These ocean dwelling animals are characterized by bilateral body symmetry, a prominent head, and a set of arms, or tentacles. There are about 800 identified species of cephalopods, a class that includes squid, octopi, nautili, and cuttlefish. With their well-developed senses and large brains, cephalopods are widely considered the most intelligent of the invertebrates. With only a few exceptions, all cephalopods have an ink sac, which can be squirted into the surrounding water to confuse predators. They are also the only mollusks with a closed circulatory system.