But Mr. Thompson has since, in a paper read before the Royal Society on the 5th of March 1835, deelarod his " discovery of the metamorphosis in the second type of the Cirripedes, namely, the Lepadea, completing the natural history of these singular animals, and confirming their affinity with the Cruatacea ;" and the Memoir, with a plate, is published in the second part of the Philosophical Transactions' for 1835. The following is the abstract of the paper:— "The discoveries made by the author of the remarkable metamor phoses which the animals composing the first family of the Cinipedes, or Balani, undergo in the progress of their development, and which he has published in the third number of his Zoological Researches,' ( p. 76), are in the present paper, which is intended as a prize essay for one of the royal medals, followed up by the report of his discovery of similar changes exhibited by three species of two other genera of the second tribe of this family, namely, the Lepades. The !erne of this tribe, like those of the have the external appearance of Bivalve 31onoculi, furnished with locomotive organs, hi the form of three pairs of members, the most anterior of which are simple, and the other bifiii. The back of the animal is covered by an ample shield, terminating anteriorly in two extended horns, and posteriorly in a single elongated epinona process. Thus they possess considerable powers of locomotion, which, with the assistance of an organ of vision, enable them to seek their future permanent place of residence. The author is led from his researches to the conclusion that the Cirripedes do not constitute, as modern naturalists have considered them, a dis tinct class of animals, but that they occupy a place intermediate between the Crtutaeca decapoda (with which the Balani have a marked affinity) and the Crustacea entomostraca, to which the Lepedes are allied; and that they have no natural affinity with the Testaceous if °Mora, as was supposed by Linnrcus, and all the older systematic writers on zoology." Mr. Thompson does not seem to have been aware of a paper by Dr. J. Martin-Saint-Ange, read at the Academy of Sciences on the 14th July 1834, and published in the Sevens Etrangera' (tome vi.), and separately by ]l,•hillil`re (1835). The following is the summary of the principal facts stated by him in the course of a very laborious and acute investigation :—The mouth of the Podunculatod Cirripedes is composed of pieces entirely comparable to those of the mouths of many Crustacea, and especially of the Phyllosomrs; the upper lip, the palpi, and the mandibles are so analogous that the resemblance extends even to the form. The three jaw-feet (piede-mfichoires), which are met with most .commonly in the Crust acre, are conjoined in a single jaw-foot which receives the nervous trunks ; at its base are always found from two to four branchim. The ten ordinary feet of the Crustacea are faithfully represented in the Anatifas (Campyloso watts); at the base of many among them arefound branchim disposed like those of certain Crustacra, and the number even is sometimes repeated. There exists in each foot a double canal, fit for establish ing a circulating current, and traversing all the articulations of the cirri. The body is composed of a certain number of rings, or of articulations, very distinct, each of which supports a pair of feet. In the interior of the body there are a dorsal vessel (like that in a great number of the Articulated Animals), and a double series of ganglions; of which the number, aceordiug to Dr. Martin-Saint-Ange's researches, is equal to that of the feet; there is besides another pair on the lateral parts of the stomach. The pedicle may be regarded as analogous to the tail of many Crustacea; it is in this cavity, and not, as has been said, on the back, that the eggs are found; these pass afterwards by a conduit, not yet indicated, in the envelope, which, by its resemblance to the mantle of the ,ifollusca, establishes the only possible analogy between the Cirripedes and the last-named animals. The organs placed upon the back, which envier described as eggs, are the generative apparatus of the male, of which the disposition is very remarkable. Finally, the stomach and intestinal canal inclose in the interior a membranous sac of a retort-shape; the disposition and use of which establish, according to the researches of 1L Serres, an additional approximation between the Cirripedes and the Aunehides. Dr. 3Iartin-Saint-Ange then proposes, as the last result of his labours, to place the class Cirripedia at the end of the Crustacea, so as to estab lish a natural link or passage between the superior Articulated Animals and the Annelida. Snell are the conclusions drawn in the Memoir of Dr. Martin-Saint-Ange, who refers with approbation to the dis coveries of Mir. Thompson, published in 1830; and before we proceed to give a further account of the structure of the an-leer/la we will state Mr. Thompson's view of the ovarial system. "In the whole of the tribe of the Cirripedes," observes Mr. Thompson, in his paper in the 'Philosophical Transactions' above quoted, "the ova, after expulsion from the ovarium, appear to be conveyed by the ovipositor into the cellular texture of the pellicle, just beneath the body of the animal, which they fill to the distance of about an inch. When first placed in this situation, they seem to be amorphous and inseparable from the pulpy substance in which they are imbedded ; but as they approach to maturity they become of an oval shape, pointed at both ends, and are easily detached. Sir Everard Home has given a very good representation of them, at this stage of their progress, in his ' Lectures on Comparative Anatomy,' from the elegant pencil of Mr. Bauer. During the stay of the ova in the pedicle, they render this part more opaque and of a bluish tint ; the ova themselves, and the cellular texture with which they are surrounded, being of a pale or azure-blue colour. It is difficult to conceive in what manner the ova aro extricated from the situation above indicated ; but it is certainly not by the means suggested by Sir Everard Home in the above-mentioned lecture, namely, by piercing outwards through the membranes of the pediele, for the ova are subsequently found forming a pair of leaflike expansions, placed between either side of the body of the animal and the lining membrane of the shells in ',pas (Pen. talasrnis), or of the ,leathery internal tunic in Cineras. These leaves have each a separate attachment at the sides of the animal to the septum, which divides the cavity occupied by the animal from that of the pedicle ; they are at first comparatively small, have a rounded outline, and pongees the same bluish colour which the ova had in the pedielo; but as the ova advance in progress theme leaves extend in every dimension, and lap over each other on the back, pursing through various lighter shades of colour into pale-pink, reel finally, when ready to hatch, become nearly white. These leaves appear to be com posed of a layer of ova irregularly placed, and imbedded in a kind of parenchymatous texture, out of which they readily fall when about to hatch, on its substance being torn asunder ; indeed, it appears at length to become so tender as to fall entirely away, so that after the period of gestation is past no vestige of these leafy conceptaclea is to be found. When the larva,, barely visible to the naked eye, burst forth from the ova, their development goes on with such rapidity that they seem to grow sensibly while under observation. The larva of the ',peaks then is a tailed Monocaus, with three pains of members, the most anterior of which are simple, the others bitid, having its back covered with an ample shield, terminating anteriorly in two extended horns, and posteriorly in a single elongated spinout process." The following observations on the development of the lame of these animals, by Mr. Darwin, are amongst the latest contributions to this interesting inquiry :—" The ova, and consequently the larva, of the Lepadidte, in the first stage, whilst within the sac of the parent, vary in length from .007 to .009 in Lopas, to .023 of an inch in Seal pelt um. My chief examination of these lame has been confined to those of Sealpelluni rulgare ; but I saw them in all the other genera. The larva is somewhat depressed, but Dearly globular; the carapace anteriorly is truncated with lateral horns ; the sternal surface is flat and broad, and formed of thinner membranes than the dorsal. The horns just alluded to are long in Lepas and short in Scalp-Bum ; their ends are either rounded and excessively transparent, or, as in Ma, furnished with an abrupt minute sharp point. Within these hones I distinctly saw a long filiformed organ, bearing excessively fine hairs in Biwa, so exactly like the long plumose spines on the prehensile ruitemire of time larvae in the last stage, that I have not the least doubt that these horns are the cases in which antenna, are in process of formation. Posteriorly to them on the sternal surface, near each
other, there are two other minute doubly-curved pointed home, about in length, directed posteriorly; and within these I again saw a most delicate articulated filiformed organ and a thicker pedicle. In an excellent drawing by Mr. C. S. Bate, of the larva, of a Chthamaltut (Balanus punctatus of British authors), after having been kept alive and moulted once, these organs are distinctly shown as articulated antennae (without a case), directed forwards : hence, before the first moult in Scalpelluin we have two pairs of internee in process of formation. Anteriorly to the bases of these mailer antenna: is seated the her:La-shaped eye (as I believe it to be), of aim inch in diameter, with apparently a single lens, surrounded, except at the apex, by dark reddish pigment-cells. In porno cases, as in some species of Lepas, the large, when first excluded from the egg,, have not an eye, or a very imperfect one. There are three pairs of limbs, seated close together in a longitudinal line, but some way apart iu a transverse direction. The first pair always consists of a single apinose mime ; it is not articulated in Scali:glum, but is multi-articulate in some genera; it is directed forwards. The other two pairs have each two mini, supported on a-common haunch or pedielo ; in both pairs the longer runes is multi-articulate, and the shorter ramiis is without articula time, or with only traces of them ; the longer apices borne on thee() limbs (at least in Scalpel, um and Chthamalus) are finely plumose. The abdomen terminates a little beyond the posterior end of the carapace in a slightly upturned horny point. A short distance anteriorly to this point, a strong epinomie forked projection depends from the abdominal endue. Messrs. V. Thompson, Goodsir, and Bate have kept alive for several days the lamer of Lepas, Conehoderma, Balanus, Verruca., and Chilamalus, and have described the changes which supervene between the first and third exuviation.s. The most connpienous new character is the great elongation of the posterior point of the carapace into an ahnost filiforrn spines° point in Lepas, Cuncloderma, Clithamalus, and Baton us, but not, according to (loodsir, in one of the species of the latter genus. The posterior point also of the abdomen becomes developed in Balanus (Gootimmir) into two very long spearlike processes, serrated on their outer sides ; in Lepas and Conchoderant, according to Thompson, into a single tapering sphere projection; and iu Chthamal us, as figured by Mr. nate, the posterior bitid point, as well as tile depending ventral fork, lucre:wee much in size. Another important change, which has been particularly attended to by Mr. Bath, is the appearance of spines,• projections and spines (some of which are thick, curved, and strongly plumose or almost pectinated along their inner sides) on the pedicles anti lower segments of the shorter rani of the two posterior pain; of limbs." In this stage of the growth of the larva, Mr. Darwin found the mouth in Scalpellurn redgare seated on a very alight prominence in a most remarkable situation, namely, in a central point between the bases of the three pairs of legs. Mr. Darwin continued :—" I traced by dissection the (esophagus for some little way until lost in time cellular and oily matter filling the whole animal, and it was directed anteriorly, which is time direction that might have been expected from the course followed by the (esophagus in the larva in the last stage, and in the mature Cirripede." The larva, in its second stage of development, is known only from a single specimen described and figured by Burmeister (`Bei trap zur Naturgeschichte der Rankenftisser,' s. 16). In its general shape and compressed form it seems to come nearer the last than the first stage. It has only three pairs of legs, situated much more posteriorly on the body than in the first stage, and all directed posteriorly. They are much shorter than in their earlier stages. They are undoubtedly the three pairs of limbs of the first stage metamorphosed. The chief development of the larva since its first stage is towards its anterior end.
In the last stage the larva have increased many times in size since their exclusion from the egg. They are now much compressed, nearly of the shape of a Cypris, or mussel-shell, with the anterior end the thickest, the sternal surface nearly or quite straight, and the dorsal arched. Almost the whole of what is externally visible consists of the carapace, the thorax and limbs being hidden and inclosed by its baCkward prolongation, and even at the anterior end of the animal the narrow sternal surface can be drawn up, so as to be likewise inclosed. The antennae are large and conspicuous. They are at first well-furnished with muscles, and serve as organs of locomotion, and apparently as feelers ; but their main function is to attach the larva, preparatory to its final metamorphosis into a Cirripede. The disc can adhere even to so smooth a surface as a glass tumbler. The attach ment is at first manifestly voluntary, but soon becomes involuntary and permanent. .Mr. Darwin makes the following remarks upon the eyes and mouth in their last stage :— " posterior and rounded margins of the basal articu lation of the above-described prehensile antennae are reflected inwards, in the form of two forked horny apodemes, together resembling two letters U U 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 Lepasaustralis,eachAth of an inch in diameter, are attached to the outer arms, thus, • U U •, in the posi tion of the two full stops. Hence the eyes are included within the cara pace. Each eye consists of eight or ten lenses, varying in diameter in the same individual from to 4-„th of an inch, inclosed 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-poiuted 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 cer tain Crustacea. In specimens just attached, in which no absorption has taken place, two long muscles with transverse striae may be found attached to the knobbed tips of the two middle arms of the two • U U • and running up to the anterodorsal surface of the carapace, where they are attached. Other muscles (without transverse stria) 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 antennae within the carapace.
"Mouth.—This is seated in exactly the same position as in the mature Cirripede, on a 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 oesophagus. Hence the larva, in this, its last stage, cannot eat. It may be called a locomotive pupa ; its whole organisation is apparently adapted for the one great end of finding a proper site for its attachment and final metamorphosis." In this stage the thorax is much compressed, the six pairs of legs are all close one behind the other. 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 ramps bears four, and in the five succeeding pairs six similar spines. The abdomen is small, and consists of only three segments : it contains only the rectum, and two delicate muscles running into two appendages, between the bases of which the anus is situated.