porifera
Porifera are sea sponges. Sea sponges are extremely basic life forms with no real structured systems in them. They just rely on the flow of water through them to take care of their needs, like oxygen and food. Sea sponges are made up of cells with a jelly like substance called mesohyl sandwiched between them.
excretory system
The excretory system of porifera is extremely simple. The cells release their waste products. Then these waste products are simply carried away by the water flowing through the organism.
examples
The tube sponge (Aplysina archeri) is one example of porifera. The center of the sponge is hollow from the top all the way to the bottom.
The common sea squirt (Ascidiacea) is another example.
One last example is the yellow sponge (Biemna anisotoxa).