Oldalképek
PDF
ePub

66

On the PROPERTIES of the VIVIPAROUS SNAIL.

[From SPALLANZANI'S MEMOIRS on RESPIRATION.]

"ERRE

§ 1. ERRESTRIAL animals, which exist always in an aëriform medium, furnished the subjects of the preceding experiments; but in this order, even among those which are furnished with calcareous shells, a much greater number inhabit rivers, ponds, lakes, and salt water. I have shown that land testacea absorb the oxygen contained in the atmosphere, and that they perish when deprived of it: but do the aquatic testacea absorb, in the same manner, the ogygen diffused in the water; and is this absorption as necessary to their existence as it is to that of the land testacea? If respiration, however different its mode of action in different species of animals, be, notwithstanding, a function essential to all living beings, we cannot doubt that nature has subjected to the same law the inhabitants of the water: it must, however, be acknowledged that we have no direct proofs on this subject, and that our ideas are altogether built on those of analogy.

"This subject is nevertheless agreeable and interesting in itself, and also of great importance in the animal economy: such were the motives and the views which induced me to enter on the investigation of aquatic snails, after having examined land snails in the same manner. The marshes of Pavia furnished me with three fresh-water crustacea.

"§ 2.-The first was the viviparous snail, Helix vivipara of Linn. Several reasons operated on me to examine this animal. It does not lay eggs, but brings forth its young.

I remarked on this occasion the following circumstance, which is pe culiar to this species of snail, and which I never observed in other animals placed, like the testacea, among the lowest order of living beings. All these viviparous or oviparous animals have a certain season when they produce their young or lay their eggs. To some of them nature seems to have appointed the spring for this operation, to others summer or autumn, and to a few even winter ; but those which multiply twice a-year are not very common. I am certain, however, that these snails, of which I have spoken, give birth to their young during every season of the year. I have kept them for seve ral years in my house, in large vases filled with water, covered at the bottom with muddy earth, by which they are nourished; and I observed young snails upon the

mud in winter as well as in the three other seasons, although they multiplied in far greater abundance during the spring.

"An attention to the anatomy of these animals corroborates this observation. If we cautiously divide the body of one of them, we discover that its uterus is a canal which opens externally, and contains a greater or less number of fœtuses or small snails, as well formed as the mothers that nourished them in their matrix. Some snails contain fifteen small ones, others twenty-five, and I have reckoned thirty-five in a third: in short, in one of them I even found seventy. The largest of them are

P

usually

usually in the lowest region of the uterus, as being the most mature, and most prepared for exclusion. "If we remove the young from the uterus into a glass filled with water, they at first fall to the bottom, because they are specifically heavier than this fluid; but it was not without surprise that I afterwards perceived them leave their shells with their antennæ pointed on their head, crawl on the bottom of the glass, climb up its sides, and ascend to the surface of the water. These snails had then arrived at maturity, and consequently were at the point of exclusion. I was furnished with an irrefragable proof of this, in observing some of them, taken from the uterus by a kind of Cæsarean operation, evolved in the vessels wherein I had placed them.

"3.-But this investigation presented another interesting pha

nomenon.

On viewing attentively the most elevated portion of the uterus, we observe the fœtuses become gradually less, and in fact they are transparent eggs, in which

we

can distinguish the fœtuses themselves with the greatest clearness. They were partly without the shell, which was already formed, and swimming in a fluid that at this period served them for nourishment. But these eggs, of a globular form, and lined with a subtle membrane under the shell, were of different sizes: in the smallest, when viewed through a microscope, we perceived already the small animal like a point of organized matter, but the shell was not yet visible.

"I discovered, then, that these small snails, which the large ones brought forth, proceeded from an egy hatched in the uterus of the

mether.

"This observation is valuable, since it shows that an animal termed viviparous, because it brings forth its young, should at first be called oviparous, since this progeny originally proceeds from an egg which is hatched in the ma

trix.

"As anatomical researches have likewise discovered other animals similar to these under the same point of view, analogy would induce us to believe, that those which are termed viviparous, generally considered, derive, in like manner, their origin from an egg.

"54.-In order not only to gratify my own curiosity, but excite that of my students, I opened, in the course of my public lectures, a great number of these snails; and I can affirm that they all contained young ones in the uterus; which made me suspect that this species, like a variety of land snails, participate of both sexes. But if this opinion be just, are they what is strictly termed hermaphrodites? that is, can they procreate their species without copulation, like the fresh-water polypusand several other worms? Or, rather, are they less strictly hermaphrodites? and must we suppose that the concurrence of two individuals is necessary to fecundation, as is the case with land snails? We observe, however, the two sexes united in each individual of these last animals, and in their intercourse they reciprocally fecundate each other. We look, however, in vain for these two sexes in the viviparous snail: must we then suppose that these snails are true hermaphrodites?

"If this be the case, copulation cannot be essential to their fecundation, and each individual must

possess

possess within itself the faculty of reproducing its species. In the constant attention I have given to these snails, with a view of studying their habits, when they were in clear and shallow water, I have always been solicitous, especially during spring and summer, to observe if they actually copulated, as we frequently witness in land snails: but I was never able to discover them in this situation; from which circumstance alone I am not, however, warranted to affirm they are really hermaphrodites, because it was very possible the act of copulation might be performed during my absence.

"Having determined to make a decisive experiment, I took, with this intention, several snails from the uterus, and placed them in small holes filled with water, so that there was only a single individual contained in each hole. I dug six of them in a place near Pavia, well supplied with water by means of subterranean springs, and which, after having been once formed, would remain full of this fluid during the whole year. In the spring the holes were filled to three-fourths of their depth, and I placed in each of them a single small snail immediately on being taken from the uterus. On emptying the holes, at the end of three months, in each of them 1 found the snail I had placed in it, but somewhat increased in bulk.

I refilled the holes with water, and again placed in them the same snails: I repeated my visits, and in the following year the four which only now remained had increased to double their original size. I conjectured that the others had perished, because it was not sup posable they would have left their natural element. In the second year there was a proportional increase of bulk; and at the commencement of the third I was enabled, by the two which only then remained, to solve the question which at first induced me to enter on this experiment.

"In the bottom of one of the two holes, which contained the remaining animals, I found three, and in the other four, small snails; but, on breaking the shell of the parent animals, I discovered in the uterus the young in all their different stages of growth, as I had formerly observed in the others, and even eggs still smaller than any which had previously fallen under my observation.

"Since, then, each of these ani mals had been constantly kept separated from all others, it affords a demonstrative proof that copula, tion is not necessary to the multiplication of this species, and conseque tly that they are true herma phrodites; and, so far as I know, this property had not hitherto been discovered in the other species of aquatic or land snails.”

[blocks in formation]

ANALYTICAL EXPERIMENTS and OBSERVATIONS on LAC.

[From Mr. HATCHETT'S MEMOIR in the PHILOSOPHICAL TRANSACTIONS.]

"THE period is uncertain when HE period is uncertain when the substance called lac, so curious in its origin and so useful to many arts, was first introduced into Europe; and although it probably was known to the antients, yet the inaccuracy of their descriptions precludes this from being stated as a positive fact.

"The natives of India have long employed it for various purposes, exclusive of those which cause it to be in request with Europeans; but many of the Indian processes are undoubtedly as yet unknown to us. "One of these, of a very useful nature, was some time since obligingly communicated to me by Charles Wilkins, Esq. F.R.S. and has been the cause of this inquiry into the nature and properties of lac.

"Mr. Wilkins informed me, that the Hindûs dissolve shell lac in water, by the mere addition of a little borax; and the solution, being then mixed with ivory-black, or lamp-black, is employed by them as an ink, which, when dry, is not easily acted upon by damp or water. Upon trial, I found the fact to-be exactly as Mr. Wilkins had stated, and therefore made other experiments; but the results of these I shall at present omit, as they will

occur with more propriety and perspicuity in the latter part of this paper.

"In respect to the natural history of lac, we are much indebted to Mr. Kerr, Mr. Saunders, and Dr. Roxburght; from whose valuable communications to this society, we learn many curious par. ticulars concerning the formation of this substance, which, from their accounts, and from inspection, evidently appears to be the nidus or comb of the insect called coccus or chermeslacca,deposited on branches of certain species of mimosa and other plants.

"Lac is distinguished into four kinds; of which, however, only three are commonly known in commerce, viz. stick lac, seed lac, and shell lac; the difference of these, with that of the fourth, called lump lac, is as follows.

"1. Stick lac, is the substance or comb in its natural state, incrusting small branches or twigs.

"2. Seed lac, is said to be only the above, which has been separated from the twigs, and reduced into small fragments; but I suspect it to have undergone some other process, as I have found the best spe cimens to be very considerably deprived of the colouring matter §.

* Natural History of the Insect which produces the Gum Lacca. By Mr. James Kerr, of Patna. Phil. Trans. for 1781, p. 374.

+ Some Account of the vegǝtable and mineral Productions of Boutan and Thibet. By Mr. Robert Saunders. Phil. Trans. for 1789, p. 107.

Chermes Lacca. By William Roxburgh, M.D. Phil. Trans. for 1791, p. 228.

$ Mr. Wilkins informs me that the crude lac, as it is taken from the branches and twigs of the trees, is usually deprived of its colouring matter by boiling, having been previously reduced, by pounding, into small fragments. In Bengal, the silk-dyers are the people who thus produce what we call the seed lac, which they do for the sake of

the colour.

"3. Lump

[ocr errors]

"S. Lump lac, is formed from seed lac, liquefied by fire, and formed into cakes. And,

"4. Shell lac, according to Mr. Kerr and Mr. Saunders, is prepared from the cells, liquefied, strained, and formed into thin transparent laminæ, in the following manner. "Separate the cells from the 'branches; break them into small 'pieces; throw them into a tub of 'water, for one day; wash off the 'red water; dry the cells, and with 'them fill a cylindrical tube of cotton cloth, two feet long, and one ' or two inches in diameter; tie both 'ends, and turn the bag above a charcoal fire; as the lac liquefies, twist the bag, and, when a sufficient quantity has transuded the 'pores of the cloth, lay it upon a smooth junk of the plantain tree, and with a strip of the plantain leaf draw it into a thin lamella; 'take it off while flexible, for in a ' minute it will be hard and brittle*.' "The degree of pressure on the plantain tree, regulates (according to Mr. Saunders) the thickness of the shell; and the quality of the bag determines its fineness and transparency.

"Assam furnishes the greatest quantity of the whole of the lac now in uset.

"Mr. Kerr (speaking of stick lac) observes, that the best lac is of a deep red colour; for, if it is pale and pierced at the top, the value is diminished, because the insects have left their cells, and consequently these can be of no use as a dye or colour, but probably may be better for varnishes.

"The seed lac which I have examined, contained but little of the colouring matter, and appeared (as I have already observed) to have undergone some process of purification; but, of all the varieties, shell ac contains the least of the tingeing substance, as may well be expected, when the mode of preparing it is considered.

"It is remarkable, that although lac has been known, and imported into Europe, during so long a time that the date cannot now be ascertained, yet it has but little attracted the attention of chemists.

"The first chemist of eminence who mentions it, and the only one who has subjected it to any thing like a regular examination, is the younger Geoffroy, whose paper is published in the Mem. de l'Acad. de Puris for the year 1714. In this paper, Mr. Geoffroy seems to have been chiefly induced to examine it on account of its tingeing substance; but he nevertheless has not neglect. ed the substance which constitutes the cells. This he considers to be a sort of wax, very distinct from the nature of gum or resin. But it is to be observed, that he formed this opinion, not so much upon the results of chemical experiments, as upon the cellular construction observed in the stick lac, which, as he justly remarks, demonstrates it to be formed by insects, after the manner that the honeycomb is formed by bees; and that it is not therefore, as some have supposed, a gum or resin, which has exuded from vegetables simply punctured by insects.

Phil. Trans. 1781, p.378. + Phil. Trans. 1789, p. 109. Observations sur la Gomme Lacque, et sur les autres Matières animales qui fournissent la Teinture de Pourpre. Par M. Geoffroy le jeune. Mém. de l'Acad. 1714, p. 121. § Mr. Kerr observes, that as a red substance is obtained by incision from the plaso tree, very analogous to lac, it is probable, that the insects have little trouble in animalizing the sap of these trees, in the formation of their cells. Phil. Trans. 1781, p. 377. P3 "Geoffroy

« ElőzőTovább »