Squid, shell-less cephalopod mollusk, order Teuthoidea. Although a few species live in coastal waters, the majority inhabit the open ocean. The squid is a streamlined animal with 10 arms around the head, facing forward. The mantle at the rear of the body houses the gills and the openings of the excretory, sex, and digestive organs. Sudden contraction of the mantle cavity produces a blast of water that can be directed forward or backward, providing the main means of propulsion. All squid can swim very rapidly and are active predators of fish, shooting out their long arms, provided with suckers and hooks, to grab their prey.