The octopus is a soft-bodied, eight-limbed mollusk of the order Octopoda. Around 300 species are recognized, 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 center point of the eight limbs ("tentacle" is used as an umbrella term for cephalopod limbs; however, within a tautological context; "arm" is used to refer to such limbs while "tentacle" is reserved for feeding appendages not found on octopuses).
The soft body can rapidly 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.
Octopuses have a closed circulatory system, in which the blood remains inside blood vessels. Octopuses have three hearts; a systemic heart that circulates blood around the body and two branchial hearts that pump it through each of the two gills. The systemic heart is inactive when the animal is swimming and thus it tires quickly and prefers to crawl. Octopus blood contains the copper-rich protein hemocyanin to transport oxygen. This makes the blood very viscous and it requires considerable pressure to pump it round the body; octopuses' blood pressures can exceed 75 mmHg. In cold conditions with low oxygen levels, hemocyanin transports oxygen more efficiently than hemoglobin. The hemocyanin is dissolved in the plasma instead of being carried within blood cells and gives the blood a bluish color.
The systemic heart has muscular contractile walls and consists of a single ventricle and two atria, one for each side of the body. The blood vessels consist of arteries, capillaries, and veins and are lined with a cellular endothelium which is quite unlike that of most other invertebrates. The blood circulates through the aorta and capillary system, to the vena cavae, after which the blood is pumped through the gills by the auxiliary hearts and back to the main heart. Much of the venous system is contractile, which helps circulate the blood.