The octopus, squid, cuttlefish, and chambered nautilus are all cephalopods. The term cephalopod means head foot. Their head is right next to their foot (tentacle end). The squid appears to be a shelless mollusc, but its shell, or the remnants of it, is inside its body. It appears as a thin, clear blade inside the mantle. It is called the pen, and it is useful only for muscle attachment.
The squid swims by forcing water out of its funnel in a manner similar to jet propulsion. With its streamlined, tapered body and broad, triangular fins that stabilize, the squid is a very good swimmer and an efficient predator. It can swim into a school of fish and, using its image-forming eyes, seek out prey. It then uses its long sucker-tipped tentacles to capture its prey. The prey is paralyzed quickly with a bite behind the head from the beak of the squid and an injection of poison. The beak is located in the center of the circle of eight arms and two tentacles. It cuts the food into pieces and transfers it to its mouth with its radula.
Located on the arms are suckers which hold tight when muscles are contracted. Notice that the tentacles are twice as long as the arms and only have suckers on their flattened ends.
The squids' enemies are mostly sea mammals or large fish that are very fast. They defend themselves by use of an ink sac that expels an inky blackness into the water when the squid is alarmed. The ink confuses the enemies' sense of sight and smell. Squids also have pigment cells in their skin that enable them to light or darken their skin color in order to blend with the sea floor or to match surface light from above. If attacked and a tentacle is torn off, a new one is regenerated.
Squids are useful to humans. They are dried and prepared for food in some cultures. Fishermen often use them for bait. In China especially, the ink sac is collected and sold as the famous India ink.