When the animal is disturbed it retires completely into its shell. From that moment, the equilibrium being changed, the shell is overturned, and the animal is nearly invisible. If frightened, it entirely submerges itself, and sinks to the bottom.
These little beings share with other Cephalopods the strange faculty of changing colour under the influence of some vivid impression; but their graceful and delicate organization redeems them from the charge we have brought against the cuttles. The Nautilus can blush, turn pale, and show through its transparent shell its body changing in sudden shades; but it never exhibits those bristling, unpleasant tubercles, the hideous inheritance of the larger and coarser Cephalopods—the tyrants of the sea.
The male Argonauts are very small, often not a tenth part of the size of the females, which alone possess the shells.
The Nautilus carries its egg in the shell, and the little ones are also hatched in this floating cradle. Four species are at present known: the species described by Aristotle and Pliny, and the more ancient naturalists; namely, A. argo, or papyracea (Figs. 327 and 329), which are inhabitants of the Mediterranean as well as the Indian Ocean and the Antilles. Two others, A. tubercula, belonging exclusively to the Indian Ocean, and A. baillant, which is met occasionally in the Pacific and Atlantic Oceans.
The nautilus belongs to the section of Octopoda, and the class of Acetabuliferous Cephalopods, having, as the name indicates, eight feet, from ὀκτὼ, eight, and ποῦς, foot; at the same time the body is entirely fleshy, and without fins. The genera of cuttles (Sepia) and Calmars (Loligo) belong to another section of the same class; namely, the Decapoda, because they have ten feet and a sort of internal osselet, with fins, &c.
THE DISTRIBUTION OF THE MOLLUSCA.
We have thought it better to treat this subject in a separate chapter, for its vast and complicated nature renders it otherwise difficult to handle, except in a space which would exceed the limits of this work.
The different genera of the organic world are peculiar to, or most frequent in, certain localities, and even species and varieties have their limits. This habit pervades the entire range of organisms, from the lowest plants to man, whose qualities are to a great extent the type of the locality he inhabits. The geography of the Mollusca is perhaps the best known to science. The labours of Mr. Louis Agassiz, Dr. Sclater, and Professor Edward Forbes, have done much towards giving us a clear idea of zoological geography. Climate alone is insufficient to account for the distribution of animals: some higher cause rules here. But while we admit this, still we must acknowledge that climate exerts considerable influence in modifying the qualities of species.
The distribution of the Mollusca may be considered from three points of view. First, as regards geography; second as regards depth; and third as regards time; the last belongs to geology.