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Animals take food into a body cavity that is continuous with the outside environment. They secrete digestive enzymes into that cavity, and the enzymes break down the food into nutrient molecules that can be absorbed by the cells lining the cavity.

The simplest digestive system is found in the simplest animals such as sponges. Water flows from the environment through the body of the sponge in water channels, and individual cells capture food particles from the water (see Figure 30.2B). A slightly more specialized digestive system is a gastrovascular cavity, which connects to the outside world through a single opening. Cnidarians, such as jellyfishes, capture prey using stinging nematocysts and use their tentacles to cram the prey into their gastrovascular cavity. Enzymes in the gastrovascular cavity partially digest the prey. Cells lining the cavity take in small food particles by endocytosis. The vesicles created by endocytosis then fuse with lysosomes containing digestive enzymes, and intracellular digestion completes the breakdown of the food. Nutrients are released to the cytoplasm as the vesicles break down.