Group of squids
There are about 300 species of squid, classified into 28 families. Squids also offer aesthetic value, as a popular item in public aquariums and as a focal point in movies and literature.
Squids not only play a key role in the marine food chains-they prey on fish, crustaceans, and other mollusks, and in turn are preyed on by fish and aquatic mammals, such as whales-but they also serve as a popular source of food for humans. Some squid species lose their tentacles in post-larval stages, and thus the adult only has eight arms (O'Shea 2006). Squids differ from the squid-like cuttlefish in that cuttlefish have an internal shell (cuttlebone) on their back. (Tentacles tend to be longer than arms and usually have suckers as their tips only.) The suckers of squids also have hooks and/or sucker rings, while octopuses have simple suckers without secondary armature (O'Shea 2006).
They differ from octopuses in that octopuses have eight arms and no tentacles, while squids and cuttlefish, at some point in their life cycle, have eight arms and two tentacles. Squids belong to the subclass Coleoidea along with octopuses, cuttlefish, and extinct belemites. Like all cephalopods, squids are characterized by bilateral symmetry, a prominent head, and a modification of the mollusk foot into the form of arms or tentacles surrounding the mouth, which has beak-like jaws. This large, diverse group of invertebrates comprise the order Teuthida (sometimes listed as order or suborder Teuthoidea) or, in some classifications, the orders Oegopsida and Myopsida (listed as suborders of Teuthida in some taxonomies). Squids are marine cephalopods (class Cephalopoda, phylum Mollusca) with ten arms and tentacles (at some point in life), secondary armature on their suckers, and lacking the internal shell specific to cuttlefish.