Increasing size brings increasing control over your environment, and allows for greater complexity. But larger and thicker organisms can no longer rely on diffusion. Cells that are too far away from the surface will starve to death, or drown in their own poisons before they can be carried off. And to make matters worse, the surface area across which gases, nutrients, and wastes must be exchanged rapidly decreases as you get larger. As organisms become larger, their volume increases much more rapidly than their surface area.
Cells become farther removed from the outside at an exponential rate.
Consider a spherical creature. The animals volume increases as a cube of its radius, but its surface area only increases as the square of the radius. So as it gets bigger, more and more volume is covered by less and less surface area. They rely on a network of tubes that runs throughout the body of the organism, a vascular system. These closed tubes can circulate water and nutrients, and carry off wastes.