Squids are marine cephalopods belonging to the phylum Mollusca and order Teuthida which encloses about 300 species. Like other cephalopods they have a well demarcated head, manle cavity, arms and bilateral symmetry. They have eight long and flexible arms arranged in pairs with two long tentacles just like the cuttlefishes. They are very strong and potent swimmers and some species are known to glide out of water for some time. They differ from the ancestral molluscs because their body plan is condensed antero-posteriorly and extended dorso-ventrally. The foot present in the ancestor has been modified in squids into a complex set of tentacles which are very efficient sense organs. The eyes are also developed in such a way that it resembles like that of the vertebrate eyes. The ancestral shell that was present in the ancestors has been lost in the squids and only gladius or pen is present. The pen is a very delicate and feather shaped structure that supports the mantle of squid and also serves as an area for the attachment of muscles. Pen is made up of a material that resembles chitin in its composition.
The bulk of the body is occupied by the mantle which bears swimming fin on each side. These fins like other marine animals are not used for locomotion. The skin bears a large number of chromatophores that enable the squid to change color according to the surroundings making it effectively invisible in the water body. The underside of the body is lighter in color in comparison to the upper side and is an efficient source of camouflage. The inside of the mantle cavity bears gills as well as openings of excretory and reproductive systems. The front of the mantle cavity is characterized by the presence of a siphon which is used for locomotion by the squid and it follows the principle of jet propulsion. During the act of locomotion water is sucked into the mantle cavity and expelled out of the siphon in fast, strong jet. The direction of siphon can be changed according to the direction of travel. Inside the mantle cavity just beyond the siphon there lies the visceral mass which is covered by a thin layer of epidermis. This visceral mass houses all the vital organs of the body.
There is a giant axon measuring about 1 mm in diameter is known to innervate the mantle cavity and controls the mechanism of jet propulsion. Squids bear high powers of intelligence among all invertebrates. The Humboldt squids use active communication system while searching their food. In females the ink gland is hidden benefit a pair of nidental glands which lie anterior to the gills. Both these organisms participate in food collection and formation of shells over the eggs. Females also bear a large and translucent ovary lying posterior to the visceral mass. Males lack these organs but possess a large testis, a spermatophoric gland and sac. This sac contains spermatophores in the mature males which are transferred into the mantle of the female during mating. The shallow water species of continental shelf and epipelagic zones bear hectocotyli which are modified arms especially for fertilizing the female's eggs. Most deep sea squids lack hectocotyli but bear large penalties for this purpose.
Squids have complex digestive systems. Stomach is highly muscular and is present in the mid-point of the visceral mass. From here the food bolus moves in the caecum for digestion. Caeum is an large, white organ present next to either testis or ovary. Finally food enters the liver present next to the siphon for absorption. Solid waste is passed out of the rectum. Beside the rectum ink gland is present which injects black ink in the mantle cavity. Squids have three hearts. Two hearts supply the blood to the gills and each encircling the systemic heart which pumps blood around the body. Blood bears copper-rich haemocyanin which is an efficient oxygen transporter. Kidneys are difficult to identify. The systemic heart is made up of one lower ventricle and two upper auricles. The head bears 8 arms and 2 tentacles both act as hydrostatic organs bearing suckers along their edges. In mature males one half of the left ventral tentacle is herterocotylized and bears a copulatory pad rather than the suckers. Mouth terminates into a sharp, horny beak made up of chitin and cross-linked proteins used for killing and teasing the prey into smaller chunks. Beak lacks teeth but radula is present in the mouth. Eyes present of either side of the head are protected by a hard lens. The lens work in the same way as found in the human eyes. They have limited powers of hearing.
They may attain a length of about 60 cm but the giant squids may be about 13 m long. Squids have derived their name from a Greek word. Many species are a part of popular cuisines among the Chinese, Italian, Korean, Turkish, Spanish, Indian and Portuguese people.