In organism or animals with hydrostatic skeletons the muscles contract the shape of the coelom and this is the reason there is the movement because of the pressure of the fluid inside the fluid filled cavity. Hydrostatic skeleton this a fluid filled gastrovascular cavity or coelom. It offers resistance and support to the contraction muscles for motility. It is also a structure found mostly in soft bodied animals that consist of a cavity filled with fluid and the coelom, surrounded by muscles.