happened even after twelve hours had elapsed from the exsection. The heart, in another experiment, being exposed and laid naked to the eye, was observed to beat spontaneously for a few seconds, then to stop for about the space of a quarter of an hour, when it again began to beat quickly and violently without any obvious cause, so that this learned physician thinks it certain that, in these animals, the motion of the heart is not beyond its control, but also voluntary or subject to the will,*- an inference in which you will not be disposed to coincide. In a small Snail just hatched, Bradley counted about three seconds between each pulsation; and in an old Snail from six to seven seconds elapsed.† Gaspard saw the heart of a Vineyard Snail (Helix pomatia) beating, in the summer, twentyfive to twenty-eight times in a minute; and that of a Pondmussel contracted, according to Pfeiffer, fifteen times in the minute. On examining a specimen of Carychium lineatum our mutual friend, Mr. Alder, observed, through the transparent shell, that its heart beat only eight times in a minute, and he was about to conclude that the slowness of locomotion, in this class of animals, resulted from the tardy circulation of the blood, when, on examining a few specimens of Vitrina pellucida, he was surprised to find that their heart beat about a hundred and twenty times in a minute! The latter was in a state of action, and the former of rest. He has since found, that the number of pulsations in the same individual is very unequal at different times,-a variableness dependent on external influences, doubtless, and principally, perhaps, on the temperature of the air or water to which the animals are exposed. §

The blood itself is of a bluish-white colour, and glutinous consistence.|| Lister tells us, that when he kept the

Exer. Anat. de Coch. p. 38. Exer. Anat. tert. p. 13.

+ Phil. Ac. of the Works of Nature, p. 129.

Tiedemann's Comp. Phys. i. 156.

Mr. Garner states that in the Lamellibranchiata the pulsations of the heart are generally from twenty to thirty in the minute.-Charlesw. Mag. N. Hist. iii. 168. Messrs. Alder & Hancock have given the number of pulsations in the following Nudibranchiata:

Sir E. Home says that the blood of the Teredines is red, Comp. Anat. i. 32; and that of the Planorbis is almost purple.-MILNE-EDWARDS, Elem. de Zool. p. 18. Milne-Edwards has found, in the neighbourhood of Palermo, an Ascidia with red blood.—Ann. and Mag. N. Hist. xv. 69.

blood of a Snail in a vessel for some days, it remained liquid and entire, not separating, in the manner of human blood, into two portions of unequal densities; but, when he applied heat, it readily congealed into an opaque bluish coagulum, just as the human serum would have done under the same circumstances. But Lister knew well that the blood of these creatures was not homogeneous; for he adds, that with a good microscope it is easily shown to consist of globules swimming in a limpid fluid; that these globules are truly round, and considerably exceed in size those of human blood; they are also heavier than the fluid part, since they gradually sink to the bottom when kept still in a glass tube.* The late experiments of Prevost and Dumas have confirmed those of the old English naturalist: they have ascertained that the globules of the Snail have a diameter one third greater than those of man† and quadrupeds; and, what is more remarkable, they found the globules to be really spherical, as Lister has asserted, although analogy would have led us to a different conclusion; for they are elliptical in birds, reptiles, and fishes, to which the mollusca are certainly more nearly allied than they are to the mammalia. The globules in the bivalve mollusca are also, according to Poli, much larger than in man; so that he considers the latter to be to the former as hemp-seed to milletseed. § The red colour of blood has been attributed to the

Exer. Anat. de Coch. p. 95, and Exer. Anat. tert. p. xxxiii.--"Si tamen in bono microscopio examinetur, id est, syphone capillari vitreo, venæ alicui intruso, globulos opacos verè orbiculares haud paucos videbis; ac sanguineos nostros globulos magnitudine plurimùm excedentes. Hi vero globuli, ut præ sanguinis globulis pauci sunt, ita aqua quædam limpida innatant, et paulatim præ gravitate ad imum syphonem descendunt. Idem quoque experimentum de succo vitali in Cochleis fluviatilibus feci; idemque coagulum subcaruleum, igni admotus, dedit."

+ The red globules of human blood, according to the observations of Mr. Bauer, as corrected by Kater and others, are one five-thousandth part of an inch in diameter.-HOME's Comp. Anat. vol. iii. p. 4., compared with p. 12. But in the foetus, the globules, say Prevost and Dumas, differ in their form and volume from those of the adult; the former being double the size of the latter. BOSTOCK's Physiology, vol. ii. p. 200; and approximating nearer, of course, to the size of those of mollusca. The fact is curious, when considered in relation to some speculations of Carus.

Zoological Journal, i. 178.

§ Rudolphi's Physiology, trans. i. p. 132.-Mr. Lister found the particles of blood in the Ascidia to be not uniform in size or shape, but they were mostly between 00025 and 0002 inch in diameter, and approaching to globular.-Phil. Trans. for 1834, p. 380. Milne-Edwards says that the bloodglobules of invertebrated animals differ greatly from those of the vertebrated: their size is very variable in the same individual, their surface appears wrinkled, neither a central nucleus nor an exterior envelope can be distinguished, and their form is in general spherical.—Elem. de Zoologie, p. 21.

existence of iron in it in combination with phosphoric acid; but it perhaps militates against this hypothesis when we find that the white blood of the mollusca, although the contrary has been asserted, contains the same mineral ingredients; for Erman has detected iron, and very probably also manganese, in the blood of the Helix pomatia and Planorbis corneus; and Poli likewise speaks of iron in the blood of Arca glycymeris. * As the following analysis may probably be applied with safety to the whole class, I extract the passage entire, notwithstanding it repeats some particulars already noticed:-"The blood of the Helix pomatia," says M. Gaspard, "is rather thick, but without viscosity; it has a faint smell, a slightly saline taste, and is so abundant that each individual contains not less than a drachm and a half. It is of a delicate blue colour, which is neither altered nor modified by change of aliment, by asphyxia, or by hybernation. It is miscible with water, but of greater specific gravity, and falls to the bottom in visible streaks or entire drops. When exposed to the atmosphere it does not spontaneously congeal, like that of vertebrated animals, but it separates by rest into two distinct fluids; the one blue, which swims at the top; the other colourless and opaque, remaining at the bottom of the vessel. In a few days it decomposes with fetor. It is unaltered by muriate of barytes and by alcohol; is simply discoloured by potash, and by vinegar and other weak acids: but acetate of lead, nitrate of silver, and, still more, nitrate of mercury, occasion a copious dense precipitate. Boiling water, sulphuric and nitric acid, coagulate it strongly, like albumen."†

Rudolphi's Physiology by How, i. 113. ↑ Zool. Journal, i. 177.

225

LETTER XIV.

ON THEIR SECRETIONS.

THIS letter you may properly consider as a continuation of the preceding; 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. SHELL.-The most important and general secretion of molluscous animals is shell, but as the structure and formation of this must occupy a separate letter, I will at present confine myself to some account of its chemical composition, with such other remarks as may not find a more appropriate place in our correspondence.

However varied in external character, shells differ very little in their chemical composition. They all consist of carbonate of lime united to a soft albuminous matter, and any variation that occurs in different shells is merely in the relative proportions of these constituent parts. Mr. Hatchett, to whom we are indebted for all our correct information on this subject, has divided shells into two classes, according to the proportion and state of their animal matter. The first class he names Porcellaneous Shells, since they resemble porcelain, are usually of a compact texture, and have an enamelled surface, which is often finely variegated. The convolute shells afford good examples of this class. They consist of carbonate of lime, cemented together by so small a portion of albumen, that, when immersed in a dilute acid, the shell is completely dissolved, and not a sensible trace of it left behind. "The shells belonging to the second class are usually covered with a strong epidermis, below which lies the shell in layers, and composed entirely of the substance well known by the name of mother-of-pearl. They have been distinguished by the name of Mother-of-pearl Shells." The fresh-water mussels, the oysters, the Haliotis, and pearly Turbines are examples of this class. "When immersed in acids they effervesce, at first strongly, but gradually more and more feebly, till

at last the emission of air-bubbles is scarcely perceptible. The acids take up only lime, and leave a number of thin membranous substances, which still retain the form of the shell." These membranes have the properties of coagulated albumen.* But the distinction between these two classes holds good only in extreme cases; for there are many shells which are intermediate, and stand on debatable ground. The compact bivalves dissolve in the menstruum entirely, as does also the Common Whelk (Buccinum undatum), but they are not properly porcellaneous; while the various landsnails leave an insoluble membrane, though they are not perlaceous shells.

Mr. Hatchett thought that he discovered some traces of phosphate of lime in the shell of the Common Garden-snail (Helix aspersa); but as he did not find any in that of the Helix hortensis, it may be doubted whether the presence of phosphate of lime should be considered as a chemical character of land-shells. According to Raspail and Prevost bivalve shells, in the first periods of their growth, consist wholly, or nearly so, of this latter earth,—a discovery as inexplicable as it was unexpected.

It has been made a question from what source the animal obtains the earthy material of its shell. Some, considering the vastness of the quantity required, or rather eliminated, by the millions of testaceous mollusca now living, and that have lived through all ages, and the enormous extent of the formations that have resulted from the deposition of their remains, have been led to ascribe to the animal a power to produce or generate lime by some interior, though yet unknown process, regulated by the allcontrolling influence of the living principle. In proof of the existence of such a power, they remind us that the proportion of lime contained in sea-water, or river-water, and in their food, is obviously too small to furnish all that is excreted; and moreover it has been ascertained that snails confined, so that during a whole summer they have had no access to any preparation of lime, yet did secrete and form very considerable portions of shell. It is dif*Thomson's Chemistry, v. 554, 555. Edinb. 1807. * Phil. Trans. abridg. xviii. 556.

To the instances formerly given, I may add another, furnished by Sir John G. Dalyell. "I have seen snails which were kept months in water only, void very sensible excretions, and increase the size of their shells, though these continued uncommonly transparent, nor could the excretions be the remains of food in the stomach, for the snails had never ate; they were young, and I had bred them from the egg."-SPALLANZANI'S Tracts, trans. i. xliii.