Oligochaetes, like earthworms, possess a hydrostatic skeleton where the fluid-filled coelom provides support and facilitates movement through muscle contractions. 

A hydrostatic skeleton is a type of skeletal system where a fluid-filled cavity (the coelom) provides support and rigidity, allowing for movement through the coordinated contraction of muscles surrounding the cavity. 

While arthropods are known for their exoskeletons, hydrostatic skeletons are found in some arthropod larvae and during the molting process, but are not the primary form of skeletal support in adult arthropods.