Squids often bring to mind fanciful images from the movie "20,000 Leagues Under the Sea," where giant squids grappled with ships. In real life, about 375 species inhabit the world's oceans. They are members of the phylum Mollusca and are related to snails. Smaller squid are around 20 to 50 cm (8 to 20 inches) long, but giant squid reach about 18 meters (60 feet) long. Squid are predators, capturing smaller animals such as fish, crustaceans and other squid. Food passes along a flow-through digestive tract, with wastes discharged into the inner cavity of the mantle and then to the outside.
Streamlined, torpedo-shaped squid have a tough, skin-like outer layer called the mantle that covers the body organs. The pen, which is all that is left of the mollusc shell, lends the mantle some rigidity. Fins help squid maneuver through the water. Squid move by jet propulsion, pumping water inside the mantle and expelling it through a narrow structure called the siphon or funnel. The head has two large eyes and 10 arms. The two longest arms, called tentacles, have suckers that often have sharp hooks that help hold prey. The eight shorter arms bring prey toward the mouth.
The Digestive Tract
A squid's digestive tract is made up of a tubular structure, with the food passing in a straightforward manner through the tube from the mouth to the anus. For this reason, it is sometimes called a pass-through digestive system. Parts of the tube are expanded into pouches or sacs, and accessory digestive organs occur along the length of the tube to aid digestion and absorb nutrients. An extensive system of valves and ducts regulate the flow and absorption of digestive juices and the nutrients that are released during digestion.
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The Beak and Tongue
Once food is captured, the tentacles and arms holds the prey against the mouth opening. There a horny parrot-like beak grabs onto it, holding it firmly so that the radula, a rough, tongue-like organ just inside the mouth, can rasp it into fine pieces. Squid can't swallow large pieces of food because the digestive tract passes through a circular hole in the middle of the squid's brain, and larger pieces might damage the brain. The tongue pushes the ground-up food from the mouth into the throat, and then into the esophagus.
Salivary glands in the region of the esophagus empty their juices into the esophagus to mix with the finely rasped food. Farther along, secretions from the elongate, brownish liver enter the mix within the esophagus. The esophagus connects to the whitish sac-like stomach, where digestion begins due to the mixing of the enzymatic digestive organ secretions. Food then enters the stomach pouch, also called the caecum, along with substances from the pancreas.
The intestine is a narrow tube that exits out of the caecum and travels through the rest of the space within the mantle cavity. Toward the end, it becomes the rectum and farther along, the anus, where it connects to the siphon to eject waste materials along with the water pumped from within the mantle for propulsion.