This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
Fig. 284. Animal of the Nautilus Pompilius. (After Owen).
* For this invaluable addition to zoological knowledge, science is indebted to George Bennett, Esq., who obtained the living animal near the island of Erromanga, New Hebrides. " It was found in Marekini Bay, floating on the surface of the water subject of a monograph by Professor Owen, who has most completely investigated its general organization and relations with other families of the Cephalopoda. The shell of the Pearly Nautilus (N. Pompilius) is extremely common, and may be met with in every conchological collection, notwithstanding the extreme rarity of the mollusk that inhabits it - a circumstance, perhaps, to be explained by the fact that the living animal dwells in deep water, and when it comes to the surface is so vigilant against surprise, that at the slightest alarm it sinks to the bottom. On making a section of the shell its cavity is found to be partitioned off by numerous shelly septa into various chambers (fig. 284, s s), in the last of which the body of the animal is situated. A long tube or siphuncle (h h), partly calcareous and partly membranous, passes through all the compartments quite to the end of the series. The membranous siphuncle is continued into the animal, and terminates in a cavity contained within its body, hereafter to be described, which is in free communication with the exterior.
(1518). Various conjectures have been indulged in concerning the end answered by the camerated condition of the shell in these Mollusca. Dr. Hooke* suggested the idea that the chambers might be filled with air generated by the Nautilus, and thus made so buoyant that the specific gravity of the animal and its shell should correspond with that of the surrounding medium, and that, acting in the same manner as the swimming-bladder of a fish, the creature would float or sink, as the air in its shell was alternately compressed or rarefied. Should this supposition be correct, it would seem probable, as Dr. Buckland has pointed out, that the simple retraction of the head, by injecting water from the chamber within its body (pericardium) into the membranous siphuncle, would cause the needful condensation of the air contained in this singular float, and allow the Nautilus to sink to the bottom; while the protrusion of its arms, by taking off the pressure, and thus allowing of the expansion of the confined air, would give every needful degree of buoyancy, even sufficient to permit the mollusk to rise like a balloon to the top of the sea.
(1519). The body of this Cephalopod is covered with a thin mantle (a a), of which a large fold (b) is reflected on the exterior of the shell. It is securely fixed to its residence by two lateral muscles, the insertion of one of which is seen at g. A large coriaceous hood (n) covers the head, and, when the creature retreats into its habitation, closes the entrance like a door, while through the infundibulum (i) the ova and excrementitious matters are expelled from the body. The most renot far distant from the ship, and resembling, as the sailors expressed it, a dead tortoiseshell cat in the water. It was captured, but not before the upper part of the shell had been broken by the boat-hook in the eagerness to take it, as the animal was sinking when caught." - (Dr. Bennett's Journal).
* Philosophical Experiments and Observations. 8vo, 1726.
(1520). Turning our attention to the anatomical structure of the Cephalopoda, we find that in all of them the exterior of the body is entirely formed by an intricate interlacement of muscular fibres. The sac that contains the viscera, itself muscular, is united to the head by strong and largely-developed fasciculi; the funnel (fig. 285, a), through which, as through a fleshy pipe, the products of excretion, as well as the eggs or seminal fluid, are ejected, is formed of a tissue similarly endowed with contractility; while the arms are composed externally of muscles disposed in various directions, and moreover have their central portion occupied by strong bands, which traverse them longitudinally from end to end, so that they are thus gifted with all needful powers of motion, and may be shortened, elongated, or bent in any direction at pleasure.
(1521). In those natatory species which, like Loligopsis, or Onycho-teuthis (fig. 282), have fins appended to the sides of the visceral sac, these organs likewise are made up of muscular substance; and, being thus converted into broad moveable paddles, they also form efficient locomotive agents.
(1522). One important circumstance observable in the class before us must not be forgotten in connexion with this portion of the history of the Cephalopoda. We may remind the student that, in the vertebrate division of animated nature, to which these creatures immediately lead us, the locomotive system is supported by an internal vascular and living skeleton, composed either of cartilage, as is the case in the most imperfect vertebrated genera, or, in the more highly organized forms, of bones articulated with each other, and possessing'within themselves the means of growth and renovation derived from the blood which permeates them in every part. The reader will remember that, in all the classes that have offered themselves to our notice, we have not hitherto observed anything at all comparable to an internal osseous framework such as Man possesses, - dead, extravascular shells, formed by successive depositions of layers of calcareous material, or jointed cuticular armour equally incapable of growth, having as yet represented the skeleton, and formed the only levers upon which the muscular system could act in producing the movements connected with locomotion.
(1523). Having, however, already had abundant opportunities of seeing how gradually nature proceeds in effecting the development of a new series of organs, we might naturally be led to expect in the creatures before us some faint indications, at least, of our approach to animals possessed of an internal bony framework; and our expectations in this particular will be found on investigation to be well-grounded. It is, in fact, in the Cephalopoda, the highest of the molluscous classes, that the rudiments of an osseous system for the first time make their appearance; not, indeed, as yet composed of perfect bone, but formed of cartilaginous pieces, - some being so disposed as to protect the ganglionic mass above the oesophagus, which now from its size well deserves the name of brain, whilst others serve to afford bases of attachment to the muscular system in different regions of the body.