Morphologically, cnidarians classify as polyps or medusae. Polyps are cylindrical and medusae are circular convex, like an umbrella. Both shapes have tentacles.
In general polyps are sessile but some species, like hydra, can move by alternating contact points on the substrate and performing somersaults. Medusae can move expelling water jets by contraction of the body.
Some cnidarians alternate polypoid and medusoid forms in their life cycle.
