Nautilus. The nautilus is a pelagic marine mollusc of the cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus. Though it more specifically refers to species Nautilus pompilius, the name chambered nautilus is also used for any of the Nautilidae. All are protected under CITES Appendix II. Nautilidae, both extant and extinct, are characterized by involute or more or less convolute shells that are generally smooth, with compressed or depressed whorl sections, straight to sinuous sutures, and a tubular, generally central siphuncle. Having survived relatively unchanged for hundreds of millions of years, nautiluses represent the only living members of the subclass nautiloidea, and are often considered living fossils. The word nautilus is derived from the Greek nautílos and originally referred to the paper nautiluses of the genus Argonauta, which are actually octopuses. The word nautílos literally means sailor, as paper nautiluses were thought to use two of their arms as sails. The tentacles of the nautiluses are actually cirri, composed of long, soft, flexible appendages which are retractable into corresponding hardened sheaths. Unlike the 8-10 head appendages of coleoid cephalopods, nautiluses have many cirri. In the early embryonic stages of nautilus development a single molluscan foot differentiates into a total of 60-90 cirri, varying even within a species. Nautilus cirri also differ from the tentacles of some coleoids in that they are non-elastic and lack pads or suckers. Instead, nautilus cirri adhere to prey by means of their ridged surface. Nautiluses have a powerful grip, and attempts to take an object already grasped by a nautilus may tear away the animal's cirri, which will remain firmly attached to the surface of the object. The main cirri emerge from sheaths which cohere into a single firm fleshy mass. Also, the pair of cirri before the eye and the pair of cirri behind the eye are separate from the others. These are more evidently grooved, with more pronounced ridges. They are extensively ciliated and are believed to serve an olfactory purpose. The radula is wide and distinctively has nine teeth. The mouth consists of a parrot-like beak made up of two interlocking jaws capable of ripping the animal's food, mostly crustaceans, from the rocks to which they are attached. Males can be superficially differentiated from females by examining the arrangement of tentacles around the buccal cone: males have a spadix organ located on the left side of the cone making the cone look irregular, whereas the buccal cone of the female is bilaterally symmetrical. The crop is the largest portion of the digestive tract, and is highly extensible. From the crop, food passes to the small muscular stomach for crushing, and then goes past a digestive caecum before entering the relatively brief intestine. Like all cephalopods, the blood of the nautilus contains hemocyanin, which is blue in its oxygenated state. There are two pairs of gills which are the only remnants of the ancestral metamerism to be visible in extant cephalopods. Oxygenated blood arrives at the heart through four ventricles and flows out to the animal's organs through distinct aortas but returns through veins which are too small and varied to be specifically described. The one exception to this is the vena cava, a single large vein running along the underside of the crop into which nearly all other vessels containing deoxygenated blood empty. All blood passes through one of the four sets of filtering organs upon leaving the vena cava and before arriving at the gills for re-oxygenation. Blood waste is emptied through a series of corresponding pores into the pallial cavity. The central component of the nautilus nervous system is the oesophageal nerve ring which is a collection of ganglia, commissures, and connectives that together form a ring around the animal's oesophagus. From this ring extend all of the nerves forward to the mouth, tentacles, and funnel; laterally to the eyes and rhinophores; and posteriorly to the remaining organs. The nerve ring does not constitute what is typically considered a cephalopod brain: the upper portion of the nerve ring lacks differentiated lobes, and most of the nervous tissue appears to focus on finding and consuming food. Nautiluses also tend to have rather short memory spans, and the nerve ring is not protected by any form of brain case.