One ortwo things connected with this circulatory system are worthy of special notice. The first is, that its direction is not constant, but reversible. Afterwe have watched this course followed with regularity for perhaps a hundred pulsations or so, all of a sudden the heart ceases to beat, and all the globules rest in their circling course, that we had supposed incessant. Strange to behold, after a pause of two or three seconds, the pulsation begins again, but at D 2 the opposite end of the heart, and proceeds with perfect regularity, just as before, but in the opposite direction. The globules, of course, obey the new impulse, enter at their former exit, and pass out at their former entrance, and perform their circulation in every respect the same as before, but in the reverse direction.

Those globules that pass through the vessel into the foot-stalk appear to accumulate there as in a reservoir, until the course is changed, when they crowd into the heart again, and perform their grand tour. Yet there is a measure of circulation here, for even in the connecting vessel one stream ascends from the reservoir into the body, as the other (and principal one) descends into it from the heart ; and so, vice versa.

I have spoken of these motions as being performed with regularity ; but if you look closely, you will see that this must be understood with some qualification. The pulsations are not quite uniform, being sometimes more languid, sometimes more vigorous ; perhaps forty beats in a minute may be the average ; but I have counted sixty, and, presently after, thirty; I have counted twenty beats in one half-minute, and only fifteen in the next. The period during which one course continues is equally uncertain ; but about two minutes may be the usual time. Sometimes the pulsation intermits for a second or so, and then goes on in the same direction ; and sometimes there is a curious variation in the heart's action,—a faint and then a strong beat, a faint and a strong one, and so alternately for some time.

The phenomena of respiration are so closely connected with those of circulation that it is not at all out of the way to turn from the latter to the former; not to say that it would be high treason against scientific curiosity if I were to remove this object without explaining to you that marvellous play of wheels that occupies the largest part of the area that you behold. As you look on the globe, you observe, hanging down from the upper extremity, and reaching nearly to the bottom in one direction and almost from side to side in another, a transparent square veil, which is indeed a flat membranous bag, having its sides pretty close together, with small openings along its edges, and an orifice at the bottom leading into the stomach.

The mouth of this sac is in close connexion with the upper or principal orifice, and therefore receives the water, which is constantly flowing in, while that aperture is expanded. This fluid then bathes the whole interior of the sac ; but a portion of it escapes by the lateral openings into the cavity of the body, between the sac and the mantle, and is discharged through the secondary, or side orifice.

The inner surface of this transparent sac is studded with rings of a long oval figure, set side by side in four rows. These rings appear to consist of a slight elevation of the general membranous surface, so as to make little shallow cells, the whole edges of which are fringed with cilia, whose movements make waves, that follow each other round the course in regular succession. In truth it is a beautiful sight to see forty or more of these rings, all set round their interior with what look like the cogs on a watch wheel, dark and distinct, running round and round with an even, moderately rapid, ceaseless motion. These black running figures, so like cogs and so well defined as they are, are merely an optical delusion ; they do not represent the cilia, but merely the waves which the cilia make : the cilia themselves are exceedingly slender close set hairs, as may be seen at the ends of the ovals, where a slight alteration of position prevents the waves from taking the tooth-like appearance. Sometimes one here and there of the ovals ceases to play, while the rest continue ; and, now and then, the whole are suddenly arrested simultaneously, as if by magic, and presently all start together again, which has a most charming effect. A still more singular circumstance is, that while in general the ciliary wave runs in the same direction in the different ovals, there will be one here and there in which the course is reversed ; and T think that the animal has the power of choosing the direction of the waves, of setting them going and of stopping them, individually as well as collectively.

The object of these ciliary wheels is to keep up a constant current in the water. This fluid, as I have said, enters from without, through the upper orifice of the body, and is hurled over the whole surface of the breathing sac by means of the ciliary waves, parting with its oxygen, as it goes, to the blood, which streams, as we saw, everywhere between the rows of wheels. But the water has another function : it carries particles of organic matter with it, which are suitable for the nourishment of the creature; these atoms are carried by the currents to the bottom of the sac, and are poured into the stomach, where they are digested ; the remains, together with the waste water, being discharged through the lateral orifice.

Thus we see how closely connected are the three great processes of circulation, respiration, and digestion.

Chapter III. Mollusca: their Shells, Tongues, Eyes, and Ears

One of the most interesting aspects of microscopic study is that in which it reveals the intimate structure of objects, which, to the unassisted eye, appear simple or nearly so, but which prove by the aid of magnifying power to be complex. Thus we are often introduced to very curious contrivances (if I may use such a word in reference to the works of God), by which difficulties are overcome, and substances, which would seem at first wholly unfit for certain duties, are in the most admirable manner adapted to fulfil them.

The combination of strength and lightness is always a difficult problem in human art ; its successful solution always excites our admiration. In the Divine mechanics, too, it is very often required, and the variety of modes in which it is accomplished are in the highest degree novel and suggestive. We lately saw one of these in the structure of a feather, in the contrivance by which extreme lightness of material was made, by a most remarkable arrangement, to offer a firm resistance to opposing force. I have now another example to show you, in which a material, in itself heavy, is by its arrangement made very light, while it preserves its strength.

You have seen many times, when walking along the yellow sands kissed by the rippling waves, the shell, or bone, as it is sometimes called, of the Cuttle-fish. You know that it consists of a shallow boat-shaped shell, the hollow of which is filled with a white substance, which can be scraped away even with the finger-nail, and which is sometimes used as pounce, to rub on paper from which writing has been erased. It is this substance of which I mean now to speak.

The possessor of this structure is a member of the numerous class Mollusca,* which are generally characterised by being inclosed in shells. Now shell, as we all knowr is a solid, stony substance, much heavier than water ; take into your hand that large Cassis on the mantel-piece, and observe its great weight and compactness. It is, in fact, real limestone; differing from that of the rocks only in this, that it has been deposited by the living organic cells of an animal, and arranged in a definite form. We will presently examine other examples. The " cuttle-bone " is a shell, not indeed inclosing the animal, but inclosed by it, being contained within a cavity in the substance of the fleshy mantle ; cut open the mantle, and the shell instantly drops out.

The Cuttle is a rapid swimmer through the open sea. A shell so large as this, if solid and compact like that of the Cassis, would condemn it to grovel on the bottom, and frustrate all the instincts of its nature. On the other hand, it needs the strength and support of a solid column. Wonderful to tell, the calcareous f shell is made not only to be no hindrance to its swimming, but to contribute greatly to its buoyancy : it is what the string of corks is. to the bather who cannot swim, it is afloat. Throw this entire cuttle-shell into water ; it floats on the surface asbuoyantly as if it were actually carved out of cork.

I cut with a keen knife a little cube out of the shell,, and, fixing it on the end of the revolving stage-needle, apply a low power, say seventy diameters, using reflected

From the Latin mollis, soft ; a name given by Cuvier to this class from their bodies being always soft, whether bearing shells or not.

T From calx, Latin for lime.

light. We are looking now at the perpendicular section ; is it not a beautiful object ? you might fancy yourself 1 looking at one of the noble icebergs that majestically navigate the polar seas, when it is rendered porous and laminated by the rains of spring. You see a number of thin horizontal tiers or' stages, perfectly parallel and equi-distant, about one-fortieth of an inch apart, rising above each other like the floors of an editice. These are connected together by an infinite multitude of thin pillars of crystal, or a Perpendicular. CUTTLE-SHELL. b Horizontal. rather leaves, some of which show their edges towards us, others their broader sides, and others are broken off at various distances, the fragments standing up from the floor, or depending from the roof, like stalactites and stalagmites in a cavern.*

This whole series of crystal floors and supporting plates is formed of calcareous matter,—limestone, in short ; but though the latter are set in such close array that the eye cannot penetrate to any appreciable distance between them, their extreme thinness renders the whole structure very light, the interstices being occupied by air.

But now if I give the stage-needle half a revolution, we shall have the horizontal section presented to the eye. In this aspect we acquire much more information as to the structure. The cut has been made very close to one of

In calcareous districts the water trickling through into caverns often forms shapeless masses on the floor or hanging like icicles from the roof; in the former case they are called stalagmites, in the latter stalactites.

the horizontal floors, which we see marked all over with a great number of lines, each of which runs hither and thither, in a very sinuous pattern. The lines are made up of a brilliant sparkling substance ; they are, in fact, the basal portions of what we saw in the other section as thin perpendicular plates ; I have cut oft* the plates close to the bottom, and what we see is their insertion into the floor.

Thus we perceive that what we took for a multitude of plates, were but the various doublings and infoldings of a single plate of great length, running quite across the floor : an arrangement by which the strength of the material is greatly augmented. You have often seen the mode in which light walls are made of corrugated iron, especially at railway stations; and are doubtless aware that the corrugation, or bending in or out, imparts a strength to it which the mere sheet iron, if set up as a smooth, plane surface, would in no wise possess. The principle is exactly the same in the two cases ; but the corrugation of the limestone plates in the cuttle-shell is far more perfect than that of the iron ; added to which there is the other advantage, that the aggregate mass of material is made highly buoyant by the large bulk of empty space that intervenes between the sinuous folds of the crystal plates.

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