We all know that unicellular organisms do not have a separate alimentary canal system. All the functions of life are carried out inside a single cell. Food is taken in directly into a cell by phagocytosis/endocytosis and then with the help of enzymes digested in a food vacuole. Figure shows the process of endocytosis in Amoeba.
Figure: Digestion in amoeba
Similar intracellular digestion occurs in sponges, some coelentrates, ctenophores and turbellarians. Although the process is called intracellular digestion, the food material is actually separated from the rest of the cellular material by a membrane which it can cross after digestion. In organisms such as cnidarians and platyhelminths, a gut or enteron is present and here along with extracellular digestion where enzymes are secreted into the cavity, intracellular digestion also takes place within the cells that line the ca2ity. However, in annelids and molluscs more extracellular than intracellular digestion takes place. Digestion is entirely extracellular in nematodes, insects, echinoderms and vertebrates.