Squid, like octopi, cuttlefish and nautilus species, are cephalopods, a branch of mollusks. Ancestral squid, which had elongated, external shells, known as orthocones, arose in the Ordovician Period (about 500 million years ago-MYA) and their fossils appear in marine sedimentary rocks throughout the Paleozoic Era. By the onset of the Mesozoic Era (225 MYA), belemnites, possessing a linear, internal shell, had evolved, sharing the seas with nautiloids.
As the Cenozoic Era dawned, 65 MYA, the belemnites diverged into the modern squid, octopi and cuttlefish lines; of the 800 species found today, the great majority are squid, ranging from less than 1 to almost 50 feet in length. Like the Mesozoic belemnites, squid have an internal, chitinous "shell," known as a gladius, that supports the structure of the mantle. Other anatomic features include a siphon for locomotion, a beaked mouth, eight arms and two long tentacles. Squid have large, prominent eyes and those of giant squid species are the largest in the animal kingdom. Like octopi and cuttlefish, squid have chromatophores on their external surface which allow the animal to change color and blend with its surroundings and an ink sac that releases a black cloud to confuse predators.
Squid are a diverse group of carnivorous invertebrates that occupy a wide variety of marine habitats, from warm, shallow seas to the deep, cold oceans; many deepwater species migrate toward the surface at night to feed on krill, shrimp and small fish. The primarily predators of squid include whales, sharks, dolphins, large fish, sea birds and humans; indeed, their massive schools are an essential component of fisheries across the globe.