An octopus is a soft-bodied, eight-limbed mollusc of the order Octopoda. The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefish, and nautiloids. Like other cephalopods, an octopus is bilaterally symmetric with two eyes and a beaked mouth at the center point of the eight limbs. The soft body can radically alter its shape, enabling octopuses to squeeze through small gaps. They trail their eight appendages behind them as they swim. The siphon is used both for respiration and for locomotion, by expelling a jet of water. Octopuses have a complex nervous system and excellent sight, and are among the most intelligent and behaviourally diverse of all invertebrates.
Octopus
A giant Pacific octopus at Echizen Matsushima Aquarium, Japan
Diagram of octopus from side, with gills, funnel, eye, ocellus (eyespot), web, arms, suckers, hectocotylus and ligula labelled.
A finned Grimpoteuthis species with its atypical octopus body plan
Mollusca is the second-largest phylum of invertebrate animals, after Arthropoda; members are known as molluscs or mollusks. Around 76,000 extant species of molluscs are recognized. The number of fossil species is estimated between 60,000 and 100,000 additional species. The proportion of undescribed species is very high. Many taxa remain poorly studied.
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Image: Octopus 2
Image: Clams on Sandy Hook beaches panoramio
Image: Tonicella lineata
