A digestive tract has two openings and food is processed through a tube-like structure containing multiple organs from mouth to anus, as seen in humans, insects, and animals. In contrast, a digestive cavity has a single opening where food enters and leaves through the same opening, and digestion occurs in a pouch-like structure without different organs, as observed in hydra and flatworms. The key differences are the number of openings, structure, organ involvement, and ability to fully digest nutrients.