Tetrabranchiata

Tetrabranchiata, an order of Cephalopoda, including the Pearly Nautilus. The characters of the order are that the animal is protected by an external shell composed of many chambers, of which the last is occupied by the body of the animal. The chambers are separated from one another by thin plates or septa, each of which is perforated by a membranous tube known as the siphuncle. In the recent species there are four gills, whence the name of the order. It is probable that the fossil forms had the same number, which may be a relic of an ancestral condition when the organs of the body were repeated in a series, as in the worms. The region of the body around the month is divided into lobes, which correspond to tbe arms of the Dibranchiata, such as the cuttle-fish. On the lobes are many tentacles, which correspond to the suckers on the arms of a cuttle-fish, The funnel is a mere slit, and not closed as a tube, and there is no ink-bag. The eye is much more primitive in structure than that of the cuttle-fish. The Nautilus has a beak-like pair of jaws; and similar structures occur in many fossil forms, and were described as Rhyncholites and Rhynchoteuthis before their real character was known. The shell in the Nautilus is a discoidal coil, but in fossil forms great variations are known; thus, in some, the coil is not in one plane, but in a spire, as in Turrilites, or it may be an open coil, as in Lytoceras; in others the shell is not coiled, but cylindrical, as in Orthoceras, in which it is straight, or Cyrtoceras, in which it is bent, or hook-shaped as in Hamites. Most of the Tetrabranchiata are extinct. The order was of great importance in Palaeozoic and Mesozoic times; in the Cainozoic period it is much rarer, and the only living genus is the Pearly Nautilus. There are two sub-orders: the Nautiloidea, of which the Nautilus is the type, and which includes most of the Palaeozoic forms, such as the Orthoceratidae, and such abnormal types as the Ascoceratidae, the second sub-order is the Ammonoidea, which includes most of the Mesozoic species, such as the Ammonites (q.v.), and also some Palaeozoic families, such as the Clymeniidae and Goniatitidae.