[sonicl]

Squid are a large, diverse group of marine cephalopods. Like all cephalopods, squid are distinguished by having a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms and two tentacles arranged in pairs.

The main body mass of the squid is enclosed in the mantle, which has two swimming fins along each side. It should be noted that these fins, unlike in other marine organisms, are not the main source of ambulation. 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 found to be lighter 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. This is done by sucking water into the mantle cavity and quickly expelling it 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 the visceral mass of the squid, which is covered in 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.

Squid, like all cephalopods, have a complex digestive system. Food is transported into a muscular stomach, found roughly midpoint in the visceral mass. The bolus is then transported into the caecum for digestion. The caecum, a long, white organ is found next to ovary or testis. In mature squid, more priority is given to reproduction and so the stomach and caecum often shrivel up towards the later stages of life. Finally, food goes to the liver (or digestive gland), found at the siphon end of the squid, for absorption. Solid waste is passed out of the rectum. Beside the rectum is the ink sac, which allows a squid to discharge a black ink into the mantle cavity at short notice.

Squid have three hearts. Two branchial hearts, feeding the gills, each surrounding the larger systemic heart that pumps blood around the body. The hearts have a faint greenish appearance and are surrounded by the renal sacs - the main excretory system of the squid. The kidneys are faint and difficult to identify and stretch from the hearts (located at the posterior side of the ink sac) to the liver. The systemic heart is made of three chambers, a lower ventricle and two upper auricles.

The head end of the squid bears 8 arms and two tentacles, each a form of muscular hydrostat containing many suckers along the edge. These tentacles do not grow back if severed. In mature male squid, one basal half of the left ventral tentacle is hectocotylised - it ends in a copulatory pad rather than suckers. This is used for intercourse between mature males and females.

The mouth of the squid is equipped with a sharp horny beak made of chitin, used to kill and tear prey into manageable pieces. 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 of the squid, found on either side of the head, contains a hard lens, which is used much like the lens of a binocular for focusing; rather than changing shape, like a human eye, it moves mechanically.