Length of hoof-like disc, measured from the apex to the } middle of the articulation with the basal segment } 9-10/6000
Breadth of ditto 5/6000
Length of ultimate segment 6/6000
Breadth of ultimate segment beneath the notch 7/20000
Breadth of ultimate segment above the notch 5/20000
I did not see the cement-ducts, which, perhaps, was owing to the corium extending from the inside of the whole animal some way into the antennae, thus rendering them rather less transparent than in common Cirripedes.
That the ducts and cement-glands exist, is certain, for the antennae in every case were enveloped in a little irregular ma.s.s or capsule of the usual, brown, transparent, laminated cement. When several of these parasites were attached close together, the cement ran up between them.
I may here state, that I found on one Scalpellum, three males very lately attached, and not as yet imbedded in the chitine border; they were white, opaque, pulpy, and full of oily globules; the lower part was considerably more pointed, and extended further beyond the prehensile antennae, than in the older and imbedded specimens. There were distinct remnants of two great reddish-brown eyes, showing that in this respect the larvae of the male in their last stage of development, are characterised like the larvae of other Lepadidae. The male larva would, probably, be a little larger than the male itself; but yet compared with the larva in the earliest stage, there can have been unusually little increase of size during the several intermediate metamorphoses; I judge of this from the dimensions of the larva of the hermaphrodite in the first stage, namely, 9/400ths of an inch, exactly the size of some of the smaller males. In the allied genus Ibla, the increase is also less than is usual, namely, from 15/1000ths of an inch, the diameter of the ovum, to only 25/1000ths of an inch, the length of the boat-shaped larva, just before its final metamorphosis.
_Habits and Concluding Remarks._--The males are imbedded in the spinose chitine border of the occludent margin of the scuta, exactly over an oblique fold or notch (fig. 15 _a_ _a_), close by the umbo. This fold has no direct relation to the males, but being present is taken advantage of by them; for it occurs in the young hermaphrodite, before the attachment of the males, and in species of the genus in which the males are attached to other parts. It occurs, also, in fossil species of Pollicipes, and in these it seems caused by the upper inner part of the valve being rendered more and more prominent during growth: in the present species, I suspect, its origin is connected with the formation of a ridge bounding the outer side of the pit for the adductor scutorum muscle: we shall see in the next species, that this fold is of the highest importance in relation to the position of the Males. The transparent chitine border of the scuta is broad, and fills up the fold in the sh.e.l.l, so that the outline of the occludent margin is not affected by it: in the drawing (fig. 9) some of the inner layers of chitine (_e_ _e_), which dipped into and filled up the fold, have been removed, that the lower part of the animal might be more plainly exhibited. The chitine bears numerous spines of various lengths, which must afford some protection to the males, rudely arranged in lines, parallel to the edge of the valve, indicating the successively-formed layers of chitine; each spine has a fine, tortuous tubulus connecting its base with the underlying corium. The extreme outer edge of the border is thin, forming a kind of lip, close beneath which the delicate tunic lining the sack is attached. During continued growth, the valve is added to in thickness, and so is the chitine border, and likewise in breadth. It appears that the larva of the male must attach itself on the under side of this border, on the edge of the tunic of the sack, and that by the action of the cement, the corium beneath is killed (as I believe always is the case with other parasitic Cirripedia), whereas on both sides, the chitine continues to be added to, so that the male, excepting the upper and always projecting portion, becomes imbedded at first laterally, and ultimately all round: I have seen specimens in several different stages of imbedment. Hence, in old specimens, with a thick and broad chitine border, it might and does come to pa.s.s that one male is imbedded (the valve being laid flat) directly beneath another.
I have examined a great number of specimens from various localities, taken at different times of the year,--some dozen specimens from Cornwall,[53] and several from unknown localities in various collections; some from Ireland, from the Shetland Islands, from Norway, and from near Naples. Every one of these specimens, with the exception of some of the Neapolitan ones, had parasitic males attached to them: I must also except very young specimens, on which they never occur. On a Cornish specimen, with a capitulum a little more than one fifth of an inch in length, it may be mentioned as unusual that there were three males. In young specimens there is generally one male on each scutum, but sometimes there are two, and sometimes none on one side. In large old Cornish specimens I have counted on the two sides together, six, seven, and eight males, and in one Irish specimen no less than ten, seven all close together on one valve and three on the other, but I do not suppose that all these were alive at the same time. In the Neapolitan specimens, however, which are the largest that I have seen, there was in no case more than two; and out of seven or eight specimens, four had not any male; so that it would appear there is something in this locality hostile to the development of the parasitic males. I have noticed only one instance (that given in fig. 9) in which the males were imbedded a little way apart; generally they touch each other, and are cemented together: where there are several males, they occur at different levels, as measured from the under or upper surface of the chitine border: in one instance of four males adhering to one valve, I distinctly perceived that the lowest one was white, pulpy, and recently attached; the two above, which were placed close together and between the same laminae of chitine, were mature; and the third still higher up, was dead, empty, transparent, and half decayed: in some other instances, I have found the uppermost parasites dead, and, together with the surrounding chitine, partially worn away.
[53] I am greatly indebted to Mr. Peach for his unwearied kindness in procuring me fresh specimens. Mr. W. Thompson allowed me to dissect one, possessing particular interest, out of his three Irish specimens. Professor Forbes procured me a specimen from the Shetland Islands, and Professor Steenstrup was so kind to take pains to send me some Scandinavian specimens.
The larva of the male must have a different instinct from the larva of the hermaphrodite; for the latter attaches itself head downwards to a coralline, whilst the male larva crawling on the scuta of the hermaphrodite, discovers, I presume by eye-sight, the fold in the sh.e.l.l beneath the translucent border of chitine, and there invariably attaches itself. Its object in choosing this particular spot, I believe, simply is that the depth or thickness of the chitine is there greater, and sufficient for its imbedment, which would hardly be the case elsewhere.
This parasite has, as we have seen, no mouth or stomach, and indeed, considering its fixed position and the non-prehensile condition of its limbs or cirri, a mouth would have been of no service to it, without it had been extraordinarily elongated. The male must live on the nourishment acquired during its locomotive larval condition; and its life no doubt is short, but yet not very short, as I infer from the depth to which mature specimens are buried in the chitine border. The full development of the spermatozoa consumes, I suppose, some considerable lapse of time. The thorax and limbs, though furnished with muscles, are obviously, as already remarked, of no use for prehension; these parts serve, probably, to defend the little creature, when its eye announces the pa.s.sing shadow of some enemy, and for this purpose they are well adapted from the extreme sharpness of the spines. The thorax, into which I traced the vesicula seminalis, no doubt also serves for the emission and first direction of the spermatozoa; and hence, perhaps, its singularly extensible structure. I have already remarked, that in specimens preserved in spirits, the thorax is often largely protruded, and bent down at right angles to the orifice. I presume this is caused by endosmose; nevertheless it deserves notice, that it was in these protruded specimens that the vesicula seminalis was most conspicuously gorged with spermatozoa. I suspect the longitudinal and transverse muscles lining the upper part of the outer integuments of the whole animal, can be of little use to the creature, without it be to aid in the protrusion of the thorax, and perhaps in the violent expulsion of the spermatozoa, thus causing them to reach the ovigerous lamellae within the sack of the hermaphrodite. It is also probable, that the action of the cirri of the hermaphrodite, would tend to draw inwards the spermatozoa in the right direction. In one specimen, the spermatozoa in the hermaphrodite and in the male were mature at the same time; in another this was not the case; and as the males, apparently, become attached at all periods of the year, this want of coincidence in maturity must often occur. Can the males retain their spermatozoa, till told by some instinct, that the ova in the sack of the often fecundated hermaphrodite are ready for impregnation; or are the spermatozoa sometimes wasted, as must annually happen with such incalculable quant.i.ties of the pollen of many dioecious plants?
This little Cirripede is, in many respects, in a partially embryonic condition. There is no separation between the capitulum and peduncle; there is no mouth; and the thorax, throughout its whole width, opens into the anterior part of the animal: the limbs differ greatly from those both of the mature Cirripede and of the larva, but come closest to the latter: the preservation of the abdomen is a well-marked embryonic character. On the other hand, the four rudimentary calcareous valves, the narrow orifice, the hirsute outer integument, the two muscular layers, the single eye, and male internal organs, are all characteristic of the fully-developed condition. The four little valves, as I believe, represent the scuta and terga, though they are placed considerably below the orifice: the little bristly points have no h.o.m.ological signification, and are absent in the male of the following closely allied species. The four pairs of limbs answer to the four posterior cirri, as may be inferred from their proximity to the abdominal lobe, and from the three posterior pairs closely resembling each other, and differing a little from the first pair; this latter pair corresponds with the third pair in the hermaphrodite form of Scalpellum. If I am right in believing that only a single vesicula seminalis is ordinarily developed in the male, this is a special and singular character.
As stated in the beginning of this description, from the one great fact of the absolute correspondence of the prehensile antennae of the parasite, with those of the hermaphrodite _Scalpellum vulgare_, together with its fixed condition, its short existence, and exclusively male s.e.x, I have thought myself justified in provisionally considering it as the Complemental Male of the Cirripede to which it is attached; but I hope final judgment will not be pa.s.sed on this view, until the whole case is summed up at the end of the genus.[54]
[54] I trust, before long, that some naturalist, with more skill than I possess, will examine these parasites on _Scalpellum vulgare_, which unfortunately is the only species of the genus that can be easily obtained. Fresh specimens, or those preserved in spirits of wine, are necessary. The action of boiling caustic potash is very useful in cleaning the prehensile antennae. If these latter organs are sought in the hermaphrodite for the sake of comparison, young specimens, adhering to clean branches of a coralline, should be procured, and caustic potash used.
2. SCALPELLUM ORNATUM. Pl. VI, fig 1.
THALIELLA ORNATA. _J. E. Gray._ Proc. Zoolog. Soc., 1848, p. 44, Annulosa, Plate.
_S. (Foem.) valvis 14, sub-rufis: lateribus superioribus quadranti-formibus, arcu crena profunda notato._
(Fem.) Capitulum with 14 reddish valves: upper latera quadrant-shaped, with the arched side deeply notched.
Mandibles with three teeth; maxillae narrow, bearing only four or five pair of spines.
MALES, two, lodged in cavities on the under sides of the scuta; pouch-formed, with four unequal, rudimentary valves: no mouth: cirri not prehensile.
Algoa Bay, South Africa. Attached to Sertularia and Plumularia.
British Museum.[55]
[55] I am greatly indebted to Mr. Bowerbank for specimens of this extremely interesting species; also to Mr. Morris, to whom Mr.
Bowerbank had given some of the original specimens.
FEMALE.
_Capitulum_ oblong, with the upper portion much produced; valves, 14, thick, naked, closely locked together, irregularly clouded with pale crimson; the membrane connecting the valves is not furnished with spines. On most of the valves there are furrows and ridges diverging from the umbones, and the lines of growth are plainly marked: in the valves of the lower whorl, the umbones are slightly protuberant.
_Scuta_, convex, unusually thick, oblong, quadrilateral, with the occludent margin the longest; lateral margin slightly hollowed out. The umbo (and primordial valve) is situated at the uppermost point of the valve, and consequently the growth is exclusively downwards. On the under side (Pl. VI, figs. 1 _b"_ and 1 _c"_), in about the middle of the valve, there is a pit (_a_) for the adductor scutorum muscle, the depth and distinctness of which varies a little; above the pit, and between it and the apex, there is a transverse, oblong, deeper depression (_b_), within which, the male is lodged. A small portion of the apex of the valve projects over the terga.
_Terga_, large, nearly equalling the scuta in area, flat and sub-triangular; the scutal margin is not quite straight. The apex of the valve is thick and solid, and must have projected freely for a length equalling one third of the occludent margin.
_Carina_, laterally broad, angularly bent; slightly widening from the apex to the base; internally, deeply concave. The position of the umbo varies, in young specimens it is seated at the uppermost point, and consequently in such there is no upward growth; in older specimens, from the junction and upward production of that part on each side of the valve, which I have called in fossil specimens the intra-parietes, the valve is added to above the umbo, but to a lesser degree than in _S.
vulgare_. Slight ridges separate the roof from the parietes, and the parietes from the intra-parietes.
_Rostrum_, minute, narrow, widening a little from the apex downwards, inserted like a wedge between the umbones of the rostral latera, and hardly projecting above their upper margins, so as to be easily overlooked: internally concave.
_Upper Latera_ (fig. 1 _a_), quadrant-shaped, with a deep square notch cut out of the arched margin, which notch receives the upper point of the carinal latera; the surface of the valve between the notch and the umbo is depressed.[56]
_Rostral Latera_, small, gradually widening from the umbo to the opposite end, which is obliquely rounded.
_Infra-median Latera_, approaching to diamond-shaped, placed obliquely to the longer axis of the capitulum; or the upper part may be described as spear-shaped.
[56] The only valve which I have seen at all like this, is a fossil specimen from the Upper Chalk of Scania; this is described in my memoir on the Fossil Lepadidae (Palaeontographical Society), under the name of _Scalpellum solidulum_ (Tab. 1, fig. 8, _e_, _f_), and is perhaps erroneously there considered as a carinal latus.
_Carinal Latera_: these appear as if formed of two valves united together; the upper portion, widening as it ascends in a curved line, terminates in a rounded margin, which enters the deep notch in the upper latera; the other and lower portion is shorter, and terminates in a square margin ab.u.t.ting against the infra-median latera; the umbones of the carinal latera project beyond the line of the carina.
_Direction of the Lines of Growth in the Valves._--This should always be carefully observed, on account of the great diversity there is in this respect between the different species, especially when the recent are compared with the older fossil species; moreover one of the chief characters between the genus Scalpellum and Pollicipes, depends on the direction of the lines of growth. In the scuta, terga, rostrum, and upper latera of the present species, the chief growth is downwards; in the carina, in mature specimens, it is both upwards and downwards; in the carinal latera, both upwards and towards the infra-median latera; in the infra-median latera chiefly upwards; and, lastly, in the rostral latera, towards the infra-median latera.
_Peduncle_, short, not half as long as the capitulum; calcareous scales imbricated as usual, tinged red, almost crescent-shaped, ac.u.minated at both ends, of remarkable length, so that in each whorl there are only four scales: a full-sized scale equals in length one of the rostral latera. The tips of two scales, in one whorl, lie under the middle points of the carina and rostrum; and in the whorl, both above and below, a single much curved scale occupies this same medial position.
The peduncle does not seem to have been attached in any definite position to the h.o.r.n.y coralline, as is the case with _S. vulgare_.
Length of capitulum in the largest specimen .2 of an inch.
The _Mouth_ is directed towards the ventral surface of the thorax. The _Labrum_ is far removed from the adductor muscle, with the upper part forming an overhanging projection; I believe there are some very minute bead-like teeth on the crest. _Palpi_, small, narrow, thinly clothed with bristles.
_Mandibles_, with three teeth, of which the first is distant from the second; inferior angle not much ac.u.minated, pectinated on both edges.
_Maxillae_, small, narrow, produced, without any notch, with two large upper spines, of which one is much thicker than the other; on the convex upper margin there are some minute tufts of very small hairs.
_Outer Maxillae_, with few bristles, arranged in a continuous line on the anterior surface; on the external surface there is a tuft of long bristles. Olfactory orifices situated laterally, forming two flattened, tubular projections.
_Cirri._--First pair placed not far from the second; the three posterior pair not very long, with their segments elongated, not protuberant, bearing four pair of non-serrated spines, with a single short bristle between each pair; dorsal tufts small, with one spine longer than the others. First cirrus rather short, segments not very broad; second cirrus with the rami nearly equal in length, anterior ramus rather thicker than the posterior ramus, with three longitudinal rows of spines.
_Caudal Appendages._--These are minute, rather broad, not half as long as the lower segments of the pedicels of the sixth cirrus, with four very long spines at the tip.
_p.e.n.i.s._--There is no trace of a probosciformed p.e.n.i.s in the four specimens examined; and as this organ is present in every ordinary cirripede, with the exception of _Ibla c.u.mingii_ which we know to be exclusively female, so we may infer with some confidence that the form here described is female, although it is impossible in specimens once dried to demonstrate the absence of the vesiculae seminales and testes.
_Affinities._--This is a very distinct species; it is, however, much more nearly related to _S. rutilum_, than to any other species; and next to this, to _S. vulgare_; from this latter species it chiefly differs in the large scales of the peduncle, in the scuta not being added to at their upper ends, and in the membrane covering and connecting the valves being spineless; but there is a greater difference in the trophi and in the cirri. The peduncle of _S. ornatum_ presents some resemblance to that of the singular cretaceous genus, _Loricula_.
MALE.
All the specimens, as already stated, were dry, but in an excellent state of preservation, so that after having been soaked in spirits, they could be minutely examined. In the four which I opened, I found, in a transverse pouch on the under side of each scutum, a male lodged; in a fifth dead and bleached specimen, the cavities in the sh.e.l.l for the reception of the males, were present; and in a sixth young specimen, also dead, cavities were in process of formation. As compared with plants, the relation of the s.e.xes in this species may be briefly given, by saying that it belongs to the cla.s.s _Diandria monogynia_. I will first describe the males themselves, and then the cavities in the sh.e.l.l of the female. The males differ in every point of detail, from the complemental males of _S. vulgare_, but yet present so close a general resemblance, that a comparative description will be most convenient.