1. Jellyfish changes the volume of the inner cavity by shrinking the outer shell and squeezing the inner cavity, squirts out the water in the cavity and moves by spraying water.

When the jellyfish tries to sink into the water, its tentacles will extend upward to form a long line. When it intends to rise and move in a certain direction, the distal end of the tentacles bends against the moving direction, and with the help of the tentacles, the moving direction is effectively changed.