How do cnidarians sense?

Answer 1

Medusae and complex swimming colonies can sense tilt and acceleration by means of chambers lined with hairs which detect the movements of internal mineral grains. If the body tilts in the wrong direction, the animal rights itself by increasing the strength of the swimming movements on the side that is too low. They also have "little eyes" known as ocelli, which can detect the direction from which light is coming. Although these probably do not form images, their lenses simply produce a clearer indication of the direction from which light is coming.

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I have to add that most species of box jellyfish have four groups of two complex eyes, and either two or four simpler ocelli, for a total of up to twenty four eyes. The complex eyes are similar to our own and have corneas, lens, and retinas. Tests have demonstrated their ability to navigate obstacles and recognize colors by way of their vision, although it has yet to be demonstrated that they can track prey. Their nervous system is not centralized, but it is a much more complex nerve net than other species of jellyfish. So far, some primitive learning and memory ability has been demonstrated in experiments.

Answer 2

They sense by using their central nervous system