Squid extract oxygen from the water using gills, just as fish do. These gills are located in the squid's mantle, the soft but muscular cavity behind the squid's head.
A squid swims by expanding the muscles that line its mantle and allowing the water around it to rush into the cavity. When the water rushes through the inside of the mantle, it passes over the squid's gills. The gills extract the oxygen from the water, and expel carbon dioxide that the squid has produced in its circulatory system into the same water.
Once oxygen has been extracted from the water, the oxygen immediately enters the squid's circulatory system by entering the blood. From there it is transported to all of the cells that need oxygen.
All of the muscles in the mantle then contract suddenly, which forces the water that the squid has just taken in and the waste carbon dioxide out of a tube behind the squid's head known as the funnel. This jet of water leaving the funnel propels the squid backwards, and this is how they move. This method of movement is so powerful that, when a squid flees a predator, it can move as fast as 25 body lengths a second.