octopus and squid

Squid and octopus are members of the class Cephalopoda, which together with sea slugs, bivalves, univalves, chitons, and other shells make up the large phylum of animals known as molluscs, more commonly called shellfish.

Mollusks form a vast group comprising more than 100,000 different species. Marine mollusks have a wide range of habitats, from above the high tide mark to the bottom of the deep sea and even drifting on plankton and floating on the ocean surface. Cephalopods, of which there are several hundred species, are entirely marine and can be grouped into three main divisions: the squid, the octopus, and the primitive nautiloids.

Squids have 10 tentacles that surround their mouth and head, two of which are very elongated. The movement is through a jet of water, a method also used by the octopus and the nautilus. Water is fed into a small hole and expelled under high pressure from a moving siphon so that the animal can move in any direction it chooses. In addition, the mantle of the squid is sometimes used as an aid in locomotion by means of an undulating movement along the sides.

Octopuses have four pairs of arms and a hard beak with a mouth in the center of the arms. Like the squid, the octopus has no internal or external skeleton, which allows them to squeeze into very tight places. Scientific tests have shown that octopuses are highly intelligent and may be the most intelligent of all invertebrates. Many species of octopus have an ink chamber that allows them to expel a dark colored fluid that gives them a cover to escape from predators.