The Medals of Creation, Volumes 1 and 2 / First Lessons in Geology and the Study of Organic Remains

by Gideon Algernon Mantell

Lign. 149.
Nautilus pompilius in its shell.

a. The animal, occupying the last or body chamber of the shell.

b. The shell cut vertically through the middle, and showing the air-chambers and the siphuncle.

The appearance and structure of the recent shell are familiar to every one; a correct knowledge of the nature of the original animal has, however, been obtained but very recently. In its general characters the animal of the Nautilus, which is an inhabitant of the seas of hot climates, resembles the naked Cephalopoda; it possesses four branchiæ, or gills, and numerous hollow arms and retractile tentacula. Its head is furnished with a muscular flattened disk, which serves as an operculum to the shell when the animal is retracted. The beaks are horny, and coated at their tips by calcareous matter. It has no ink-bag, and is destitute of fins or other organs for swimming. The body occupies the ample outer cell of the shell, to which it is firmly attached by two lateral muscles; and it has a siphuncle, that passes from the posterior part of the animal through the shelly tube, and by which communication is maintained with the entire series of cells or chambers. The siphuncle is provided with a small artery and vein, and traverses the entire series of chambers, thus maintaining the vitality of the shell. Mr. Edwards considers that "it may be looked upon as an elongated cæcum, and that it is not under any circumstances used by the animal as a hydrostatic balance."[409]

[409] The reader interested in this subject should consult the Memoirs by M. Valenciennes, Mem. de l’Inst.; M. Vander Hoven, in the Proceedings of the Zoological Society; Prof. Owen’s Memoir on the Pearly Nautilus, Mr. Gray’s paper in the An. Nat. Hist., Mr. Edward’s Monograph in the Palæont. Soc., and Mr. Woodward’s Manual.

Upon making a vertical section of the shell, the inner volutions are exposed, and the cavity is seen divided at regular intervals into cells, by smooth, concave, nacreous septa; these vary in number according to the age of the individual; there are about thirty-five in an adult specimen. The partitions are pierced in the centre by a shelly tube, which traverses each cell to within a short distance of the next partition; and this tube is rendered a continuous channel in the living animal, by the membranous siphuncle. This series of air-chambers constitutes an apparatus which renders the Nautilus nearly of the same specific gravity as the surrounding water, and enables it to rise to the surface of the sea, or sink to the bottom, by a very small amount of muscular exertion. The Nautilus swims, like the Cuttle-fish, by expelling the water from its respiratory chamber; the walls of which are very thick and powerful muscles.

From this very general description of the only living representative of the numerous genera of tetrabranchiate Cephalopoda, which swarmed in such prodigious numbers in the ancient seas, we may pass to the consideration of the fossil Nautili, and their related congeners. Our remarks must be limited to the genera that will serve to demonstrate the most important modifications of structure, and explain the nature of the fossil remains of this extensive class of extinct beings.

The genera into which these shells are distributed are founded upon the mode in which the shell is coiled, its form, the character of the partitions or septa, and the situation of the siphuncle. A little reflection will enable the student to understand the principles of this classification. The essential character of all the shells of this class, is to have an external chamber larger than the inner chambers, and which contains the body of the animal; to be divided internally into different compartments, by partitions (concavo-convex, with the concavity outwards); and to have a pipe or tube extending from the outer open chamber to the innermost cell. They are divided into three groups or families.

1. The Nautilidæ (Bd. pl. xxxi.): in these the septa are smooth, or but slightly undulated, and the siphuncle either traverses the centre of the cell-partitions, or is situated towards the inner margin or turn of the spire.