Squid are soft-bodied creatures belonging to phylum Mollusca, class Cephalopoda. Other members of this class include octopuses, cuttlefish and nautiluses. Cephalopods have large brains relative to their body size and are considered to be the most intelligent invertebrates. They also have well-developed eyes. Unlike an octopus, which has eight arms and no tentacles, a squid has eight arms and two tentacles. The inner surfaces of the arms are covered entirely with suckers, whereas the tentacles, which are longer, usually have suckers only at the end. To capture prey, a squid rapidly extends its tentacles, grasps the prey, then brings it to its mouth. A squid’s mouth is located in the center of its ring of arms and contains a hard beak that is used to bite off pieces of the prey.
The body of the squid is covered with skin containing pigment cells called chromatophores. Squid and other cephalopods have the amazing ability to control their chromatophores by contracting and relaxing the muscles around these cells. They can rapidly change from one color to another; some can become striped, and some can even undulate with color. They may change color and pattern as a warning or during different behaviors, such as feeding and mating. Squid have an ink sac that they use as a means of defense. They expel ink to confuse predators, then escape by jetting away.