The food molecules then diffuse or are actively transported into the ground substance of the cytoplasm where the food is utilized. Digestion that occurs in food vacuoles within a cell's cytoplasm is called intracellular digestion. Digestive waste is eliminated from the food vacuoles of collar cells by exocytosis. Amoebocytes are the food distribution system of sponges. The sponge body is composed of two layers of cells.
Collar cells form one layer, and epidermal cells for the other.
These two layers are separated by a jelly-like material called mesoglea. Amoebocytes are present within the mesoglea. These cells obtain food molecules from collar cells. They then move to the epidermal cells and release food to these cells. Nitrogen waste and carbon dioxide leave sponge cells by diffusion into the water moving through the sponge.