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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 wyith 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,f supposing it to be the receptacle only of a fluid secretion, interchanging its place alternately from the siphuncle to the pericardium,J would be subsidiary to the movements of the animal, both at the surface, and bottom of the sea.

* See Sup. Note.

t 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 siphuncle had no appearance of muscular fibres.

t 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 PI. 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.

t It appears from the figure of the animal, PI. 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 pericar. dium, 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 caurc 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, lias 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 (PI. 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 ,-f and Parkinson (Organic Remains, vol. iii. p 102,) admitting that these chambers were not accessible to water, thinks that the act of rising or sinking depends on the alternate introduction of air or water into the siphuncle; but he is at a loss to find the source from which this air could be obtained at the bottom of the sea, or to explain "in what manner the animal effected those modifications of the tube and its contained air, on which the variation of its buoyancy depended."* The theory which supposes the chambers of the shell to be permanently filled with air alone, and the siphuncle to be the organ which regulates the rising or sinking of the animal, by changing the place of the pericardial fluid, seems adequate to satisfy every hydraulic condition of a Problem that has hitherto received no satisfactory solution.

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 branchiz, and the movements of the heart will proceed 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 branchiz 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 malic 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.

j• If the chambers were filled with water, the shell could not be thus suspended without muscular exertion, and instead of being poised vertically over the body, in a position of case and safety, would be continually tending to fall flat upon its side; thus exposing itself to injury by friction, and the animal to attacks from its enemies. Kumphius states, that at the bottom, He creeps with his boat above him, and with his head and barbs (tentacula) on the ground, making a tolerably quick progress. I have observed that a similar vertical position is maintained by the shell of the Planorbis corneus, whilst in the act of crawling at the bottom.

I have dwelt thus long upon this subject, on account of its importance, in explaining the complex structure, and hitherto imperfectly understood functions, of all the numerous and widely disseminated families of fossil chambered shells, that possessed siphunculi.f If, in all these families, it can be shown that the same principles of mechanism, under various modifications, have prevailed from the first commencement of organic life unto the present hour, we can hardly avoid the conclusion which would refer such unity of organizations to the will and agency of one and the same intelligent First Cause, and lead us to regard them all as "emanations of that Infinite Wisdom, that appears in the shape and structure of all other created beings."J

* The recent observations of Mr. Owen show, that there is no gland connected with the siphuncle, similar to that which is supposed to secrete air in the air-bladder of fishes.

t See Sup. Note.

t Dr. Hook's Experiments, p. 306.

SECTION IV.

AMMONITES.

Having entered thus largely into the history of the Mechanism of the shells of Nautili, we have hereby prepared ourselves for the consideration of that of the kindred family of Ammonites, in which all the essential parts are so similar in principle to those of the shells of Nautili, as to leave no doubt that they were subservient to a like purpose in the economy of the numerous extinct species of Cephalopodous Mollusks, from which these Ammonites have been derived.

Geological Distribution of Ammonites.

The family of Ammonites extends through the entire series of the fossiliferous Formations, from the Transition strata to the Chalk inclusive. M. Brochant, in his Translation of De la Beche's Manuel of Geology, enumerates 270 species; these species differ* according to the age of the strata in

* Thus one of the first forms under which this family appeared, the Ammonites Henslovvi, (PI. 40, Fig. 1,) ceased with the Transition formation J the A. Uodosus (PL 40, Figs. 4, 5.) began and terminated its period of existence with the Muschelkalk. Other genera and species of Ammonites, in like manner, begin and end with certain definite strata, in the Oolitic and Cretaceous formations; e. g. the A. Buckl&ndi (PI. 37, Fig. 6.) is peculiar to the Lias; the A. Goodhalli to the Greensand; and the A. Rusticusto the Chalk. There are few, if any, species which extend through the whole of the Secondary periods, or which have passed into the Secondary, from the Transition period.

The following Tabular Arrangement of the distribution of Ammonites, in different geological formations, is given by Professor Phillips in his Guide to Geology, 1834, p. 77.

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