Water is a denser medium than air. It is difficult to move rapidly through it. As a result, active aquatic animals Like fish have streamlined bodies. Water , however, helps to support the body mass of large aquatic animals. This is not the case with large land plants and animals as air is a much lighter medium than water. Hence, large terrestrial plants and animals have better developed supporting systems (vascular xylem and skeleton) than aquatic ones. These adaptations are mainly structural.
