Fig. 252.—Nautilus pompilius L., in section, showing the septa (s, s), the septal necks (s.n, s.n), the siphuncle dotted in (si), and the large body chamber (ch).
The septa are traversed by a membranous tube known as the siphuncle, which in Nautilus is said by Owen to connect ultimately with the pericardium. The septal necks, or short tubular prolongations of the septa where they are perforated by the siphuncle, are in the great majority of the Nautiloidea directed backwards (Fig. [252]), i.e., they project from the front wall of each chamber, while in nearly all Ammonoidea they are directed forwards. When the siphuncle is narrow, as in the Ammonoidea, it is simple, but when wide, as in many of the Nautiloidea, its walls are often thickened by the deposition of masses of calcareous matter, or by rings and radiating lamellae of the same material. In position, the siphuncle is sometimes central, sometimes sub-central, sometimes (Ammonoidea) marginal. In some cases its position is believed to change during the growth of the individual. The precise object served by the siphuncle is at present unknown. Some hold that it preserves the vitality of the unoccupied chambers, by connecting them with the soft parts of the animal; others have regarded it as a means for lightening the shell by the passage of some gas into the chambers.
Fig. 253.—Ammonites (Cadoceras) sublaevis Sowb., Kellaway’s Rock, showing the marginal position of the siphuncle (si).
The initial chamber in Nautiloidea consists of an obtuse incurved cone, marked on the outer surface of its posterior wall by a small scar known as the cicatrix, which may be slit-like, round, oval, or cruciform in shape. It has been held that the cicatrix originally communicated with the protoconch or larval shell, which probably dropped off as development proceeded. In the Ammonoidea, on the other hand, there is no cicatrix, and the initial chamber probably represents the protoconch, as seen in the nucleus of many Gasteropoda.
Sub-order 1. Nautiloidea.—Shell straight, bent, or coiled, aperture simple or contracted; siphuncle often narrowed by internal deposits, position variable; septal necks short, usually directed backwards; septa concave towards the aperture; initial chamber conical, with a cicatrix on the posterior wall.
The Nautiloidea, of which Nautilus is the sole living representative, date back to the Cambrian epoch, and attain their maximum in the Silurian and Devonian. At the close of the Palaeozoic era, every family, with the sole exceptions of the Orthoceratidae and Nautilidae, appears to have become extinct. The former disappear with the Trias, and after the lapse of the whole Secondary era, Aturia, a form closely related to Nautilus, makes its appearance.
(a) Retrosiphonata: septal necks directed backwards.
Fam. 1. Orthoceratidae.[401]—Shell straight or slightly curved, aperture simple, body-chamber large; siphuncle cylindrical, position variable. Single genus, Orthoceras (Fig. [254]). Cambrian to Trias.
Fam. 2. Endoceratidae.—Shell straight, siphuncle wide, marginal, septal necks produced into tubes fitting into one another. Principal genera: Endoceras (specimens of which occur six feet long), and Piloceras—Ordovician.