Cnidarians have a special kind of skeleton, it is called a hydrostatic skeleton.
This skeleton is usually found in only soft bodied animals like cnidaria.
This skeleton is formed by a fluid filed cavity within the body sometimes called the coelom. In the case of Cnidaria, the gastrovacular cavity is the coelom in Cnidarian's hydrostatic skeleton. (www.boundless.com)
Musculature
In medusas, they have a circular muscle on the base of their bell to that contracts and pushes water out of the bell to propel them forward. (bril2002.narod.ru.html)
Hydras have musculoepithelial cells that cover the outer surface of the body that have longitudinal muscle fibers. In sea anemones all the mucels are in gastrodermis (the inner layer) (bril2002.narod.ru.html)