Gas exchange occurs through its body cavity. The circulation passes through the nerve net so it is more efficient, which also passes through the body cavity. Nutrients and food enter Cnidaria due to flagella in choanocytes. The flagella create a current bringing in food and tentacles capture it. The amoebocytes inside are specialized cells that digest food and circulate nutrients.
Respiration happens that way too.
Food and nutrients are captured by the tentacles which bring them into the mouth of the Cnidaria. After that, the nutritients are spread through the animal through the body cavity. All waste is disposed through the anus, which is where it came in. Cnidaria respires using hydrolysis. They would take in dissolved O2 and release CO2 by opening the mouth.