A muscular hydrostat is a biological structure found in animals. It is used to manipulate items or to move its host about and consists mainly of muscles with no skeletal support. It performs its hydraulic movement without fluid in a separate compartment, as in a hydrostatic skeleton.

A hydrostatic skeleton or hydroskeleton is a type of skeleton supported by hydrostatic fluid pressure, common among soft-bodied invertebrate animals colloquially referred to as "worms". While more advanced organisms can be considered hydrostatic, they are sometimes referred to as hydrostatic for their possession of a hydrostatic organ instead of a hydrostatic skeleton, where the two may have the same capabilities but are not the same. As the prefix hydro- meaning "water", being hydrostatic means being fluid-filled.