The shell of the ancestor has been lost, with only an internal gladius, or pen, remaining. The pen is a feather-shaped internal structure that supports the mantle of squid and serves as a site for muscle attachment. It is made of a chitin-like substance.
The main body mass of the squid is enclosed in the mantle, which has a swimming fin along each side. It should be noted that these fins, unlike in other marine organisms, are not the main source of ambulation in most species.
The skin of the squid is covered in chromatophores, which enable the squid to change color to suit its surroundings. The underside of the squid is also almost always lighter in color than the topside, in order to provide camouflage from both prey and predator.
Under the body are openings to the mantle cavity, which contains the gills (ctenidia) and openings to the excretory and reproductive systems. At the front of the mantle cavity lies the siphon, which the squid uses for locomotion via precise jet propulsion. In this form of locomotion, water is sucked into the mantle cavity and expelled out of the siphon in a fast, strong jet. The direction of the siphon can be changed, in order to suit the direction of travel.
Inside the mantle cavity, beyond the siphon, lies the visceral mass of the squid, which is covered by a thin, membranous epidermis. Under this are all the major internal organs of the squid.
In female squid, the ink sac is hidden from view by a pair of white nidamental glands, which lie anterior to the gills. There are also red-spotted accessory nidamental glands. Both of these organs are associated with manufacture of food supplies and shells for the eggs. Females also have a large translucent ovary, situated towards the posterior of the visceral mass.
Male squid do not possess these organs, but instead have a large testis in place of the ovary, and a spermatophoric gland and sac. In mature males, this sac may contain spermatophores, which are placed inside the mantle of the female during mating.
The mouth of the squid is equipped with a sharp horny beak mainly made of chitin and cross-linked proteins, and is used to kill and tear prey into manageable pieces. The beak is very robust, but does not contain any minerals, unlike the teeth and jaws of many other organisms, including marine species. Captured whales often have squid beaks in their stomachs, the beak being the only indigestible part of the squid. The mouth contains the radula (the rough tongue common to all molluscs except bivalvia and aplacophora).
The eyes, found on either side of the head, each contain a hard lens. The lens is focused through movement, much like the lens of a camera or telescope, rather than changing shape as the lens in the human eye does.
In 1978, the "NOFOUL" rubber coating of the AN/SQS-26 SONAR dome of USS Stein (FF-1065) was damaged by multiple cuts over 8 percent of the dome surface. Nearly all of the cuts contained remnants of sharp, curved claws found on the rims of suction cups of some squid tentacles. The claws were much larger than those of any squid that had been discovered at that time.
In 2003, a large specimen of an abundant but poorly understood species, Mesonychoteuthis hamiltoni (the Colossal Squid), was discovered. This species may grow to in length, making it the largest invertebrate. It also possesses the largest eyes in the animal kingdom. Giant squid are often featured in literature and folklore with a frightening connotation. The Kraken is a legendary tentacled monster possibly based on sightings of real giant squid.
In February 2007, a Colossal Squid weighing 495 kg (1,091 lb) and measuring around 10 metres (33 ft) in length was caught by a New Zealand fishing vessel off the coast of Antarctica.
Squid are members of the class Cephalopoda, subclass Coleoidea, order Teuthida, of which there are two major suborders, Myopsina and Oegopsina (including the giant squids like Architeuthis dux). Teuthida is the largest of the cephalopod orders, edging out the octopuses (order Octopoda) for total number of species, with around 300 classified into 29 families.
The order Teuthida is a member of the superorder Decapodiformes (from the Greek for "ten legs"). Two other orders of decapodiform cephalopods are also called squid, although they are taxonomically distinct from Teuthida and differ recognizably in their gross anatomical features. They are the bobtail squid of order Sepiolida and the Ram's Horn Squid of the monotypic order Spirulida. The Vampire Squid, however, is more closely related to the octopuses than to any of the squid.
|World squid catch in 2002|
|Species||Family||Common name|| Catch|
|Loligo gahi||Loliginidae||Patagonian squid||24,976||1.1|
|Loligo pealei||Loliginidae||Longfin squid||16,684||0.8|
|Common squids nei||Loliginidae||225,958||10.3|
|Ommastrephes bartrami||Ommastrephidae||Neon flying squid||22,483||1.0|
|Illex argentinus||Ommastrephidae||Argentine shortfin squid||511,087||23.3|
|Dosidicus gigas||Ommastrephidae||Jumbo flying squid||406,356||18.6|
|Todarodes pacificus||Ommastrephidae||Japanese flying squid||504,438||23.0|
|Nototoda russloani||Ommastrephidae||Wellington flying squid||62,234||2.8|
Individual species of squid are found abundantly in certain areas, and provide large catches for fisheries.
The body of squid can be stuffed whole, cut into flat pieces or sliced into rings. The arms, tentacles and ink are also edible; in fact, the only parts of the squid that are not eaten are its beak and gladius (pen).