Here's why:
* Simple Body Plan: Cnidarians have a very simple body plan, often only two cell layers thick. They lack specialized organs for excretion.
* Diffusion: They rely primarily on diffusion for waste removal. Waste products like ammonia simply diffuse out of their cells directly into the surrounding water.
* Gastrovascular Cavity: The gastrovascular cavity, which serves as their digestive system, also plays a role in waste removal. Undigested materials are expelled through the mouth.
In summary: Cnidarians don't have specific organs for excretion. They utilize the simple processes of diffusion and their gastrovascular cavity for waste removal.
