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.