Octopuses and squid belong to the order Octopoda, which is in the class Cephalopods, phylum Mollusca, which includes snails and clams. They are related to other mollusks but have no internal or external shell. There are more than one thousand species of octopuses and squid alive today. They have two eyes and eight (octopuses) to ten (some squid) tentacles that are attached directly to the head. The tentacles are covered with sucker pads that help the animal move along the ocean floor, its normal place of residence. They have the most complex brain of the invertebrates, and also have long- and short-term memories, much like vertebrates. The genus Octopus contains several different species, which vary greatly in size. The smallest is Octopus arborescens, a species that averages two inches in length. The largest species, Octopus dofleini, commonly known as the giant Pacific octopus, can grow to sixteen feet in length and weigh up to three hundred pounds, with a tentacle span of thirty feet. The best known and most widely distributed species is Octopus vulgaris, a medium-sized octopus found in every ocean. This species is fromtwo to three feet in length and lives in holes on the ocean floor. It feeds mainly on crabs, lobsters, and other crustaceans. It is very intelligent: One Japanese scientist taught an octopus how to open a sealed jar in order to get at its contents, a spiny lobster. An octopus can change its skin color quickly when it is frightened or threatened. Its changed color provides camouflage to help it blend in with the background so that the octopus becomes almost invisible to predators. Scientists have discovered that octopuses change colors not only for defense, but also to reveal their moods and emotions. Angry members of the species turn a deep red in color, while during mating season, both males and females display stripes and colors that reflect their inner excitement. Some species squirt out clouds of a dark inky substance, which hangs in the water for several seconds. This inkblot has the same size and shape as the octopus, and while it draws the attention of the predator, the octopus escapes. When an enemy, such as a moray eel, attacks an octopus, it may lose one of its eight arms in the fight, and while the eel watches the twitching arm the injured octopus can swim away. The tentacle grows back very quickly. Squid are cephalopods and belong to the order Teuthoidea (ten-armed), which has many species. They are found in every ocean and range in size fromless than one inch to more than sixty-five feet in length. Adult squid of some species can race through the water at speeds of up to twenty-five miles per hour. They are aggressive hunters, equipped with two more tentacles than octopus. The squid use these extra arms to catch their food. Some squid hunt in packs and use their ability to change color to lure their prey. They are also bioluminescent, which means they can light up. Actually, bacteria living under their skin produce the light in squid that live close to the surface. Deep-water squid, on the other hand, living thousands of feet below the surface, make their own light. These species have light-producing organs called photophores that make two chemicals: a protein called luciferin and an enzyme known as luciferase. When the two chemicals are mixed, the enzyme breaks down the protein, releasing a pale, blue-green light. This is the same process used by fireflies. Some squid species can squirt out clouds of glowing bacteria when they are endangered. This light show distracts the predator and helps the squid escape.
Thanks for description - Animal life club