Sponges are simple multicellular organisms. They do have aggregate cellular level of body organization. All major body functions in the sponge such as exchange of gases, circulation, and excretion are carried out by diffusion between the cells located at the pore (opening) of the sponge and the water which passes through the pore. All the cell present within the sponge take oxygen from water through diffusion. Similarly, carbon dioxide is released into water diffusion. The excretory product produced as a by-product as a result of the breakdown of protein is diffused out by individual cells into the water as it passes through the sponge.
Contractile vacuoles are organelles that are responsible for excretion function. They perform osmoregulation which manages wastes, water and other needed materials in the cell. They are found in some prokaryoric organisms like the algae, protists, amoeba and other unicellular organisms.
