Common octopus (Octopus vulgaris) on guard

   The common octopus (Octopus vulgaris) can change color to blend in with its surroundings. The octopus's siphon is a muscular organ used for propulsion, respiration, defense, and manipulation. By expelling water from the mantle cavity through the siphon, the octopus swims rapidly and releases an inky mixture. The octopus's digestive tract is located at the front of the body, opening into the mantle cavity. Waste products are expelled through the siphon.