Octopuses have muscular bodies that are usually composed of eight arms, a beak and a mantle. The octopus uses its arms to move in the water, to defend itself, to hunt and to mate. Octopuses eat and hunt with their beaks, and their mantles house their internal organs.
Inside the mantle, most octopuses have a brain, a digestive gland, salivary glands, gonads, a kidney, an anus, an ink sac, a funnel and three hearts. Some species of octopus have vestigial shells inside their mantles, although octopuses generally do not have bones or skeletons of any kind. Octopuses have eyes very similar to human eyes and have excellent vision that helps them target and hunt their prey. The rows of suckers on their muscular arms enable octopuses to move easily across the ocean floor and hang on tightly to their next meal. If an octopus needs to move quickly, it can take in water through its mantle and then seal off the mantle and expel the water forcefully through its funnel. This propels the octopus through the water at speeds of up to 25 miles per hour. Octopuses are extraordinarily adept at camouflage because their skin contains pigmented cells called chromatophores, which can change color when the octopus contracts or relaxes the muscles surrounding each cell.