Cnidaria
The cnidaria phylum encompasses organisms like jellyfish, anemones, and stony corals. Their excretory systems are extremely basic.
excretory system
The cnidaria excretory system is extremely basic. When a cnidarian captures its prey it swallows it up through its mouth and into a digestive cavity. Then, digestive enzymes are released that turn the prey into a slurry like substance. From there the cnidarian absorbs the needed nutrients. Then it come out the same way it came in. The remainder is spit out of the mouth where the water's current takes it away. Here you can see a jellyfish eating a small fish.
examples
The terrifying box jellyfish (Chironex fleckeri) is one example of a cnidarian. Its extremely painful sting can incapacitate or kill a person. The transparency of it does a great job at showing the simplicity of how it works. Food comes in, its eaten up, and then spit out.
This here is an anemone (Anemonia sulcata). While it does not look like a jellyfish its excretory system works the same.
A final example is a type of stony coral called brain coral (Faviidae). This too excretes waste like anemones and jellyfish.