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unadulterated before the aperture is shut. This process is not so obvious in the branchial Mollásca, 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 Cephalópoda, in which inspiration and exspiration are well marked. “The first 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 ventricösus, “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 13 in. in length, I saw respire 18 times per minute; and the larger one, mentioned above, respired 10 times per minute. The time seemed to be pretty equally divided between inspiration and exspiration.” In the bivalves, whose cloak forms a shut sac, the water is sucked in through the siphonal 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 Mollásca 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. (Cuvier, Mollus. Mem. xx. p. 17.; Coldstream, in Edin. New Phil. Journ. for July, 1830, p. 240.)

I have told you that the respiration of the Mollásca 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, Dr. Sharpey has discovered that, in “ the Mollusca, and other inferior tribes of aquatic animals, the external covering of the body generally, but especially of the respiratory organs, possesses the power of impelling the water contiguous to it in a determinate direction along the surface, by which means a constant current is kept up, and the blood exposed to the influence of successive portions of the surrounding element: this peculiar provision effecting, in those creatures, the same purpose as the respiratory muscles in the more perfect animals." These currents, in the Mollusca at least, and probably in all the animals in which they have been detected, are produced by the action of minute cilia, visible only with a glass, which are in constant motion, and clothe all the surfaces along which the currents are excited. Similar cilia had been observed on the eggs and organs of many zoophytes by previous naturalists, and in a few naked Mollusca, by Dr. Fleming; but the merit of proving their existence in all the great families of the Mollusca, with the exception of the Cephalopoda and the Tunicàta, and of pointing out their use, is due to Dr. Sharpey. Carus came near the discovery; for he observed the currents in question, but left uninvestigated their cause; or, rather, he attributed the phenomena to one which has probably little efficiency. His words are :

“ In a living bivalve, it is easy to observe that the water gains access to the branchial laminæ by the fissure in the cloak, and escapes by the anal tube, which serves also to evacuate excrement and ova. It has not, however, been hitherto noticed, that this current is uninterrupted, and that thus these animals, when not too deeply immersed, form an eddy on the surface of the water. But as, in almost all other animals, the influx of air or water to the respiratory organs is intermittent, the simultaneous and continuous current into the fissure of the cloak and out of its tube, of which I have satisfied myself by numerous observations, 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.” (Comp. Anat., transl. vol. ii.

p. 148.)

As this discovery appears to me the most important which has been made of late years in the physiology of these animals, you will permit nie to transcribe, for your perusal, a paragraph of considerable length from Dr. Sharpey's Essay, with a view of giving some farther illustration of the process.

“When a live muscle (Mytilus edulis) is attentively examined in a vessel of sea water, it is soon observed to open its shell in a slight degree, and about the same time a commotion may be perceived in the water in its vicinity. This is occasioned by the water entering at the posterior or large extremity of the animal, into the cavity in which the gills are lodged, and coming out, near the same place, by a separate orifice, in a continued stream. This current is obviously intended for the purpose of renewing the water required for the respiration and nutrition of the animal; but, though it is now a well-established fact in the history of the muscle, the mechanism by which it is produced has not, so far as I know, been satisfactorily explained. Some have contented themselves with ascribing it to an alternate opening and shutting of the shell; but, as no such motion takes place in the shell, except at distant and irregular intervals, it is evident that the constant passage of the water cannot be explained in this way. Others, who saw the insufficiency of this explanation, have endeavoured to account for it by assuming peculiar contractions and dilatations of the mantle in virtue of its muscular power, or, like M. de Blainville, have supposed that the triangular labial appendages placed round the mouth excited the current by their constant motion. After meeting with the currents in the tadpole, it struck me that the entrance and exit of the water in the bivalve Mollásca might not improbably be owing to a similar cause; and that the surface of the respiratory organs, and other parts over which the water passed, might have the power of exciting currents in it, the combined effect of which would give rise to the entering and returning stream. “This conjecture proved, on actual examination, to be right. Having cut off a portion of the gill, I found that a current was excited along its surface in a determinate direction, and that it moved itself through the water in an opposite one, exactly as in the case of the tadpole. The whole surface of the gills and labial appendages or accessory gills, the inner surface of the cloak, and some other parts, produced this effect. The currents on the gills are of two kinds. When finely powdered charcoal is put on any part of their surface, a great portion of it soon disappears, having penetrated through the interstices of the vessels into the space between the two layers of the gill. On arriving here, a part is forced out again at the base of the gill from under the border of the unattached layer, but most of it is conveyed rapidly backwards in the interior of the gill between the two layers, and almost immediately escapes at the excretory orifice, or that from which the general current already mentioned is observed to come out. That portion of the powder which remains outside the gill is carried along its surface in straight lines from the base to the margin, along which it then advances onwards towards the fore part of the animal. As the spaces between the layers of all the gills terminate directly or indirectly at the excretory orifice, it is easily conceivable that the water, penetrating by the entire surface of these organs, may, by their concentrated effect, give rise to the powerful current which is observed to come out from the animal. “On examining a portion of the gills with a powerful lens, I perceived that it was beset with minute cilia, which are evidently instrumental in producing the different currents. Most of them are ranged along the anterior and posterior margin of each of the vessels composing the gills, in two sets: one nearer the surface, consisting of longer and more opaque cilia; the other close to the first, but a little deeper, in which they are shorter and nearly transparent. Both sets are in constant motion, but of this it is difficult to convey a correct idea by description. The more opaque cilia, or those of the exterior range, appear and disappear by turns, as if they either were alternately pushed out and retracted, or were continually changing from a horizontal to a vertical direction. The motion of the other set appears to consist in a succession of undulations, which proceed in a uniform manner along the margin of the vessel from one end to the other. It resembles a good deal the apparent progression of the turns of a spiral when it revolves on its axis, and might very easily be mistaken for the circulation of a fluid in the interior of a canal, more particularly as the course of the undulations is different on the two edges of the vessel, being directed on the one towards the margin of the gill, and on the other towards the base. But, besides that the undulations continue to go on for some time in small pieces cut off from the gill, which is inconsistent with the progression of a fluid in a canal, the cilia are easily distinguished when the undulatory motion has become languid. When it has entirely stopped, they remain in contact with each other, so as to present the appearance of a membrane attached to the edge of the vessel. “It is very remarkable, that, when the gill is immersed in fresh water, both the currents and the motion of the cilia are almost instantaneously stopped.” " * On a peculiar Motion excited in Fluids by the Surfaces of certain

Animals; by William Sharpey, M.D. Edin. Med. and Surg. Journal, vol. xxxiv. p. 118, &c.

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 Mollásca, 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 proportion 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 Mollásca under the same circumstances; for they not only consume all the oxygen, but actually continue afterwards to exspire carbonic acid gas: consequently, after the respiration 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.” (Edin. New Phil. Journ., April, 1833, p. 383.") These observations may serve to

"...The Rev. Mr. Guilding has conjectured that some Mollásca may even purity water:—“Neritinae are destroyed with great difficulty: some,

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