which baffle all conjecture, they begin to pass over this valley every morning in a westerly direction, and return in the evening to their eastern roosting-place in the woods of Nostell Priory.

Rooks are observed to keep up a very close and friendly intercourse with starlings [VII. 183.] and jackdaws [VI. 394. 516.]; but, on looking at them in the fields, the observer will perceive, that, while the jackdaws mix promiscuously with the rooks, both in their flight and in searching for food, the starlings always keep in their own flock. This circumstance has long engaged my attention; but I am no farther advanced in the investigation than I was on the first day on which I set out. It is one of the many secrets in the habits of birds, which will, perhaps, be for ever concealed from our view. Walton Hall, Nov. 27. 1833. CHARLES WATERTON.

[FOR remarks, by Mr. Waterton, "on the nudity on the forehead and at the base of the bill of the rook," see V. 241 -245.; and for observations, also by Mr. Waterton, "on the supposed pouch under the bill of the rook," see V. 512515. In Captain Brown's edition of White's Natural History of Selborne, which is noticed in VI. 133., there is a figure of a domestic hen in male plumage ;" and in p. 93, 94., in a long note, Captain Brown has adduced some instances of this phenomenon which he had read of, or seen.]

66

ART. III. An Introduction to the Natural History of Molluscous Animals. In a Series of Letters. By G. J.

Letter 12. On their Respiration.

THE respiration of the Mollúsca 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 Mollúsca, 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 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 ventricòsus, "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 1 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 Mollúsca tunicàta. 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 Mollúsca 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 Mollúsca, 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 Mollúsca 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 Mollúsca, with the exception of the Cephalopoda and the Tunicata, 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 me 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 (Mýtilus 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 Mollúsca 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 ex

cretory 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.