Most animals that have simple body plans possess a gastrovascular cavity . Digestive sac with a single opening. Function in both digestion and nutrient distribution. Hydra is a carnivore which captures prey. Food items are immobilized by stings from nematocysts on the tentacles.
Tentacles then force prey through the mouth into the gastrovascular cavity.
Specialized gastrodermal cells secrete digestive enzymes that fragment the soft tissues of the prey into tiny pieces. Some gastrodermal cells also possess flagella whose movement prevents settling of food particles and distributes them through the cavity. The small pieces are phagocytized by nutritive gastrodermal cells and surrounded by food vacuoles. Hydrolysis is completed by intracellular digestion.