Cnidarians have a digestive system with only one opening through which both digested and undigested material passes. This type of system is called a gastrovascular cavity. Cnidarians use their stinging organelles to stun and kill their prey. Once captured, the food is passed into the opening of the gastrovascular cavity.
The smell of fluids from the wounded prey causes the tentacles in cnidarians to fold inward toward the opening of the digestive cavity where the prey is deposited. When the food enters the gastrovascular cavity, glands release enzymes that turn the prey to slurry within a few hours depending on the size. The slurry circulates through the digestive cavity as gastroderm cells on the inner walls absorb the nutrients. Absorption takes a few more hours, and it takes a few days for the cells to fully process the nutrients. The circulation of nutrients is propelled by currents created by cilia in the gastroderm, by circular movements or by both processes. Indigestible portions of the prey are expelled through the same opening in which the prey was taken in.
Cnidarians also feed by absorbing organic material dissolved in water, filtering food particles present in the water and through symbiotic processes with algae that inhabit their cells.