Crustaceans breath oxygen, as does (nearly) every other creature on earth. As they are aquatic creatures, they use the same respiratory system as fish and breath oxygen through gills. The diagram shows the apparatus that can be used to investigate the exchange of gases/respiration in small aerobic crustaceans. Carbon dioxide is produced by the process of respiration and affects the volume and pressure of gas in the tube.In tube P, O2​ is taken in and CO2​ released by crustaceans, is absorbed by soda lime reducing the pressure exerted by the gas. Hence, liquid marker moves to left.Whereas in tube Q, glass beads are present instead of soda lime, so there is no absorption of CO2​, the pressure exerted by CO2​ here is balanced by that of oxygen and hence the liquid marker stood still.