Movement

Cnidarians are motile at least one time in their life.

As you know, adult polyps do not move, they are sessile. But when they are young they are they are a larvae which swims a short distance to find a good environment to live in.

With its translucent parts, the jellyfish are nearly invisible in the water. Their bell-shaped top, flaps and dangling tentacles appear like the infamous Medusa, Greek mythological character with poisonous snakes instead of hair. One type of jellyfish moves through the water by attaching itself in the current to seaweed or other material, and catch prey at that spot. Others use jet propulsion by using specialized coronal muscles that are located on the bottom of the bell, and they force the water out of the bell to push the jellyfish forward. The ocean’s currents are the greatest determinant of the jellyfish’s movements, since even the ones that use jet propulsion cannot move against currents and waves. More significantly, without eyes or a brain, only nerve cells are directing the jellyfish to find food or avoid danger, with sensing organs identifying their location and distance from light. In order to balance in the water, sacs located on the bell rim stimulate nerves to contract muscles to keep the jellyfish stable.