe within a separate partition, to the ventral surface, occupying the same position as the mass of ovarian cæca in the female. The eye lies on the line of junction between the testis and the vesicula seminalis, and on their ventral side. The testis is rounded, and consists of a mass of cells, on an average 1/5000th of an inch in diameter. The vesicula seminalis varies extremely in condition, being either a mere rather broad vessel, enlarged where it joins the testis, or a bag fully as large as the testis itself, and distended with spermatozoa, all arranged parallel to its longer axis. There was an evident relation between the size of the vesicula seminalis and that of the testis, the number of the cells in the latter decreasing as the mass of the spermatozoa increased: there was also an evident relation between the age of the male and the state of these organs; younger and more opaque individuals, having their testes of large size; and older specimens, with the lower end of the peduncle arrived at its full dimensions, having the vesicula distended. Some few old specimens had evidently discharged their spermatozoa. By dissection I more than once distinctly traced the vesicula seminalis entering the broad lower end of the penis. The membrane, forming the vesicula, is ringed, and I presume is, as in other cirripedes, contractile, so as to expel the spermatozoa. The probosciformed penis (_m_) is of extraordinary length: it is plainly ringed, or rather articulated, in this respect resembling that organ in Ibla and Alepas; it tapers gradually, and terminates (as usual) with a brush of fine bristles; it is furnished with delicate voluntary muscles, arising from the body round its basis, and extending no doubt up to the apex, but too fine to be traced all the way. Its broad lower end is attached in a slight depression, on the ventral side of the sack, a little above the point of attachment of the pupal antennæ. According to all analogy, the spot whence the penis springs must be considered as representing the thorax and abdomen; and the outer membrane of the penis is here, as on this view it should be, reflexed and is continuous with that lining the sack. Ordinarily the penis lies coiled up in complicated folds, appearing like a large intestinal worm, and fills the lobed part of the peduncle, which apparently serves for no other purpose than its reception. In one case in which I dissected out the penis, I found it in its contracted state; 41/1000th of an inch in length, equal to that of the entire capitulum and peduncle; in a specimen, in which the penis had been naturally exserted, the part which protruded (_m_) was by itself rather longer than the whole animal; and as this specimen had been placed in spirits of wine, the organ no doubt was contracted; hence I think it probable that the probosciformed penis, when fully stretched out, would equal twice the length of the entire animal.

There must be a nervous system; and there must likewise be a gland (homologous with the ovaria) for secreting the cement; but I could not distinguish parts so small. Certainly there is no mouth, or stomach, or thorax, or limbs of any kind, or abdomen.

It is obvious that these males must be very short-lived: they perform their masculine functions and then perish. We have seen, however, that after the act of metamorphosis they do grow a little, and I have reason to suspect that this is effected, as with other Cirripedes, by moulting. The growth must be absolutely dependent on the store of nutriment laid up within the pupa. The young male, immediately after the exuviation of the integuments, thorax, natatory legs, abdomen, and eyes of the pupa, consists of a pulpy cellular mass, without any internal organs as yet formed.

Judging from the different sizes of the females which included perfectly developed ova, I infer that they must breed more than once during their lives; and therefore, that successive sets of males, as in the genus Scalpellum, must become attached to them. I was not, however, able to discover the prehensile antennæ or other remains of the old males adherent to the females; a circumstance which I presume is accounted for by their attachment being weak. Considering the very small size of the male, it is not surprising that so many,--in one case fourteen,--are required to impregnate the numerous ova of a single female. How the males know the proper period when the ova, lying in a sheet at the very base of the sack of the female, are ready for impregnation, I cannot say, without it be that they perceive the moulting of the external membrane, close to the edge of which they are attached; for this moulting would indicate the period when the ovigerous lamella came to the surface of the sack, and the ova would then be soon ready for impregnation. From the position in which the males are attached, and from the extraordinary length of the probosciformed penis, capable of voluntary movements, I have no doubt the males can insert the tip of this organ within the lower edge of the orifice of the sack, and there discharge the spermatozoa, which, by their own movements, must pass down the sides of the sack of the female till they reach their proper destination. The position of the males, with respect to the female's body, is almost exactly the same as that occupied by the complemental males of _Scalpellum Peronii_ and _villosum_; the lower and narrow end of the fissure, worn in the gasteropod shell, here affording that protection to the males, which the edges of the opposed scuta afford to the _complemental_ males of the above two species of Scalpellum. We cannot doubt that these latter males aid in the impregnation of the ova of the hermaphrodites, but they are not furnished with a very long penis, probably for the very reason that they are _complemental_ males, and therefore not so absolutely necessary for the impregnation of the ova as are the males of Alcippe.

I have, in my former volume, expressed my astonishment at the extent to which abortion had been carried in the male Ibla; but it has been carried much further in the male Alcippe. In Ibla, the thorax is reduced to a mere flap, and only two pairs of cirri exist in a most useless and rudimentary state, but there is a well organised mouth, stomach, and anus. In the males of _Scalpellum vulgare_, _ornatum_, and _rutilum_ there is no mouth or stomach, but there is a thorax with four pairs of minute, modified cirri, and a large abdominal lobe. Here, in the male Alcippe, all these negatives are united, we have no mouth, no stomach, no thorax, no cirri, no abdomen! The archetype crustacean consists of twenty-one segments; of these the seventeen anterior segments can be clearly made out in the archetype Cirripede: now, in the male Alcippe, the first three segments are largely developed, forming all that is externally visible, but the remaining fourteen segments are absolutely aborted, but in idea may be considered as forming the membranous depression whence the probosciformed penis springs; for this organ normally arises at the extremity of the seventeenth segment. To show the wonderful diversity of nature, even in the same sub-class, I may be permitted to remark, that whilst in Alcippe only the three anterior segments are developed, the fourteen succeeding segments being rudimentary, in Proteolepas (hereafter to be described) these fourteen segments are all largely developed, whilst the three anterior segments are quite aborted, being represented only by a thin envelope to the two threads by which this Cirripede is attached to the supporting object.[149]

[149] It may be worth stating, that in order to procure perfect
specimens of the female and male Alcippe, pieces of the shell
inhabited by them should be dissolved in weak acids.

ORDER II.--ABDOMINALIA.

_Cirripedia, having a flask-shaped carapace; body consisting of one cephalic, seven thoracic, and three abdominal segments; the latter bearing three pairs of cirri; the thoracic segments without limbs; mouth with the labrum greatly produced, and capable of independent movements; œsophagus armed with teeth at its lower end: larva, firstly egg-like, without external limbs or an eye; lastly binocular, without thoracic legs, but with abdominal appendages._

I feel compelled to form an Order for the one genus and species, namely, _Cryptophialus minutus_, to be here described. We must, I conceive, attribute much greater value, in classification, to internal parts and organs, at least where such are not known to vary, than to external structure. Now in Cryptophialus, the body consists of eight segments, of which the first two are not developed in any cirripede hitherto described. Of the eight, the seven posterior or thoracic segments are quite free, or detached from the carapace, and do not bear any appendages; whereas in all the foregoing cirripedes of the order Thoracica there are (at least in the normal sex) six pairs of cirri; Alcippe alone must be excepted, in which there are only two pairs. Again, in the Thoracica there are no abdominal appendages, excepting the terminal or caudal, whereas in Cryptophialus the abdomen bears three pairs of biramous cirri. In the crest of the labrum, being produced into a special, lancet-formed organ, articulated at its base and capable of movement, and in the palpi projecting straight upwards, we have a great difference from all other cirripedes; and these organs, we have every reason to believe, possess a high classificatory importance. The œsophagus in Cryptophialus, where it enters the stomach, is armed with teeth and hairs, moved by muscles, forming a beautiful structure, of which we have not a trace in any other cirripede. Lastly, and perhaps most importantly of all, the metamorphosis is different; for the early larval stages are passed under an egg-like condition within the sack of the parent; and the pupa differs from the pupæ of all other cirripedes, in not having natatory thoracic limbs, and is therefore only able to crawl about by the aid of its great prehensile antennæ.

Thus far the evidence is decisive in favour of Cryptophialus being placed in a separate Order; but if we were to trust to the characters derived from the external covering or carapace,--and such characters are of high importance, as we may safely infer from the natural arrangement of the foregoing families which depends on the structure of the carapace,--we should place Cryptophialus close to Alcippe amongst the Lepadidæ. These genera agree in their burrowing habits,--in their attachment by a horny rostral disc,--in the external membrane being covered with triturating points,--in the spinose and notched orifice, with an external lateral bar on each side, and in the inner tunic of the sack being protected by hairs and spines. They agree to a considerable extent in shape, and in the peculiar arrangement of the muscles of the whole external covering of the animal: they agree, also, in their manner of growth, and in the sack extending down to their basal extremity. Some of these resemblances may possibly be analogical, and due to similarity of habits, and not to affinity; and we must attribute to mere similarity in function, a certain amount of resemblance in their labrums, for this part is essentially different in the two genera; and to the same cause, the resemblance between the brush formed by the two pairs of thoracic cirri and caudal appendages at the end of part of the thorax in Alcippe, with the three pairs of abdominal cirri at the end of the whole thorax in Cryptophialus. I allude to this latter resemblance, for it was owing to it, and to the similarity in the habits of Cryptophialus and Alcippe, that I stated, in the introduction to my former volume on the Lepadidæ, that the two genera would probably fall into the same order. In the structure of all the parts of the mouth and of the cirri, in the digestive organs and in the metamorphoses, Cryptophialus is not more closely related to Alcippe than to any other genus whatever amongst the Lepadidæ.

Nevertheless I am confirmed in the view that the external resemblances between these two genera are due to some real affinity, and are not merely analogical, by a very remarkable fact,--namely, that Alcippe and Cryptophialus are both bisexual, and have males, several in number, attached exactly in the same position, and which males are so closely similar that, considered by themselves, they might absolutely be almost classed as species of the same genus! For they agree in the absence of all internal organs and parts, excepting the single testis, vesicula seminalis, and immensely long probosciformed penis; and they agree, also in manner of growth, in the arrangement of the muscles, and even in shape. The whole case seems to me very singular, and, as far as my knowledge extends, unique: we have two animals, of which the females, if classed by their external parts (homologically consisting of the three anterior segments of the head), would be placed alongside each other in the same family; but when classed by the whole rest of their organisation, certainly must be ranked in distinct orders;[150] yet the males of these very same animals might almost stand in the same genus. If it be asked why the position of Cryptophialus in the system should not be determined by the male, instead of by the female, the answer would be that the male is here abnormal and rudimentary in its whole structure; and I believe systematists are agreed that less perfect parts (and therefore a less perfect whole) offer less valuable characters than the more perfect parts or whole. We see this conclusion plainly verified in the case of the hermaphrodite _Scalpellum vulgare_ and _Ibla quadrivalvis_, for there can be no doubt where these species should be arranged, yet if we attempted to place them by their complemental males, we should utterly fail: exactly in the same manner, if _Ibla Cumingii_ and _Scalpellum ornatum_ were ranked by their males, they would be quite misplaced. So again, if we were to attempt to class the six species of Scalpellum by their males and complemental males, undoubtedly the first three and last three species of the genus would have to stand in distinct orders! Hence we may reject the males as a foundation for classification, but no doubt they serve to show that the resemblances in the carapaces of Alcippe and Cryptophialus, are not merely analogical or functional, but evince a true affinity, though these genera differ so greatly, in mouth, body, œsophagus, cirri, and especially in their metamorphoses.

[150] M. Milne Edwards would, perhaps, in accordance with the
profound views lately propounded by him on classification, consider
Cryptophialus as an extremely modified, and, to a certain extent,
degraded member or satellite of the _type_ of the Lepadidæ; but
I do not myself feel able to draw a line of distinction between
the being a very abnormal member of one group, and belonging to
a distinct group. I may add that I have several times tried to
persuade myself, with no success, into the belief that I have
somehow misunderstood the homologies of the thoracic segments and
cirri of Alcippe and Cryptophialus; for if this were so, the two
genera could be brought into much closer relationship; but with
any conceivable amount of error on my part, there remains the
great difference in the metamorphosis, not to mention the palpable
differences in the cirri, the parts of the mouth, and in the whole
course of the alimentary canal.

CRYPTOPHIALUS MINUTUS. Pl. 23, 24, fig. 1 to 19.

_Hab._--Chonos Archipelago, Southern Chile; imbedded in the
_Concholepas Peruviana_.

FEMALE. Fig. 1-18.

_General Appearance._[151]--This, the smallest known cirripede, is flask-shaped and compressed, with a small orifice, on a more or less produced neck, placed at one corner: one of the narrow sides of this carapace is somewhat flattened or depressed, with its superior edge prolonged a little upwards; by this side, the animal is attached to the cavity in the shell, within which it is lodged. There is here no distinction between a peduncle and capitulum, that is between the lower or anterior, and the upper or posterior end of the animal, as seen externally. The small orifice is toothed and hairy: it is coloured purple, as is likewise the projecting labrum; the rest of the animal being tinted only by the muscles and internal parts seen through the outer integuments. The largest specimen (fig. 2) which I have measured did not quite attain the length of one tenth of an inch. This cirripede inhabits, in vast numbers, the shells of the living Concholepas Peruviana, amongst the Chonos islands; the whole outside of the shell being sometimes completely drilled by its cavities, almost touching each other, as happens in the case of Alcippe with the shells of Buccinum. The oval aperture leading into the shell-cavity, in full-sized specimens, is between (2-3)/100 of an inch in length: it is generally surrounded by a narrow, internal, calcareous rim, which apparently has the same inorganic origin, as in Alcippe. The toothed orifice of the carapace leading into the sack, fills up the orifice of the shell-cavity; but it can be voluntarily withdrawn a little: when opened, and the animal is in action, the lancet-formed, moveable crest of the labrum, and the abdominal cirri, are exserted.

[151] I am greatly indebted to Dr. Hooker, for having several years
ago, when I examined this my first cirripede, aided me in many
ways, and shown me how to dissect the more difficult parts, and for
having made for me several very correct drawings, which, with some
subsequent alterations, are now engraved.

_Integuments._--The external membrane is colourless, thin, but strong; it is studded with minute bifid, trifid, and quadrifid points of hard chitine, which are the agents of excavation: they are directed upwards, except towards the lower end, where they are directed from the disc or surface of attachment. These points beneath the orifice, and on each side close along the lateral bar, are larger than elsewhere. There are no points on the disc or surface of attachment, which is formed of somewhat thickened, yellowish membrane, and is not moulted, like the rest of the external membrane, but is formed of successive layers extending beyond each other; the lines of growth, however, being obscure, and only occasionally distinguishable. The disc is oval, not extending to the lower end of the animal, and with the upper edge thinning out and produced upwards (fig. 1). The animal, during its growth, moves a little downwards in its cavity, by means of the new layers of the attached disc being formed, not symmetrically with respect to the old layers, but beyond them or at a greater depth in the shell; hence when the animal is removed quite perfect, by the Concholepas being dissolved in acid, the upper and deserted margin of the disc or surface of attachment generally projects as a free edge, but in a tattered and worn condition. In full grown specimens, which have ceased burrowing downwards, nearly the whole disc, though occupying the same position relatively to the animal, becomes in fact deserted, and is lined by membrane continuous with, and like that, investing the rest of the body, but furnished only with simple blunt points, instead of with the sharp bifid and trifid triturating points.

The orifice is formed on each side by a toothed rim of hard chitine (fig. 3, 4), which can be opened and shut, owing to its being flexible at the rostral end, and folded inwards at the carinal or posterior end. The teeth vary in arrangement and sharpness: generally they form, taking a rim on one side (fig. 3), two prominences or groups of points at the rostral or anterior end, always separated by a broad notch, from the bottom of which the lateral bar extends downwards, from the posterior, larger and less regularly toothed half of the rim. Of these teeth the larger ones project nearly straight up, and the smaller and lower teeth outwards, graduating into the smaller teeth, just below the rim, which again graduate into the minute points, studded over the whole surface. These outer teeth probably serve to prevent any creature crawling into the cavity, between the shell and animal. Scattered bristles rise from all over the rim. The lateral bar, above alluded to, consists of the general membrane of the body, thickened, hardened, rendered elastic, coloured yellow, and apparently formed into a fold: where attached to the under side of the rim, at the above-mentioned notch, the bar is thinner and more flexible than elsewhere: it runs half way down the animal, first straight and then curved towards and closely approaching the disc or surface of attachment. At the lower end, the bar, or more strictly the thickened membranous margin of the bar (_b′_, fig. 3), expands into an oblong, slightly rigid plate, studded with from three or four to ten or twelve points, which have their ends expanded and truncated, or even slightly bifid. The extremity of this plate projects freely from the general surface of the body. We shall afterwards refer to the use of this extremely peculiar bar and plate.

The two rims forming the orifice cannot be quite closed; but ingress by any foreign object into the sack is beautifully prevented by an internal membranous lip on each side (_d′_, fig. 3, 4), and by a third inwardly folded lip (_d′_) at the posterior and broad end of the orifice. These three lips can be brought together, and form a valve. The lateral lips are very narrow at the mouth or rostral end of the orifice, where the hairy lancet-shaped crest of the labrum closes the orifice, and largely expand towards the posterior end: they are produced from the inner tunic of the sack: they appear formed of the finest hairs, placed parallel and approximate, but when examined under the highest powers, these hairs (for they still appear such) are found to be united by delicate membrane, which has its extreme edge fimbriated. The third, or posterior and inwardly folded lip, differs in being composed of much coarser, flattened hairs, which are united towards their bases, and are free at their extremities, where they are serrated or coarsely plumose on both sides.

The sack extends down to the lower end of the animal. It is lined by delicate membrane. At the orifice on each side, a little posteriorly to the lateral external bars, this inner membrane is strengthened by a pair of thin yellowish bars (_c′_, fig. 3), which run parallel to the straight portions of the external bars. These inner bars at their lower ends become pointed and die out: at their upper ends, and close to the rim, they are broader, but more flexible, and so transparent as hardly to be distinguished from the rest of the membrane. At the rostral end of the orifice, in a medial line, this same inner tunic of the sack is thickened for a short space downwards, so as to form a fifth bar (_c′′_); which separating from the inner tunic, runs inwards between the outer and inner membranes of the carapace (_i. e._ between _b_ and _c_, fig. 3, 5), behind the mouth, as far down as opposite to the lower end of the œsophagus, and there becoming thinner and ligamentous, gives attachment to some powerful muscles.

At each exuviation, the external membrane with the dentated hardened orifice, the lateral bars, the inner tunic of the sack with its bars, are all moulted, together with the usual integuments of the animal's body. New and sharp triturating points are thus periodically formed for the work of excavation. The whole animal increases during growth in every direction, and therefore, at its lower or basal end, as was the case with Alcippe. The disc or surface of attachment, is added to by new underlying layers, extending beyond the old layers at the lower end and on the sides, but not at the upper end, where, as in the case of the calcareous discs of Lithotrya, the old layers are deserted and worn away. I saw what I believed to be little globules or patches of cement; but I was not able to discover any cement-ducts.

_Muscles of Sack and Orifice._--The animal is surrounded by rather strong longitudinal muscles, not running up close to the orifice: these muscles exhibited, to my surprise, distinct traces of transverse striæ: there are no external transverse muscles, as in all the Lepadidæ. Attached to both sides of the inward fold or hinge, at the posterior end of the orifice, some striated or voluntary muscles run for a short distance downwards, diverging like a fan: their contraction would cause the dentated rim to open: a strictly homologous muscle occurs only in Alcippe. At the opposite end of the orifice, a remarkably powerful voluntary muscle is attached to the ligamentous bar above described as proceeding from the rostro-medial end of the toothed rim (_c′′_); and at its lower expanded end, it is attached under rather above the middle of the disc: this muscle corresponds with a similar one in Alcippe, and with some much weaker muscles in other Lepadidæ. Its action would be to draw down within the shell-cavity the whole dentated rim, and likewise to close the orifice; and here, I believe, come into use the lateral elastic horny bars with their curious basal projecting plates, furnished with expanded points, for much friction would thus be caused by, yet some play be allowed for, the several movements; the elasticity of the bar bringing up the dentated orifice, when the powerful muscles attached to the rostral end of the latter became relaxed. Round the space where the just-mentioned muscles are attached to the horny disc, a sheet of other muscles radiate, a few on both sides obliquely upwards, but the greater number transversely and within the first-described longitudinal muscles; they extend on both sides about half round the animal. There are similar muscles in Alcippe, but not extending so far round the animal. Their action must be to draw the whole carapace towards the surface of attachment; the action of the longitudinal muscles being to shorten it; the orifice supported by the lateral horny bars, serving as the fulcrum for the contraction of the longitudinal muscles. I could not see any adductor scutorum muscle, although I looked particularly under the expanded plates at the ends of the lateral external horny bars.

_Body._--This is laterally compressed: it is widest and thickest at the upper end, and thence tapers to the lower or posterior end. The last three or four thoracic segments are bent under the anterior segments, giving the whole something of the appearance of certain crustaceans, divested of their legs. The somewhat conical mouth, with its singular labrum, is very large. The body consists of eight segments. The first segment (fig. 5, 1), or that succeeding the mouth, is the seventh or last cephalic segment of the archetype crustacean; it is the largest of all eight segments; it is joined by its dorsal surface to the carapace or external covering of the animal, and the membrane with which it is invested is prolonged upwards and downwards (_c_, _c_, fig. 5), and so forms the inner tunic of the sack. The succeeding seven segments are thoracic; they are free, and are destitute of limbs; the articulations separating them are transverse. The first and second thoracic segments give rise, on their medial dorsal surfaces, each to a remarkable tapering curved appendage, presently to be described. At the end of the last thoracic segment, there is a minute abdomen, bearing three pairs of biramous cirri.

The _Mouth_ consists of three pairs of organs, namely, the outer maxillæ, maxillæ, mandibles with their palpi, and of a great and very curious labrum. These organs, by the fusion (as in other cirripedes) of their lower segments, form a large, somewhat conical, projecting mouth, which is separated on the ventral surface from the rest of the body by a distinct fold or articulation, where the muscles proceeding to the above gnathites are attached. The labrum (_e_, fig. 5, 9) is of large size; and the crest close over the opening of the œsophagus is produced into a great, lancet-shaped, moveable organ, wholly unlike anything occurring in any other cirripede: it is coloured purple, and is thickly fringed in the upper part by very fine hairs: it is bowed a little backwards from the mouth: the base, which rather overhangs the œsophagus, is a little contracted, and is transversely marked by an articulation: two small, parallel, voluntary muscles (with transverse striæ) are attached at their lower ends close beneath the articulation, and extend about one third up the organ: their contraction would serve to erect it; and their relaxation would, apparently, allow it to fall backwards on a little knob (_e′_, fig. 5) behind. This little knob resembles a similar projection in many of the Lepadidæ. As the labrum is formed of similar membrane with that of the succeeding segment of the body, its limit downward, beneath the knob, can be told only by a small apodeme which projects inwards, at a little distance within the line where the membrane of the body is reflexed upwards (_c_), so as to form the inner tunic of the sack.

The mandibles, palpi, and maxillæ, all project more than is usual. The _Palpi_ (_f_, fig. 9, 5) are narrow, flattened, and taper a little; they support a few long bristles on their tips, and on one of their sides. In every other cirripede (in which the palpi are developed) they are directed transversely across the mouth, one towards the other, and are for a considerable space united to the labrum: here they project straight up, and seem to rise exteriorly to the bases of the mandibles; they are, however, united to the basal lateral edges of the labrum, and when the latter is torn from the rest of the mouth, the palpi separate with it. I could not distinguish the knob on which, in every other cirripede, the palpi are articulated. The _Mandibles_ (fig. 8) have an upper, lower, and middle tooth, with some finer intermediate points and hairs. The _Maxillæ_ (fig. 7) are narrowed in at their spinose edge, where there are three large spines and several finer bristles, together forming a flattened brush: this organ is remarkable from the apodeme (_a_) being bent into the shape of a scythe, with the terminal or blade-portion a little expanded, and directed backwards and inwards. The _Outer maxillæ_ are sub-triangular in outline, with several bristles on their summits and along their outer surfaces.

_Segments of Body._--I have stated that the mouth is succeeded by eight segments. As in all the cirripedes hitherto described, the body consists of only six segments, the number eight at first seems very improbable, and therefore I may be permitted to state that both Dr. Hooker and myself, when first examining this animal, and having no notion whatever regarding its homologies or the structure of other cirripedes, came to the conclusion, judging only from external appearances, that is, from the transverse folds, and from the lines of movement when the body was bent by a needle, that there were really eight segments. I have since carefully looked to this point: when the outer membrane is cleaned and examined, the four posterior segments are very plain, owing to a dorsal medial line, being alternately either thickened and coloured yellowish, or thin and colourless: the four anterior segments are less plain, but yet the membrane on the dorsal surface, on the line of each fold or articulation, does present some difference, from being destitute of the fine, transverse, toothed scales which occur on other parts. But I lay most stress on the fact, that all these eight articulations were used for the attachment of muscles. Hence I conclude that the eight segments are real; and we shall see, in the next order, that the very same eight segments are as plain in Proteolepas, as in the larva of an insect or as in an annelid. There is good reason to believe that the general covering or carapace consists in all cirripedes of the three anterior segments, and that the mouth (judging from its appendages) also consists of three segments, consequently the first segment of the body in Cryptophialus must be the seventh or last cephalic segment, and the seven next free segments must be the normal seven thoracic segments.

The first segment of the body (_i. e._ last cephalic, fig. 5, 1) is, as stated, the largest, and is attached dorsally to the carapace: its ventral surface is flattened, and is formed of somewhat thickened membrane: on each side, a little below the articulation separating this segment from the mouth, there is a small blunt projection, with the free part only 1/500th of an inch in length. Each of these appendages bears four or five bristles on one side near the summit, and a few on the other side, lower down: from their position I believe them to be rudiments of a first pair of maxillipeds (tetartognathites of Milne Edwards), of which no trace occurs in any other cirripede. The differences between this segment and the seven succeeding segments, is of interest, as offering some confirmation of the belief, lately disputed by some naturalists, that the cephalic and thoracic segments in the class Crustacea, do differ in their nature,--a conclusion which we shall see further confirmed under Proteolepas. The second and third (_i. e._ first and second thoracic) segments (2, 3, fig. 5) are the next largest, and are remarkable from supporting singular appendages, already alluded to. The sides of the second segment are formed of thickened yellowish membrane. The fifth and sixth segments are the smallest, and mark the point of chief flexure of the body. The eighth segment is a little elongated, formed of thicker membrane than the other segments, and dorsally is indented by the anus.