Squids use fins located on their heads to propel themselves when swimming at low speeds. These fins steer and stabilize the squids when moving slowly, and wrap around the body when they move quickly, by way of jet propulsion. Most octopuses do not have fins as adults. Some deepwater octopuses are exceptions. Octopuses are solitary animals that live alone in dens.
Some squids live in large schools, others are solitary.
Some squids school when young and become solitary later in life. The male octopus uses a specialized arm called a hectocotylus to transfer sperm to the mantle cavity of a receptive female. The female lays strings of fertilized eggs on the roof of her den. She guards, cleans and aerates the eggs with water expelled from her siphon until hatching anywhere from 30 days to a year, depending on the species. The female builds a wall of rocks to seal off the den and remains in the den until just before she dies, after the eggs have hatched.