Octopods Nautilids Cuttlefishes Squids and Relatives: Cephalopoda

The bodies of all cephalopods (SEF-oh-lo-pahd) remain firm thanks to a system of muscles that maintain fluid pressure inside. Their bodies are more or less divided into three regions: the armlike tentacles surrounding the mouth; the head that has a pair of large, distinctive eyes, one on each side; and the body, or mantle, sometimes with a pair of fins on the sides. Some species, like nautiluses, have sixty tentacles arranged in two rings around the mouth, while others have a single ring of eight tentacles. Of these species, some have a pair of additional tentaclelike appendages and appear as though they have ten tentacles. The head has beaklike mouthparts and a scraping or drilllike radula (RAE-jeh-leh). The radula is a tonguelike organ covered with rows of very hard teeth. Inside the head is a highly complex brain. Inside the head and mantle of cephalopods is a highly developed nervous system, although it is less developed in the nautiluses.

The mantles of the smallest adult cephalopods measure only 0.23 to 0.31 inches (6 to 8 millimeters) in length, while the mantle length of giant squids (Architeuthis may reach 71 inches (1.8 meters). These giants are believed to weigh up to 661.3 pounds (300 kilograms). Several kinds of squid and at least two species of octopuses grow larger than an adult human.

Many cephalopods have the ability to change their colors rapidly. They do this with several different kinds of color-producing organs in their skin that are controlled by the nervous system. Together these organs can create different colors and patterns in an instant. Some species have light-producing organs. These organs either produce blue green light by mixing chemicals together or rely on special bacteria that live inside chambers associated with the ink sac. The ink sac produces a thick inky fluid that is squirted into the water and helps them to hide from predators.