Overview: The Octopus Body Plan
The octopus body is divided into two main regions: the mantle (the rounded head-body containing all major organs) and the arms (eight equal-length limbs that extend from below the eyes). Unlike most complex animals, octopuses have no skeleton — their only hard structure is a small beak used for biting and drilling into prey.
The Three Hearts
An octopus has one systemic heart (circulates oxygenated blood through the body) and two branchial hearts (one per gill, pushing deoxygenated blood through the gills for oxygenation). Full article →
Blue Blood and Hemocyanin
Instead of iron-based hemoglobin, octopuses use copper-based hemocyanin to transport oxygen. This gives their blood a vivid blue color and works efficiently in cold, oxygen-poor water. Full article →
The Distributed Nervous System (9 Brains)
The central brain is donut-shaped and wraps around the esophagus. Each arm contains a brachial ganglion with 40–50 million neurons that can process sensory data and control movement independently. Full article →
The Skin
Octopus skin contains three types of specialized cells:
- Chromatophores — pigment sacs controlled by muscles, for color change
- Iridophores — reflective plates that create iridescent effects
- Papillae — muscular bumps that change skin texture
Eyes
Octopus eyes are camera-type eyes — structurally similar to human eyes, though they evolved completely independently (convergent evolution). They have a distinctive W-shaped pupil that may help them sense color despite having only one type of photoreceptor.
Arms and Suckers
Each arm is lined with suckers along its entire length — up to 240 suckers per arm, each capable of independent movement, taste, and grip. The suckers can detect chemical signals (taste) and pressure simultaneously.
The Ink Sac
Located near the digestive gland, the ink sac produces a melanin-rich fluid containing tyrosinase, dopamine, and mucus. Ink is expelled through the siphon as either a decoy blob or a diffuse chemical cloud. Full article →
The Siphon (Mantle Funnel)
The siphon is a muscular tube used for jet propulsion, expelling ink, releasing waste, and spawning. It can be aimed in any direction, giving the octopus precise directional control of its swimming.