Nautilus in the context of "Exoskeleton"

In mathematics, a spiral is a curve which emanates from a point, moving further away as it revolves around the point. It is a subtype of whorled patterns, a broad group that also includes concentric objects.

A cephalopod /ˈsɛfələpɒd/ is any member of the molluscan class Cephalopoda /sɛfəˈlɒpədə/ (Greek plural κεφαλόποδες, kephalópodes; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles (muscular hydrostats) modified from the primitive molluscan foot. Fishers and Swedes sometimes call cephalopods "inkfish", referring to their common ability to squirt ink. The study of cephalopods is a branch of malacology known as teuthology.

Cephalopods became dominant during the Ordovician period, represented by primitive nautiloids. The class now contains two, only distantly related, extant subclasses: Coleoidea, which includes octopuses, squid, and cuttlefish; and Nautiloidea, represented by Nautilus and Allonautilus. In the Coleoidea, the molluscan shell has been internalized or is absent, whereas in the Nautiloidea, the external shell remains. About 800 living species of cephalopods have been identified. Two important extinct taxa are the Ammonoidea (ammonites) and Belemnoidea (belemnites). Extant cephalopods range in size from the 10 mm (0.3 in) Idiosepius thailandicus to the 700 kilograms (1,500 lb) heavy colossal squid, the largest extant invertebrate.

Cuttlefish, or cuttles, are marine molluscs of the family Sepiidae. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of buoyancy.

Cuttlefish have large, W-shaped pupils, eight arms, and two tentacles furnished with denticulated suckers, with which they secure their prey. They generally range in size from 15 to 25 cm (6 to 10 in), with the largest species, the giant cuttlefish (Sepia apama), reaching 50 cm (20 in) in mantle length and over 10.5 kg (23 lb) in mass.

13°54′29″S 146°36′55″E / 13.90806°S 146.61528°E / -13.90806; 146.61528

Osprey Reef is a submerged atoll in the Coral Sea, northeast of Queensland, Australia. It is part of the Northwestern Group of the Coral Sea Islands. Osprey Reef is roughly oval in shape, measuring 25 by 12 kilometres (16 by 7 mi), and covers around 195 square kilometres (75 sq mi). It has a perimeter of 69.5 kilometres (43.2 mi). The central lagoon is only 30 metres (98 ft) deep.

Nacre (/ˈneɪkər/ NAY-kər, also /ˈnækrə/ NAK-rə), also known as mother-of-pearl, is an organic–inorganic composite material produced by some molluscs as an inner shell layer. It is also the material of which pearls are composed. It is strong, resilient, and iridescent.

Nacre is found in some of the most ancient lineages of bivalves, gastropods, and cephalopods. However, the inner layer in the great majority of mollusc shells is porcellaneous, not nacreous, and this usually results in a non-iridescent shine, or more rarely in non-nacreous iridescence such as flame structure as is found in conch pearls.

In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work mainly like muscular hydrostats. Most forms of tentacles are used for grasping and feeding. Many are sensory organs, variously receptive to touch, vision, or to the smell or taste of particular foods or threats. Examples of such tentacles are the eyestalks of various kinds of snails. Some kinds of tentacles have both sensory and manipulatory functions.

A tentacle is similar to a cirrus, but a cirrus is an organ that usually lacks the tentacle's strength, size, flexibility, or sensitivity. A nautilus has cirri, but a squid has tentacles.