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LETTER XV.

ON THEIR RESPIRATION.

THE respiration of the mollusca is so slow, so little obvious, and so easily suspended for a time, that it is possible you may never have observed the process even in those species which daily cross your path. You will, therefore, in your next walk, please to examine the snail or the slug while they are in progression, and you will see them at intervals open wide a circular hole on the side of the neck, and near the margin of the shield or collar, and, after dilating it to the utmost, they will close it again until its place becomes imperceptible; this they do about four times in a minute, expelling at each time the effete air, and inhaling a fresh supply. In like manner, the aquatic tribes, while crawling along the surface, raise from time to time the pulmonary aperture, in order to emit the vitiated air, sometimes even with a crackling noise, and to receive an equal quantity unadulterated before the aperture is shut. This process is not so obvious in the branchial mollusca, and in many of them, from the position of the gills, such a function is not necessary to renew the water around them. Where, however, the gills are strictly internal, it seems probable that the water is regularly changed when the creatures are in their natural habitats and undisturbed: we know that such is the case with the Cephalopoda, in which inspiration and expiration are well marked. "The first (inspiration) is effected by a gradual dilatation of the sac in every direction, but particularly at the sides, accompanied by a subsidence of the lateral valves, collapse of the walls of the funnel, and a rush of water through the lateral openings into the sac. Inspiration being completed, the lateral valves are closed, the sac is gradually contracted, the funnel erected and dilated, and the water expelled through it with great force, and in a continued stream." Dr. Coldstream, from whose letter I quote the preceding sentence, has seen the stream emitted by an individual of the Octopus ventricosus, "whose sac measured about four inches in length, carry light bodies to the distance of eleven inches from the orifice of the funnel. Respiration is performed more frequently in young than in

adult individuals. One, whose sac measured an inch and a half in length, I saw respire eighteen times per minute; and the larger one, mentioned above, respired ten times per minute. The time seemed to be pretty equally divided between inspiration and exspiration." In those bivalves whose cloak forms a shut sac, the water is sucked in through the branchial tube, when the capacity of the sac is increased by its own expansion, or by the opening of the shells; and by its muscular contraction, aided sometimes by the closure of the shells, it is again expelled in a stream from the anal siphon but there is no regularity in the process in such species as I have observed in confinement. It is the same with the Tunicata. The branchial sac is muscular, and just as its capacity is enlarged, apparently by the contraction of its longitudinal fibres, the water flows in to fill the space in a slow and uniform current, through the branchial aperture only, for none can be detected entering by the anal orifice. It is, after a space, expelled again by a contraction of the annular fibres of the sac, but the voluntary contractions for this purpose, as stated above, take place at irregular intervals of time, and, for the most part, not oftener than once in a minute.*

I have told you that the respiration of the mollusca is at all times slow, and easily suspended for a long period; but, to obviate the inconveniences which might result from this, and to supply the place of that regularly alternate and ceaseless play of the respiratory muscles of the vertebrates, it has been discovered, principally from the researches of Dr. Sharpey, that in the Gasteropod and Acephalous mollusca, the surface of the respiratory organs, and of the cavity in which they are contained, is clothed with minute cilia, which, by their regulated motions, impel the water along the surface in a determinate direction; and by this means a constant current is kept up, and the blood exposed to the influence of successive portions of the surrounding element. Similar cilia clothe many parts of the external surface, and of the inner surface of the alimentary canal; and similar aqueous currents are, by this means, made to flow over them; as if nature meant to avail herself of every exposed part to plant there an auxiliary to the gills, so that under no circumstances should the important function of ventilating the blood and the other fluids be suspended or weakened. You may readily observe these interesting and beautiful phenomena †

Cuvier, Mem. Mollusc. xx. 17 : Coldstream in Edinb. New Phil. Journ. July, 1830, p. 240.

+ Leeuwenhoek, who had witnessed them in the common Mussel, with

by placing a very little naked Gasteropod in a watch-glass of sea-water under the microscope, when, on looking attentively, you will see the water flowing in a rapid even stream over the body and along the tentacula, always in one and the same direction; and a little experience in the use of the glass will soon enable you to discover the minute cilia by which this motion is produced. If you have not by you a living mollusk small enough for the experiment, you may cut off a portion from the branchiæ of any species you can most easily procure-no matter to what class it may belong-and you will find the detached piece exhibiting the same phenomena, and even swimming about in the current it has itself created; *-only this you must remember, that if the animal experimented upon is a marine one, sea-water must be used, for the action of the cilia and impulsion of the fluid are instantly stopped by putting the parts into fresh water.† The important discovery of these ciliary motions enables us to explain satisfactorily some appearances which puzzled the earlier anatomists. Thus Carus observed that the respiratory current of water that flows in upon the bivalves is not intermittent, as in almost all other animals, but uninterrupted, so that these animals, when not too deeply immersed, form an eddy on the surface of the water. Now as such a current cannot be caused by the alternate opening and shutting of the shell, Carus was induced to conjecture that it "must depend on a very peculiar mechanism, which consists chiefly in the muscularity of the cloak, but partly also in the mobility of the gills themselves, and may be compared to the mechanism of certain bellows, which produce an uninterrupted current of air by means of double bags." Blainville, on the contrary, supposed that the triangular labial appen

out, however, having detected the cilia, says "Upon examining that part of the Mussel which is called the beard, I not only found it of a wonderful make, but the motion I saw in the small component parts of it was so incredibly great, that I could not be satisfied with the spectacle; and it is not in the mind of man to conceive all the motions which I beheld within the compass of a grain of sand."-Select Works, i. 77. I agree with the fine old Dutchman; and the impression of the spectacle, as I saw it in the microscope of my friend Mr. Bowerbank, only deepens with reflection.

The soft gelatinous substance which supports the cilia on the branchia of the mollusca is readily detached in flakes by compression or friction. These little detached flakes move about, agitating their cilia, like living Infusoria; and Müller has described them as species of his genera Trichoda and Leucophra.-DUJARDIN. Hist. des Infus. p. 147 and 677.

Sharpey in Edinb. Med. and Surg. Journ. xxxiv. 118, &c. Edinb. Journ. Nat. and Geogr. Sc. ii. 334. Cyclop. Anat. and Phys. i. 619, &c. Edinb. New Phil. Journ. ix. 383.

Comp. Anat. trans. ii. 148.

dages placed round the mouth excited the current by their constant motion,-a very inadequate means, even were it true, which it is not, that these organs are in continual motion. They are, however, like the gills, clothed with cilia; and therefore are not undeserving the appellation they have sometimes got of "accessory gills."

The purpose of the respiratory organs and of the currents just described, is, to expose the blood freely to the purificative action of the atmospherical air, that it may be purged of some noxious qualities which it has acquired during its circulation through the venous system, and fitted again for the continuance of the life of the individual. In the vertebrate animals the blood is altered, even in its outward appearance, by this process,-from a dark it becomes a bright red fluid, but no perceptible change is operated on the white serous blood of the mollusca. Yet that it has experienced a similar purification is not to be doubted; for the air breathed by these creatures is similarly deteriorated, as it would have been had it been breathed by the quadruped or bird; the oxygen has disappeared, and its place become occupied by an equal bulk of carbonic acid gas. This had been proved by the well-known experiments of Spallanzani and other physiologists; and though, in general, the proposition holds good, yet it appears, from the recent experiments of Treviranus, that the absorption of oxygen is not always proportional to the excretion of carbonic acid, the proportion of the one to the other depending on the strength of the respiration, the time of its continuance while the respirability of the air is diminishing, and the volume of the air in which the respiration is performed. "The more carbonic acid," says Treviranus, "there is developed while breathing in the open air, and the less the power of continuing in a medium deficient in oxygen, the less is the proportion of the consumption of oxygen to the production of carbonic acid gas, whence a small quantity of atmospheric air is respired for a moderate period. But when the respiration is continued for a longer period in the same air, and the strength of the individual begins to sink, the excretion of the latter diminishes more rapidly than the absorption of the former. We know that the higher classes of animals, when enclosed in a certain quantity of air, die long before all its oxygen has been exhausted. The case is very different with many of the mollusca under the same circumstances; for they not only consume all the oxygen, but actually continue afterwards to exspire carbonic acid gas; consequently, after the respi

ration has been continued for some time, there has been more of the latter excreted than there has been consumed of the former; nay, sometimes this occurs even before all the oxygen has been consumed."* These observations may serve to explain, in some degree, the apparent apathy of the mollusca generally to a temporary deprivation of their respiratory media, for snails may be immersed in water for many hours without injury; † and the purely aquatic species will survive as long a time exposed to the atmosphere. "The species of Voluta and Buccinum generally (particularly B. oliva and B. harpa,") says Mr. C. Collier, "die in a few hours; those of Strombus and Murex survive thirty-six, forty-eight, and even sixty hours; Trochus niloticus and turritus live yet longer; and Strombus palustris will live several days." Oysters and mussels, as every one knows, and probably all the Conchifera, will live for three or four days without any more water to breathe in than what may lie in the concavity of their shells; and Mr. Boyle has some experiments which illustrate, in a remarkable manner, their tenacity of life even in vacuum. He found that two oysters put “into a very small receiver," exhausted of air, were alive at the end of twenty-four hours; "but how long afterwards they continued so I did not observe." § Another oyster was put into a vial full of water before being enclosed in the receiver, "that, through the liquor the motion of the (air) bubbles, expected from the fish, might be more pleasantly seen and considered. This oyster proved so strong as to keep itself close shut, and repressed the eruption of the bubbles, that in the other did force open the shells from time to time; and kept in its own air as long as we had occasion to continue the trials." || Shelled snails (Helices) appeared to be not more disordered in vacuity; and even the slugs (Limax) endured the privation for many hours. The same illustrious philosopher included two of the latter "in a

* Edinb. New Phil. Journ. April, 1833, p. 383. The Rev. Mr. Guilding has conjectured that some mollusca may even purify water:-"Neritinæ are destroyed with great difficulty: some, which were even kept close in salt water, seemed to have the power of purifying it, and rendering it fit for respiration; while many large air-bubbles were generated in the glass. Some power of this kind would be very valuable to those species which inhabit maritime ponds, the waters of which, nearly dried up at certain seasons, must be stagnant and unwholesome."―Zoological Journal, v. 33.

+ Müller mentions that a variety of Helix nemoralis lived a whole summer at the bottom of a rivulet.-Verm. Fluv. et Ter. Hist. ii. pref. xi. Edinb. New Phil. Journ. vii. 230.

Phil. Trans. 1670, p. 2023.

| Ibid. p. 2024.