Why don't some creatures have skeletons?

Some creatures lack skeletons because they have evolved different structural adaptations to support their bodies. For example, many invertebrates, like jellyfish and worms, rely on hydrostatic pressure, which uses fluid-filled cavities to maintain shape and facilitate movement. Others, like octopuses, possess soft bodies that allow for greater flexibility and adaptability in their environments. This evolutionary path can be advantageous in various ecological niches, reducing the energy costs associated with building and maintaining a hard skeletal structure.