Several hundred cephalopod species belonging the phylum mollosca. These creatures range in size from a few centimeters in length to eighty feet in the case of Architeuthis, the giant squid. Squid possess a body plan similar to that of the octopus, cuttlefish, and nautilus, with ten tentacles (8 short / 2 long) and a beak-like mouth at one end of the body and a mantle at the other which holds the internal organs. They are distinguished from these other cephalopods by their rigid mantles and lack of shells.

Squid are universally carnivorous, feeding on just about anything they can successfully catch and eat. Although typically social animals, they are known to engage in cannibalism, eating smaller members of their own species.

These animals also possess a well developed nervous system and are considered to be some of the most intelligent invertebrates. They are capable of communication with other members of their own species through their ability to change the color of their skin, and have been studied with some success in laboratories to explore the learning capabilities of marine invertebrates. They also possess a highly evolved eye similar to that of vertebrates.

Squid are very fast creatures and are capable of propelling themselves through the water by sucking water into the mantle and then pushing it out through an opening called a "siphon". This sort of "jet propulsion" allows them to effectively elude predators. They also produce "ink" which they secrete into the water to hide themselves as they flee.

The blood of squid is based on copper, rather than iron. As such, it is blue in color and requires cool temperatures to effectively bond to oxygen in the water. Changes in water temperature can be fatal to squid, and weather phenomena such as El Nino are known to trigger squid migrations.

The fossil record suggests that the progenitors of the modern squid evolved a similar body plan in the early Paleozoic era. However, those creatures possessed shells not unlike the nautilus, but similar in shape to a modern squid's mantle. It is suggested that squid lost their shells in the Mesozoic era.