"From these facts, I have been led to conclude that a pearl is formed upon the external surface of an ovum; which, having been blighted, does not pass with the others into the oviduct, but remains attached to its pedicle in the ovarium, and in the following season receives a coat of nacre at the same time that the internal surface of the shell receives its annual supply. This conclusion," he adds, "is verified by some pearls being spherical; others having a pyramidal form, from the pedicle having received a coat of nacre as well as the ovum."* This conclusion, however, is far from being true. I will not deny that the fact may be, in not a few instances, as stated by Sir E. Home, for the ovum may accidentally fall into a situation where it shall become a source of irritation like any other extraneous substance; but that this is often the case is contradicted by numerous observations, and by the true theory of the formation of pearls. Professor Baer, of Koenigsberg, aware of Home's theory, undertook an investigation of it in the freshwater mussels of Germany, and the result was, that he never met with pearls either in the ovaries, liver, kidney, or any of the internal organs. The pearls were always situated either in or under the skin of the back, where it is close to the shell. + I shall conclude what I have to say concerning pearls with the following extract from the paper of Mr. Gray quoted in the preceding page:-"The pearls are usually of the colour of the part of the shell to which they are attached. I have observed them white, rose-coloured, purple, and black; and they are said to be sometimes of a green colour.+ They have also been found of two colours; that is, white with a dark nucleus, which is occasioned by their being first formed on the dark margin of the shell before it is covered with the white and pearly coat of the disk, which, when it becomes extended over them and the margin, gives them that appearance. "Pearls vary greatly in their transparency. The pink are the most transparent; and in this particular they agree with the internal coat of the shell from which they are *Comp. Anat. v. 302; and Phil. Trans. 1826, p. 339. + Edinb. New Phil. Journ. xiv. 186. There is a very interesting paper on the growth and structure of pearls in the Edinb. Phil. Journal, xi. 39, &c. See also Reaumur in Mém. de l'Acad. Roy. des Sciences, an. 1717, p. 243, &c. "There are, besides, [in Britain] several sorts of shellfish, among which are mussels, containing pearls often of the best kind, and of every colour; that is, red, purple, violet, green (prasini), but principally white, as we find in the Venerable Bede's Ecclesiastical History."-RICHARD OF CIRENCESTER, trans. p. 28. formed; for these pearls are only formed on the Pinnæ, which internally are pink and semi-transparent, and the black and purple specimens are generally more or less opaque. "Their lustre, which is derived from the reflection of the light from their peculiar surface produced by the curious disposition of their fibres, and from their semi-transparency and form, greatly depends on the uniformity of their texture and the colour of the concentric coats of which they are formed. That their lustre does depend on their radiating fibres, may be distinctly proved by the inequality of the lustre of the Colombian pearls,' which are filed out of the thick part near the hinge of the Pearl-oyster (Avicula margaritifera); so that they are formed, like that shell, of transverse laminæ, and they consequently exhibit a plate of lustre on one side which is usually flat, and are surrounded by brilliant concentric zones, which show the places of the other plates instead of the even, beautiful, soft lustre of the true pearls." 3. COLOURS.-The colouring of the shell is a part of the theory of its formation, which will be more fittingly discussed hereafter; but since their colours depend on the secretion of a peculiar matter in the mucous skin, this may be not an improper place to introduce some facts I have collected relative to the colours of the animal considered abstractedly, and which are principally intended to prove to you that there is not any strict correspondency between those of the tenant and its testaceous covering. Those parts of the body of the mollusk which are constantly covered are usually of a uniform white, a straw, or a greyish colour; and the dark spots with which they are clouded are almost always occasioned by the opacity of the internal viscera or their contents; but the organs extended beyond the shell, and which have felt the influence of the light, are very often vividly tinted and variegated; and you may deem it very probable that the intensity of the colours will be deepened or mitigated by the higher or lower latitude of the shore inhabited by even the same species. Our native Cowry (Cypræa europæa) is a plain white shell, but its snail is a very elegant creature. The proboscis is dark vermilion; the tentacula yellowish red, spotted with yellow; the upper part of the foot streaked longitudinally with yellow and brown; and the mantle greenish brown, edged with brownish red; but, notwithstanding, the shell is a uniform dull white. Similar discrepancies between the colour of the shell and its owner are often met with thus, the Cypræa voluta of Montagu (Marginella voluta, Fleming) has its fins or lateral expansions elegantly speckled with bright yellow, and the fleshy part of its body with pink. The long proboscis of the Aporrhais pes-pelicani is pink, dotted over with milk-white spots; and the animal of the white Scalaria clathrus is mottled black and white. Mr. Collier says, of some tropical species, that the foot is "blackish red in the Murices generally; green in Strombus, and some species of Trochus; black in Bulla ovum; deep red with faint designs, like those of the shell, in Conus tulipa, marmoreus, and its varieties; spotted, in Buccinum harpa; bright yellow, in Buccinum cassis; mottled, in Oliva; and deep brown, from spots, in some species of Voluta."+ The snail of the beautifully marbled Harp-shell (Harpa ventricosa, Lamarck) glories in a rich vermilion red skin. "In the Mauritius, it is the amusement of the place to watch over the trim apparatus of lines hung over some sand-bank to tempt the various brilliant species of Oliva which there abound, or to wait for the more rare approach of the harpshell, till the rich hues of its inhabitant are seen glowing through the clear blue water in the rays of a tropical rising sun." M. Rang has discovered that the animal of a species of Sigaretus, described in the Bulletin of the Linnæan Society of Bordeaux, changes its colour three or four times during its life, a circumstance which may easily lead to error, by inducing observers to consider as distinct species what are merely varieties dependent upon age; and it appears that Sigaretus is not a solitary instance of such mutability among the Gasteropods. § The colours of the naked mollusca are very various : there are black, white, grey, brown, yellow, red, and even green species; and the colours are sometimes uniform and single, but more commonly mixed, and disposed in freckles * Montagu Test. Brit. p. 204. + Edinb. Phil. Journ. Oct. 1829, p. 228. Mr. Collier uses the Linnæan names. Broderip in Zool. Journ. ii. 199. "L'animal des Tridacnes et des Hippopes offre de fort belles couleurs. Celui de la Tridacne safranée, décrit par MM. Quoy et Gaimard, est d'un superbe bleu de roi sur les bords, linéolé en travers de bleu de ciel; plus en dedans est une rangée de lunules d'un jaune verdâtre; le centre est d'un violet clair, avec des lignes longitudinales ponctuées de brun. On a sous les yeux l'un des plus charmants spectacles que l'on puisse voir, lorsque, par une petite profondeur, un grand nombre de ces animaux étalent le velouté de leurs brillantes couleurs, et varient les nuances de ces parterres sous-marins. Comme on n'aperçoit que leur ouverture bâillante, on ne peut pas se figurer ce que c'est au premier aspect." -CHENU Trait. de Conchyl. 87. § Edinb. Journ. of Nat. and Geogr. Science, i. 455. or clouds. Cranchia bonnellii affords probably the most remarkable example of colouring in the class. This bizarre Cephalopod, whose form involuntarily reminds us of those fantastic beings with which the genius of Callot has peopled hell, and which the opera sometimes imitates in its marvellous scenery, appears to rival the butterflies of tropical suns in gaudiness and brilliancy. A broad membrane unites its six upper arms into a large veil of a very rich purple colour, adorned with six double rows of buttons of sapphire-stone formed by the cups or suckers. The ventral surface of the sac, of the head, and of the two inferior arms, is studded with yellow spots arranged in quincunx, and near each spot there is another, in relief, of blue. These yellow and blue spots lie on a reddish ground sprinkled with purple dots, and have such a lustre in the living animal that they resemble as many topazes, near each of which a sapphire has been mounted. To dwell, however, on this subject would be useless; and I pass on to notice the very curious phenomena exhibited in the coloured spots of the Cephalopoda. The surface of these remarkable creatures, particularly the back and sides, is speckled with numberless minute coloured dots, which vary in size, tint, and arrangement, in the different species; and in the same species are liable to change, in the same respects, according to their degree of developement. These dots are properly follicles, or little bladders, seated in the mucous web (rete mucosum), and consequently covered by the epidermis, which is smooth and transparent. "During life, when the animal is in a state of repose, the vesicles are contracted, and are not visible. When it is excited, by being touched with the hand, or otherwise irritated, the coloured vesicles show themselves and are instantly in motion, appearing and disappearing with the velocity of lightning: sometimes they are like spots on different parts of the body; and sometimes like waves, which rapidly move across its surface." These appearances are produced "by the rapid and simultaneous. contraction which takes place in all the vesicles of a particular part of the body, and from the sudden and simultaneous expansion of all the vesicles on another part; " but the process may go on until the whole body is covered, and its natural colour become changed for that of the vesicles. Even long after death these vesicles may be made to exhibit the same alternate contractions and expansions on the application of slight irritation. * Ann. des Sc. Nat. n. s. iii. 344. Naturalists have been long acquainted in some degree with these singular phenomena. Pliny tells us that the cuttle-fish change their colour through fear,* adapting it, cameleon-like, to that of the place they are in; and some expedience-dogmatists have meanly recommended us to imitate their accommodating quality— "Apud homines cum eris, tibi in mentem veniat polypi, None of the older authors, however, attempted to investigate their cause; but, of late, several theories have been offered, and two of these are founded on a minute inquiry into the structure of the vesicles. Cuvier said, conjecturally, that the appearances were dependent on the effusion of a coloured fluid in the areolar tissue of the skin; ‡ and Professor Grant refined upon this hypothesis, by supposing the fluid to pass repeatedly to and from the minute vesicles: § but this conjecture has been apparently disproved, for the spots have no connection with any vascular system, nor do the vesicles contain any encysted fluid. Dr. G. San Giovanni, of Naples, an intelligent comparative anatomist, offered *Hist. Nat. ix. 46. + Plutarch compares flatterers to the Polype. In his Orat. de Patient. et Tolerant., he says, "Nor must the wiles and fraud of the Polype be passed over in silence. To whatever rock it adheres it imitates and assumes its colour, and thus many fishes while swimming strike against the polype as against a rock, and become the prey of its craftiness. Similar in manner are those who always frequent the company of those who have power and command, and who so accommodate themselves to times and occasions, that they never remain permanently of one opinion, but carry themselves hither and thither and change their opinions at any one's will and pleasure." Clearchus's advice is,— "Polypi, mi fili, Amphilocles heros, mentem habe, Mém. i. 7. SEdinb. New Phil. Journ., xvi. 313.-This opinion is also adopted by H. Milne-Edwards, who seems to rest it upon experiments which are certainly at variance with those of Dr. Coldstream. H. Milne-Edwards says, "The skin of these animals is furnished with a number of differently-coloured spots, which alternately appear and disappear; and if a portion is put under a microscope, it may be perceived that these changes depend on the contraction of small vesicles filled with a coloured liquid, which reach from the surface of the skin to a considerable depth. When one of these spots appears, the liquid corresponding here to the pigment in the other case is propelled towards the superficial part of the vesicle, and there displays itself; whilst during its disappearance it is forced into the deeper parts by the contraction of this superficial point itself, which then becomes almost invisible."-Edinb. New Phil. Journ. xvii. 319. |