not so glaringly have exposed his ignorance of the internal structure of the queen bee.

The male eggs, according to Strube, are placed in the small cells, and in the first part are bred working bees: the female eggs, however, as soon as the bees have brooded some of them in acorn-formed cells, become queens, which can breed both sexes, and from them spring the principal race of all the chief mother bees. The majority, however, of these female eggs are placed in small cells, and are bred as degraded queens. The remaining part of the working bees are those which, in the end, are only able to breed drones. The fructification of the queen is performed by the male working bees, and not by the drones: the drones, which are bred in the middle of the month of May, derive their existence from the degraded queens. The ovarium of these female bees cannot properly develope itself in the small cells; independently of which, they are in their nature considerably weakened. The impulse to breeding becomes thereby greatly moderated; and it is only in the warm months, that the freshness of the honey and the heat of the hive instigate these weak imperfect mothers to propagation. They mingle with the few drones which are at this time to be found in the hive, and contribute to the population of the republic. The eggs from which the first drones arise are laid towards the end of autumn; and, as during the winter they lie beyond the central heat of the hive, they do not develope themselves before the spring. It is only when there is a deficiency of male working bees that the queen is fructified by the drones.

(To be resumed.)

ART. V. An Introduction to the Natural History of Molluscous Animals. In a Series of Letters.

By G. J.

Letter 10. On their Secretions.

(Continued from p. 43.)

THIS letter you may properly consider as a continuation of the preceding (p. 31.); for the blood is the material out of which not only the growth and repairs of animal bodies, but likewise all the secretions, are directly derived; the latter obtaining their peculiar properties from differences in the structure and action of the various glands or vessels through which they percolate.

1. The most important and general secretion of molluscous animals is shell; but I have already had occasion to write so

fully of its constitution and formation [Letter 5., in Vol. III. p. 335-348.], that at present I shall confine myself to some additional remarks on internal shells, or such as are constantly covered by the cutaneous envelopes. So far as I remember, all of these are white, and generally thin; but they do not differ from other shells in their composition, unless we include among them the horny plates which strengthen the back of some Cephalopodes (Loligo), or protect the viscera of a few of the Gasteropodes (Lìmax); for these contain very little, if any, cretaceous matter. The plate of the slugs, which lies under the shield on the anterior part of the body, has been called the snail's stone, and used to be esteemed a serviceable medicine in the gravel and strangury. It varies in size and consistency in the different species; a fact of which Swammerdam does not seem to have been aware; for, to account for large slugs having "very small and membranous" plates, while smaller ones had them often much larger, and formed of "solid stone," he was inclined to think "that the snails change this their little stone yearly, in the same manner as crawfish change those two semiconvex and plain stones, which are likewise placed in their thorax, and are improperly called crab's eyes." (Book of Nature, p. 71.) But the most interesting of internal shells are, the Spírula Perònii of Lamarck, and what is commonly called cuttlefish bone. The former (fig. 104. b) is a pretty multilocular species, rolled up in a

spiral form, which lies partially embedded in the posterior part of a Cephalopode (a) that inhabits many warm seas; the latter is a calcareous production, of so singular a structure, that, according to Cuvier, it is unexampled in the animal kingdom, and may, therefore, merit a detailed description. The bone is placed in the back of the cuttlefish (Spia officinalis), within a membranous capsule, from the inner surface of which it is secreted, but with which it has no connection further than a slight adhesion from contact; for neither blood-vessels nor nerves penetrate the interior, and it is truly as unorganised, and as little within the influence of the living powers, as other shells are. In the very young Sèpiæ, the bone is horny; but in the mature cuttle it is entirely calcareous, if we except a thin membranous border. It is very light, of an elliptical figure, upwards of 6 in. in length, and not quite 3 in. in

breadth; slightly convex on the back, and considerably protuberant on the anterior side, where it is so soft that the nail, applied with a moderate force, cuts it easily. The figure a (fig. 105.) is a front view of this production, reduced to nearly

two ninths of the natural size: the convexity is greatest in the middle, whence it gradually declines towards both ends; the upper half being smooth and obtuse, while the lower is marked with imbricated curved lines, and contracts rather suddenly to a narrow point. But this point is surrounded with a broad prominent concave border, consisting of two portions closely united: the first opaque, and scored longitudinally with fine lines; the other perlaceous, and faintly lined transversely. The latter is continued not only along the margins, but extends over the whole opposite surface (b), which, from this circumstance, is much harder than the anterior; and it is also all over roughened with slightly raised knobs or granulations, arranged in obscure curved lines. To complete the view, I give two sections of the bone (c, a perpendicular, and d, a transverse, section), which will likewise disclose in part the internal structure. You perceive that the bellied portion is composed of very thin parallel and convex plates, which run slopingly from the anterior to the posterior surface; and that the intermediate spaces are filled with a much less compact material, formed of fibres in a crystalline state, placed perpendicularly, as is more clearly exhibited in the following figures (e and f). The plates are of unequal lengths; the lowermost, which are also the first formed, being the shortest, while those of the latest formation are the longest: and yet, however, they cover only half of the whole surface. The fibres of the intermediate substance are loosely connected together by other fibres running in the direction of the longitudinal plates, as is very obvious when a small and thin section is viewed through a magnifier (f): the

perpendicular fibres themselves are marked with many cross lines, and they are thickish, and of equal lengths (e), Cuvier says, positively, that they are little hollow columns, placed in a quincunx order (Mém., vol. i. p. 47.); but, according to Dr. Fleming, this is an erroneous account; and he describes them as "narrow lamina," which, when viewed laterally, appear like fine parallel threads; but, when examined vertically, are found to be waved, and fold upon themselves. Next the plate they are thin, and not much flattened; but towards their other extremity they become thicker, striated across, and more folded, with irregular margins," (Phil. of Zool., vol. ii. p. 436.); and this description agrees pretty exactly with my own ob

servations.

2. Pearl is another calcareous secretion of molluscous animals deserving notice. It is secreted only by the fish of bivalves; and principally by such as inhabit shells of foliated structure, as sea and freshwater muscles, oysters, the Pinna, &c. A pearl consists of carbonate of lime, in the form of nacre, and animal matter arranged in concentric layers around a nucleus, as it is well shown in our figure (fig. 106.),

106

copied from a beautiful engraving appended to a paper of Sir E. Home's in the Philosophical Transactions. Each layer is presumed, but I know not on what grounds, to be annual; so that a pearl must be of slow growth, and those of large size can only be found in fullgrown oysters. "It is the nacral lining of the central cell that produces the lustre peculiar to the pearl, which cannot be given to artificial ones." *

In a previous letter [Letter 5., Vol. III. p. 346.] it is stated that Dr. Brewster had proved the peculiar lustre of the pearl to be the effect of light reflected from the surface; an opinion first entertained, I believe, by the celebrated Robert Hooke. "Which, by the way, may hint us the reason of that so much admired appearance of those so highly esteemed bodies (pearls), as also of the like in mother-of-pearl shells, and in multitudes of other shelly sea-substances; for they each of them, consisting of an infinite number of very thin shells or laminated orbiculations, cause such multitudes of reflections, that the compositions of them, together with the reflections of others that are so thin as to afford colours (of which I elsewhere give the reason), give a very pleasant reflection of light: and that this is the true cause seems likely, first, because all those so appearing bodies are compounded of multitudes of plated substances; and, next, that, by ordering any transparent substance after this manner, the like pheno mena may be produced. This will be made very obvious by the blowing of glass into exceeding thin shells, and then breaking them into scales, which any lampworker will presently do; for a good quantity of these scales, laid in a heap together, have much the same resemblance of pearls."

Pearls, as Mr. Gray justly observes, are merely the internal nacred coat of the shell, which has been forced, by some extraneous cause, to assume a spherical form. They are, therefore, not properly " a distemper in the creature that produces them," and cannot, under any view, be compared with calculi in the kidneys of man (Lister, Hist. An. Ang., p. 150.); for, though accidental formations, and, of course, not always to be found in the shellfish which are known usually to contain them, still they are the products of a regular secretion, applied, however, in an unusual way, either to avert harm or allay irritation. That in many instances they are formed by the oyster, to protect itself against aggression, is evident; for, with a plug of this nacred and solid material it shuts out worms and other intruders which have perforated the softer shell, and are intent on making prey of the hapless inmate : and it was apparently the knowledge of this fact that suggested to Linnæus his method of producing pearls at pleasure, by puncturing the shell with a pointed wire. But this explanation, it is obvious, accounts only for the origin of such pearls as are attached to the shell; while we know that the best and the greatest number, and, indeed, the only ones which can be strung, have no such attachment, and are formed in the body of the animal itself. "The small and middling pearls," says Sir Alexander Johnston, "are formed in the thickest part of the flesh of the oyster, near the union of the two shells; the large pearls almost loose in that part called the beard." (Home's Lect. Comp. Anat., vol. v. p. 308.) Now, these may be the effect merely of an excess in the supply of calcareous matter, of which the oyster wishes to get rid; or they may be formed by an effusion of pearl, to cover some irritating and extraneous body. The reality of the latter theory is, perhaps, proved by a practice of the modern Chinese, who force the swan muscle (A ́nodon cýgneus) to make pearls, by

(Micrographia, p. 209., folio, 1667.) Sir E. Home disproves this explanation of Hooke and Brewster, so far as the same is applied to pearls, by the following experiment: -“ Úpon taking a split pearl, and putting a candle behind the cell, the surface of the pearl became immediately illuminated; so that the fallacy of my philosophical friend's opinion was made self-evident....The error my friends fell into was, taking for granted that the pearl was a solid body; and therefore, considering the subject mathematically, the brilliancy must be produced by the reflection from the nacral surface; but this reasoning was entirely inapplicable when applied to a sphere that is hollow." (Lectures on Comp. Anatomy, vol. v. p. 306., and Phil. Trans. 1826, part iii. p. 339.)

Lin. Corresp. by Smith, vol. ii. p. 429. Pearls formed somewhat in this manner, by the freshwater muscle, are preserved in the Hunterian Museum. Home's Lect. Comp. Anat, vol. vi. p. 296.