Absorption is the third stage of the process and involves the uptake of the small molecules resulting from digestion. Elimination is the fourth, and last, stage where undigested material passes out of the digestive compartment. Digestion occurs in food vacuoles, gastrovascular cavities, and alimentary canals Intracellular Digestion in Food Vacuoles Food vacuoles are the simplest digestive compartments. They are organelles where a single cell digests its food without hydrolytic enzymes mixing with the cell's cytoplasm. Protozoa have food vacuoles that form around food by endocytosis.
Sponges differ from other animals in that all digestion is by the intracellular mechanism.
Extracellular digestion occurs within compartments that are continuous via passages with the outside of the body. At least some hydrolysis occurs in most animals by this mechanism. Most animals that have simple body plans possess a gastrovascular cavity . Digestive sac with a single opening. Function in both digestion and nutrient distribution.