Gills are the respiratory organs of large aquatic animals. These organs are specifically adapted to their function. In Osteichthyes, the gills have four gill arches, Chondrichthyes have two, fish has 7 gill baskets with a long bony cover for the gills which is used to push water.
Which one of the following animals has no blood but respires?
Hydra doesn't have blood. It is a single-cellular animal. In the hydra, respiration is accomplished via diffusion of molecules of oxygen and carbon dioxide through the skin whereas earthworm, cockroach and octopus has blood. Thus, option B is correct.