some living beings (small insects, jelly fish, starfish, and microorganisams) do not have skeleton. how do they perform body movement?

Dear student.

The organism which does not have skeleton have other support systems that facilitate their movement. For example: Microbes mostly bear cilia and flagella that help them in movement, most of the insects have jointed limbs or other appendages,  starfish have tiny tube feet which help them to move along sea floor.

Regards.