Stasek (1972) regarded the Coleoidea as the result of evolution of a nautiloid lineage, modified in the direction of loss of shell “and increased streamlining, speed, and muscularization of the mantle. There are about 650 species of living Coleoidea. In terms of lineage, therefore, squids and octopuses are the most recently evolved of the major groups of mollusks. Nautiloids appear later, and the Coleoidea (squids and related forms) later still. These were gastropods (snails), followed by bivalves. The first fossil mollusks originated during the primary radiation of the Precambrian era. Some species were up to 3 ft (1 m) in diameter. They were apparently active predators, with a coiled shell similar to the nautilus. Ammonoids became extinct at the end of the Cretaceous period. Mollusks in the subclass Coleoidea are separate from the chambered nautilus (subclass Nautiloidea) and the ammonoids (subclass Ammonoidea). Squids are classified in the class Cephalopoda and subclass Coleoidea (Dibranchiata), which places them with octopuses and cuttlefishes. The skin contains many pigment cells or chromatophores, capable of changing the color of the animal through expansion and contraction. The body of squids has a pair of flexible, roughly triangular fins. The squids of the Mediterranean are called calamares, from the Greek kalamos and Latin calamus, meaning writing-reed or pen. Squids have no external shell, but have an internal stiffening structure known as the rod or pen. These appendages are equipped with small suction-cups, and surround the mouth opening. About one-half the length (24-36 in 60-90 cm) of the common North Atlantic species Loligo pealei consists of its streamlined cylindrical body, and the other half is its set of eight arms and two arm-like tentacles. Squid is the common name for a group of marine mollusks (phylum Mollusca) with highly developed eyes and brain, and complex swimming behavior.
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