Octopus Anatomy from Octopus.com, Part I

OCTOPUS PHYSIOLOGY
The donut-shaped brain of the octopus contains only part of its complex nervous system: at least two-thirds of an octopus’s neurons are actually located in the nerve cords of its arms. The arms themselves are boneless and highly flexible appendages which appear to have three excitatory neuronal inputs with fairly large synaptic input values.

Octopuses have no bone structure being invertebrates but do have a skull, a shell rudiment and a beak. Despite having no skeletal structure they are very strong and extremely flexible.

Octopuses have three hearts - one that pumps blue blood throughout their extensive vascular system similar to other molluscs, and two branchial hearts which pump blood to the gills for oxygenation.