Single-celled organisms that live in seawater have an external environment that provides all of the needs of the organisms, such as food, disposal of excreted wastes, and relatively con­stant conditions for maintenance of life.

As the complexity of organisms increases, the problem of supplying each cell with a proper environ­ment becomes more acute. Higher forms of animals have developed circulating blood, and the fluids derived from it, as a means of main­taining a relatively constant environment for all cells.

More on the topic Single-celled organisms that live in seawater have an external environment that provides all of the needs of the organisms, such as food, disposal of excreted wastes, and relatively con­stant conditions for maintenance of life.: