The octopus has two eyes and a beak, with its mouth at the center point of the eight limbs (traditionally called "arms", also mistakenly called "tentacles").

Octopuses have a closed circulatory system, in which the blood remains inside blood vessels. Octopuses have three hearts; a systemic heart that circulates blood round 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 haemocyanin 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, haemocyanin transports oxygen more efficiently than haemoglobin. The haemocyanin is dissolved in the plasma instead of being carried within blood cells, and gives the blood a bluish colour.

An octopus also has nine brains (well, sort of). There’s one ‘main’ brain where all the analysis and decision making takes place and eight ancillary brains (one at the base of each arm) that function as preprocessors for all the information obtained by that arm. Two-thirds of an octopus’ neurons reside in its arms, which can independently figure out how to open a shellfish, for instance, while the main brain is busy doing something else.

More Info: en.wikipedia.org