Question: what group of animals do octopus belong to?
Answer: class Cephalopoda

The octopus ( / ˈɒktəpəs / or ~ / pʊs / ) is a soft - bodied , eight - armed mollusc of the order Octopoda . Around 300 species are recognised and the order is grouped within the class Cephalopoda with squids , cuttlefish and nautiloids . Like other cephalopods , the octopus is bilaterally symmetric with two eyes and a beak , with its mouth at the centre point of the arms ( which are sometimes mistakenly called '' tentacles '' ) . The soft body can rapidly alter its shape , enabling octopuses to squeeze through small gaps . They trail their eight arms 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 .