What is the respiratory system of sea cucumber, crustacean and frogs?

Crustaceans are a group of animals whose skeletons are on the outside of their bodies. They live in saltwater and breathe through gills, just like fish. They have many pairs of legs, and their bodies are jointed, like a human鈥檚 elbows or knees. Crustaceans use feelers for touching, feeling and smelling.