Behind the octopus's head, directly opposite the arms, is its mantle. The mantle is a highly muscled structure that houses all of the animal's organs. Its gills, hearts, digestive system and reproductive glands are all crammed into this one space. The strong muscles in the mantle protect the organs and help with respiration and contraction. The octopus also has a funnel, sometimes called a siphon, which is a tubular opening that serves as a pathway for water. You'll find out the purpose for this funnel later in the article.
In place of the protective shell found in many other mollusks, the octopus brandishes a startling array of defense mechanisms. One researcher who calls octopuses "the wizards of camouflage" goes so far as to say that chameleons are humdrum by comparison [source: Stewart]. Masters of disguise, octopuses can alter their skin to take on a diversity of colors and textures to blend in with their surroundings.