How do polyps use muscles and water pressure to change shape?

In polyps, the water-filled coelenteron functions as a hydrostatic skeleton, which works alongside the elastic mesoglea to maintain the animal's form and facilitate movement.12

Polyps change shape through the interaction of two primary muscle layers:
* Elongation: When circular muscles—typically located in the gastrodermis—contract against the fluid in the gut cavity, the body becomes longer and thinner.12
* Shortening: When longitudinal muscles—found in the ectoderm or gastrodermis—contract, the body becomes shorter and fatter.12
* Bending: This occurs due to unequal contractions of longitudinal muscles on opposite sides of the body.12

When the mouth is closed, the trapped water provides the necessary pressure for these movements. Some polyps, such as the hydra, can also reduce their total volume by using muscles to expel water through the mouth, later reinflating the gut cavity using cilia to circulate water back inside.2