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tion to the successive stages of growth of the outer shell, maintains its efficacy as a float, enlarging gradually and periodically until the animal has arrived at full maturity.*

A fifth consideration is had of mechanical advantage, in disposing the Distance at which these successive transverse Plates are set from one another. (See Pl. 31. Fig. 1. and Pl. 32, Fig, 1, 2.) Had these distances increased in the same proportion as the area of the air-chambers, the external shell would have been without due support beneath those sides of the largest chambers, where the pressure is greatest for this a remedy is provided in the simple contrivance of placing the transverse plates proportionally nearer to one another, as the chambers, from becoming larger, require an increased degree of support.

Sixthly, The last contrivance, which I shall here notice, is that which regulates the ascent and descent of the animal by the mechanism of the Siphuncle. The use of this organ has never yet been satisfactorily made out ; even Mr. Owen's most important Memoir leaves its manner of operation uncertain; but the appearances which it occasionally presents in a fossil state, (see Pl. 32, Fig, 2, 3.,† and Pl. 33,) supply evidence, which taken in conjunction with Mr. Owen's re

* In a young Nautilus Pompilius in the collection of Mr. Broderip, there are only seventeen Sepia. Dr. Hook says that he has found in some shells as many as forty. A cast, expressing the form of a single air-chamber, of the Nautilus Hexagonus is represented in Pl. 42, Fig. 1.

† Pl. 32, Fig. 2, represents a fractured portion of the interior of a Nautilus Hexagonus, having the transverse plates (c. c'.) encrusted with calcareous spar; the Siphuncle also is similarly encrusted, and distended in a manner which illustrates the action of this organ. (Pl. 32, Fig. 2, a a1. a2. a3. d. e. f, and Fig. 3, d. e. f.) The fracture at Fig. 2, b. shows the diameter of the siphuncle, where it passes through a transverse plate, to be much smaller than it is midway between these Plates (at d. e. f.) The transverse sections at Fig. 2, a. and b, and the longitudinal sections at Fig. 2, d. e. f. and Fig. 3, d. e. f., show that the interior of the siphuncle is filled with stone, of the same nature with the stratum in which the shell was lodged. These earthy materials, having entered the orifice of the pipe at a in a soft and plastic state, have formed a cast which shows the interior of this pipe, when distended, to have resembled a string of

presentation of its termination in a large sac (P. 34, p, p,) surrounding the heart of the animal, (a. a.,) appears sufficient to decide this long disputed question. If we suppose this sac (p, p.) to contain a pericardial fluid, the place of which is alternately changed from the pericardium (p, p.)

oval beads, connected at their ends by a narrow neck, and enlarged at their centre to nearly double the diameter of this neck.

A similar distension of nearly the entire siphuncle by the stony material of the rock in which the shell was imbedded, is seen in the specimen of Nautilus striatus from the Lias of Whitby, represented at Pl. 33. The Lias which fills this pipe, must have entered it in the state of liquid mud, to the same extent that the pericardial fluid entered, during the hydraulic action of the siphuncle in the act of sinking; not one of the air-chambers has admitted the smallest particle of this mud; they are all filled with calcareous spar, subsequently introduced by gradual infiltration, and at successive periods which are marked by changes in the colour of the spar. In both these fossil Nautili, the entire series of the earthy casts within the siphuncle represents the bulk of fluid which this pipe could hold.

The sections, Pl. 32, Fig. 3, d. e. f., show the edges of the calcareous sheath surrounding the oval casts of three compartments of the expanded siphuncle. This calcareous sheath was probably flexible, like that surrounding the membranous pipe of the recent Nautilus Pompilius. (Pl. 31, Fig. 1, b. d. e.) The continuity of this sheath across the air-chambers, (Pl. 32, Fig. 2, d. e. f. Fig. 3, d. e. f. and Pl. 33,) shows that there was no communication for the passage of any fluid from the siphuncle into these chambers: had any such existed, some portion of the fine earthy matter, which, in these two fossils forms the casts of the siphuncle, must have passed through it into these chambers. Nothing has entered them, but pure crystallized spar, introduced by infiltration through the pores of the shell, after it had undergone sufficient decomposition to be percolated by water, holding in solution carbonate of lime.

The same argument applies to the solid casts of pure crystallized carbonate of lime, which have entirely filled the chambers of the specimen Pl. 32, Fig. 1; and to all fossil Nautili and Ammonites, in which the airchambers are either wholly void, or partially, or entirely filled with pure crystallized carbonate of lime. (See Pl. 42, Fig. 1, 2, 3, and Pl. 36.) In all such cases, it is clear that no communication existed, by which water could pass from the interior of the siphon to the air-chambers. When the pipe was ruptured, or the external shell broken, the earthy sediment, in which such broken shells were lodged, finding through these fractures admission to the air-chambers, has filled them with clay, or sand or limestone.

to the siphuncle, (n.,) we shall find in these organs an hy draulic apparatus for varying the specific gravity of the shell; so that it sinks when the pericardial fluid is forced into the siphuncle, and becomes buoyant, whenever this fluid returns to the pericardium. On this hypothesis also the chambers would be permanently filled with air alone, the elasticity of which would readily admit of the alternate expansion and contraction of the siphuncle, in the act of admitting or rejecting the pericardial fluid.

The principle to which we thus refer the rising and sinking of the living Nautilus, has been already stated (P. 241) to be the same which regulates the ascent and descent of the Water Balloon: the application of external pressure through a membrane that covers the column of water in a tall glass, forces a portion of this water into the cavity, or single air-chamber of the balloon, which immediately begins to sink; on the removal of this pressure, the elasticity of the compressed air causing it to return to its former volume, again expels the water, and the balloon begins to rise.*

I shall conclude this attempt to illustrate the structure and economy of fossil Nautili by those of the living species, by showing in what manner the chambers of the pearly Nautilus, supposing them to be permanently filled only with air, and the action of the siphuncle,† supposing it to be the receptacle only of a fluid secretion, interchanging its place alternately from the siphuncle to the pericardium, would be subsidiary to the movements of the animal, both at the surface, and bottom of the sea.

* See Sup. Note.

† The substance of the siphuncle is a thin and strong membrane, with no appearance of muscular fibres, by which it could contract or expand itself; its functions, therefore, must have been entirely passive, in the process of admitting or ejecting any fluid to or from its interior.-(See Owen's Memoir, p. 10) In our first edition it was stated erroneously that the siphùncle had no appearance of muscular fibres.

See Sup. Note.

First, The animal was seen and captured by Mr. Bennett, floating at the surface, with the upper portion of the shell raised above the water, and kept in a vertical position by means of the included air (See Pl. 31. Fig. 1.;) this position is best adapted to the retrograde motion, which a Sepia derives from the violent ejection of water through its funnel (k;)* thus far, the air-chambers, serve to maintain both the shell and body of the animal in a state of equilibrium at the surface.

Secondly, The next point to be considered is the mode of operation of the siphuncle and air-chambers, in the act of sinking suddenly from the surface to the bottom. These are explained in the note subjoined.†

*See Sup. Note.

It appears from the figure of the animal, Pl. 34, with which I have been favoured by Mr. Owen, that the upper extremity of the siphuncle marked by the insertion of the probe b., terminates in the cavity of the pericardium p, p. As this cavity contains a fluid, more dense than water, excreted by the glandular follicles d. d., and is apparently of such a size that its contents would suffice to fill the siphuncle, it is probable that this fluid forms the circulating medium of adjustment, and regulates the ascent or descent of the animal by its interchange of place from the pericardium to the siphuncle.

When the arms and body are expanded, the fluid remains in the pericardium, and the siphuncle is empty, and collapsed, and surrounded by the portions of air that are permanently confined within each air-chamber; in this state, the specific gravity of the body and shell together is such as to cause the animal to rise, and be sustained floating at the surface.

When, on any alarm, the arms and body are contracted, and withdrawn into the shell, the retraction of these parts, causing pressure on the exterior of the pericardium, forces its fluid contents downwards into the siphuncle; and the bulk of the body being thus diminished, without increasing the bulk of the shell, into whose cavities the fluid is withdrawn, the specific gravity of the whole mass is suddenly increased, and the animal begins to sink.

The air within each chamber remains under compression, as long as the siphuncle continues distended by the pericardial fluid; and returning, by its elasticity, to its former state, as soon as the pressure of the arms and body is withdrawn from the pericardium, forces the fluid back again into the cavity of this organ; and thus the shell, diminished as to its specfic gravity, has a tendency to rise.

Thirdly, it remains to consider the effect of the air, supposing it to be retained continually within the chambers, at the bottom of the sea. Here, if the position of the moving animal be beneath the mouth of the shell, like that of a snail as it crawls along the ground, the air within the chambers would maintain the shell, buoyant, and floating at ease above the body; and the tendency of the shell to rise to the surface would be counteracted by the strong muscular disk (Pl. 31, n.,) with which the creature crawls, and adheres to the bottom, using freely its tentacula to seize its prey.*

Dr. Hook considered (Hook's Experiments, 8vo. 1726, page 308) that the air chambers were filled alternately with air or water; and Parkinson (Organic Remains, vol. iii. p

The place of the pericardial fluid, therefore, will be always in the pericardium, excepting when it is forced into and retained in the siphuncle, by muscular pressure, during the contraction of the arms and body closed up within the shell. When these are expanded, either on the surface, or at the bottom of the sea, the water will have free access to the branchiæ, and the movements of the heart will procced freely in the distended pericardium; which will be emptied of its fluid at those times only, when the body is closed, and the access of water to the branchia consequently impeded.

The following experiments show that the weight of fluid requisite to be added to the shell of a Nautilus, in order to make it sink, is about half an

ounce.

I took two perfect shells of a Nautilus Pompilius, each weighing about six onuces and a half in air, and measuring about seven inches across their largest diameter; and having stopped the siphuncle with wax, I found that each shell, when placed in fresh-water, required the weight of a few grains more than an ounce to make it sink. As the shell, when attached to the living animal, was probably a quarter of an ounce heavier than these dry dead shells, and the specific gravity of the body of the animal may have exceeded that of water to the amount of another quarter of an ounce, there remains about half an ounce, for the weight of fluid which being introduced into the siphuncle, would cause the shell to sink; and this quantity seems well proportioned to the capacity both of the pericardium, and of the distended siphuncle.

* See Sup. Note.

If the chambers were filled with water, the shell could not be thus suspended without muscular exertion, and instead of being poised verti. cally over the body, in a position of case and safety, would be continually

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