What is the advantage of the cnidarian gastrovascular cavity

What is the advantage of the cnidarian gastrovascular cavity (and extracellular digestion) relative to phagocytosis (and intracellular digestion) as observed in the sponges?

Solution

Sponges ahve a basic spongocoel cavity through which the water get filtered. Sponges capture phytoplanktons by this water filtration through the cells. In the cells the digestion of planktons is done by phagocytosis that mainly occurs inside the cells. The body is spongy to absorb oxygen and water and expel the remaining waste material.

In Cniderians the digestion occurs outside the body cells. Their food is bigger in size like fishes and other small sized aquatic animals. Food is brought into the cavity and the tissues surrounding the cavity have enzymes in cnidocyt to digest food. Nutrients are absorbed after digestion and later on distributed to all the body parts by diffusion process. The cavity has just one opening for both intake and excretion of waste material.

Cniderian gastrovascular cavity is advanced in evolution towards animals performing extracellular digestion.