Eyes.—Close behind the basal articulations of the antennæ, the sternal surface consists of two approximate, elongated, narrow, flat pieces, or segments. These Burmeister considers as the basal segments of the antennæ: as they are not cylindrical, I do not see the grounds for this conclusion: their posterior ends are rounded, and the membrane forming them is reflected inwards, in the form of two, forked, horny apodemes, together resembling two letters, UU, close together; these project up, inside the animal, for at least one third of its thickness from the sternal to the dorsal surface. The two great, almost spherical eyes in L. australis, each 1/150th of an inch in diameter, are attached to the outer arms, thus, °UU°, in the position of the two full stops. Hence the eyes are included within the carapace. Each eye consists of eight or ten lenses, varying in diameter in the same individual from 1/2000 to 3/2000th of an inch, enclosed in a common membranous bag or cornea, and thus attached to the outer apodemes. The lenses are surrounded half way up by a layer of dark pigment-cells. The nerve does not enter the bluntly-pointed basal end of the common eye, but on one side of the apodeme. The structure here described is exactly that found, according to Milne Edwards, in certain crustacea. In specimens just attached, in which no absorption has taken place, two long muscles with transverse striæ may be found attached to the knobbed tips of the two middle arms of the two °UU°, and running up to the antero-dorsal surface of the carapace, where they are attached; other muscles (without transverse striæ) are attached round the bases, on both sides of both forks. The action of these muscles would inevitably move the eyes, but I suspect that their function may be to draw up the narrow, deeply folded, sternal surface, and thus cause the retraction of the great prehensile antennæ within the carapace.

Mouth.—This is seated in exactly the same position as in the mature Cirripede, on a slight prominence, fronting the thoracic limbs, and so far within the carapace, that it was obviously quite unfitted for the seizure of prey; and it was equally obvious, that the limbs were natatory, and incapable of carrying food to the mouth. This enigma was at once explained by an examination of the mouth, which was found to be in a rudimentary condition and absolutely closed, so that there would be no use in prey being seized. Underneath this slightly prominent and closed mouth, I found all the masticatory organs of a Cirripede, in an immature condition. The state of the mouth will be at once understood, if we suppose very fluid matter to be poured over the protuberant mouth of a Cirripede, so as to run a little way down, in the shape of internal crests, between the different parts, and in the shape of a short, shrivelled, certainly closed tube, a little way (.008 of an inch in L. australis) down the œsophagus. Hence, the larva in this, its last stage, cannot eat; it may be called a locomotive Pupa;[11] its whole organisation is apparently adapted for the one great end of finding a proper site for its attachment and final metamorphosis.

[11] M. Dujardin has lately (‘Comptes Rendus,’ Feb. 5, 1850, as cited in ‘Annals of Nat. History,’ vol. v, p. 318,) discovered that the “Hypopi are Acari with eight feet, without either mouth or intestine, and which, being deprived of all means of alimentation, fix themselves at will, so as to undergo a final metamorphosis, and they become Gamasi or Uropodi.” Here, then, we have an almost exactly analogous case. M. Dujardin asks—“Ought, therefore, the Hypopi to be called larvæ, when, under that denomination, have hitherto been comprised animals capable of nourishing themselves?”

Thorax and Limbs.—The thorax is much compressed, and consists of six segments, corresponding with the six pair of natatory legs; the anterior segments are much plainer (even the first being distinctly separated by a fold from the mouth), than the posterior segments, which is exactly the reverse of what takes place in the mature Cirripede; in the latter, the first segment is confounded with the part bearing the mouth. The epimeral elements of the thorax are distinguishable; the sternal surface is very narrow, and is covered with complicated folds and ridges. The six pair of legs are all close, one behind the other, and all are alike in having a haunch or pedicel of two segments, directed forwards, bearing two arms or rami, each composed of two segments, the outer ramus being a little longer than the inner one. On the lower segments in both rami of all the limbs, there is a single spine. In all the limbs, the obliquely truncated summit of the terminal segment of the inner ramus bears three very long, beautifully plumose spines: in the first pair, the summit of the outer ramus bears four, and in the five succeeding pair, six similar spines. This difference, small as it is, is interesting, as recalling the much greater difference between the first and succeeding pairs, in the first and second stage of development. The terminal segments of all the rami, bearing the long plumose spines, are directed backwards. The limbs and thorax are well furnished with striated muscles. The animal, according to Mr. King, swims with great rapidity, back downwards. The limbs can be withdrawn within the carapace.

Abdomen and Caudal Appendages.—The abdomen is small, and its structure might easily be overlooked without careful dissection of the different parts: it consists of three segments; the first can be seen to be distinct from the last thoracic segment, bearing the sixth pair of limbs, only from the fold of the epimeral element, and from its difference in shape; the second segment is very short, but quite distinct; the third is four or five times as long as the second, and bears at the end two little appendages, each consisting of two segments, the lower one with a single spine, and the upper one with three, very long, plumose spines, like those on the rami of the thoracic limbs. The abdomen contains only the rectum and two delicate muscles running into the two appendages, between the bases of which the anus is seated.

Internal Viscera.—Within the body, in front of the mouth, it was easy to find the stomach (with two pear-shaped cæca at the upper end), running first anteriorly, and then curving back and reaching the anus by a long rectum, difficult to be followed: it appeared, however, to me, that this stomach had more relation to the young Cirripede, of which every part could now generally be traced, than to the larva, with its closed and rudimentary mouth: the fact, however, of its being prolonged to the anus, which is in a different position in the larva and mature state, shows that the stomach serves, at least, as an excretory channel. Besides the stomach, the several muscles already alluded to, and much pulpy and oily matter, the only other internal organs consist of two long, rather thick, gut-formed masses, into the anterior ends of which the cement-ducts running from the prehensile antennæ could be traced. These masses are formed of irregular orange balls, about .001 of an inch in diameter, made up of rather large cells, so to have a grape-like appearance, held together by a transparent pale yellowish substance, but apparently not enclosed in a membrane: these masses lie rather obliquely, and approach each other at their anterior ends; they extend from above the compound eyes, to the cæca of the stomach to which they cohere, but in young specimens, they extend some way beyond the cæca, between the folds of the carapace. The two cement-ducts, at the points where they enter these bodies, expand and are lost; at this point, also, the little orange-coloured masses of cells have the appearance of being broken down into a finer substance. Within the cement-ducts I saw a distinct chord of rather opaque cellular matter. We shall presently see, that these gut-formed masses are the incipient ovaria.

The Young Cirripede within the Larva.—Several times I succeeded in dissecting off the integuments of the lately-attached larva, and in displaying the young Lepas australis entire. The following description applies to the Cirripede in this state; but for convenience sake, I shall occasionally refer to its condition when a little more advanced. I may premise, and the fact in itself is curious, that the bivalve-like shell of the larva, together with the compound eyes, is first moulted, and some time afterwards, the inner lining of the sack, together with the integuments of the thorax and of the natatory legs: hence, I often found specimens, which externally seemed to have perfected their metamorphoses, but which, within their sacks, retained all the characters of the natatory larva. According to Mr. King, the larva of Lepas throws off its external shell five days after becoming attached. Whilst the young Lepas is closely packed within the larva, the capitulum, as known by the five valves, about equals in length the peduncle. The peduncle occupies the anterior half of the larva; when fully stretched, it becomes narrower and slightly longer than the capitulum; the separation between the capitulum and peduncle is almost arbitrary in the mature animal, and corresponds with no particular line in the larva. Even at this early period, the muscles of the peduncle are quite distinct. No vestige is preserved in the outer integument, of the sternal and dorsal sutures of the larval carapace; but in the corium of the peduncle, three coloured marks which occur near the eyes, and two little curled marks which occur near the acoustic orifices of the larva, are all preserved for some time after maturity. The compound eyes, as we have seen, are attached to apodemes, springing from the sternal surface of the larval carapace, and are consequently cast off with it: whilst the young Cirripede is packed within the larva, the outer integument of its peduncle necessarily forms a deep transverse fold passing over the eyes and apodemes, and this, as we shall presently see, plays an important part in the future position of the animal. The antennæ are not moulted with the carapace, but left cemented to the surface of attachment; their muscles are converted into sinewy fibres, the corium after a short period is absorbed, and they are then preserved in a functionless condition. No trace of the two acoustic sacks can be perceived in the corium of the young Cirripede, excepting the coloured marks above alluded to.

In the young capitulum, the five valves stand some way apart from each other; they are elegant objects under the microscope; they are not calcified, but consist exclusively of chitine; they are rather thick, composed of an outer membrane lined by hexagonal prisms, quite unlike any other membrane in the animal. These valves, which I have called primordial valves, resemble pretty closely in shape the valves of the mature animal; the fork of the carina, however, is indicated only by a slight constriction above the lower end. After the exuviation of the larval integuments, and when calcification commences, the first layer of shell is deposited under, and then round these primordial valves. The latter, in well preserved old specimens, may often be detected on the umbones of the scuta, terga, and carina, but not on the umbones of any other valves.

The mouth seems one of the earliest parts developed: in the youngest larva dissected, I could make out at least points corresponding with each organ; and, at the period when the young Cirripede could be dissected out of its larval envelopes, their general details were quite plain. The labrum, however, had not become bullate. The mouth, as we have seen, is formed under the rudimentary mouth of the larva, and at the same relative spot occupied by the probosciformed mouth of the larva in the second stage. Thus far, in the young Cirripede and larva, there has been no great change in the relative positions of the parts: the rudimentary eyes, however, of the former are developed posteriorly to (or above, as applied to a Cirripede,) the cast-off compound eyes of the larva; but the position of the mouth, of the antennæ, and of the several coloured marks in the corium, prove to demonstration, the correspondence in both of part to part. The case is rather different with what follows.

The Cirri are developed at first of considerable length, so that the young animal may soon provide itself with food; in [Lepas australis] they are of great length, the sixth pair consisting of seventeen or eighteen obscure segments. The extreme tips of the twenty-four rami of the six pair of cirri, are formed within the twenty-four, corresponding, little, bi-segmental rami of the six pair of natatory legs; but as the cirri are many times longer than these legs, they occupy in a bundle the whole thorax of the larva; no part whatever of the thorax of the Cirripede is formed within the thorax of the larva, but (together with the pedicels of the anterior cirri) within the cephalic cavity. As a consequence of this, the longitudinal axis of the thorax of the young Cirripede lies almost transversely to the longitudinal axis of the larva; and the Cirripede, from this transverse position of its thorax, comes to be, as it were, internally, almost cut in twain, and the sack thus produced. As soon as the young Cirripede is free and can move itself, the cirri are curled up, and the thorax is advanced towards the orifice of the capitulum, its longitudinal axis resuming the position of approximate parallelism to the longitudinal axis of the whole body, which it had in the larval condition. The reader will, perhaps, understand what I mean, if he will look at the mature Cirripede, figured in [Pl. IX], [fig. 4]. In this, he will see that the body or thorax is united to the peduncle only by a small part below the mouth; on the other hand, if he imagines the whole bottom of the body (as high up as the letter h) united and blended into the peduncle, he will see the state in which these parts exist in the larva. Now, let him greatly shorten the cirri, so as to resemble the natatory legs of the larva, and then imagine a young Cirripede, with cirri of full length, formed within the old one, he will see that the new thorax supporting the cirri will have to be developed in an almost transverse position,—the animal consequently being internally almost separated into twain.