Mechanism of ingestion in unicellular organisms (such as amoeba)
When food particles come in contact with the cell surface of amoeba, it slowly engulfs the whole food with the help of pseudopodia. This process takes place in approximately 2 min.
Different modes of ingestion reported in amoeba are as follows:

• Circumvallation: With the help of pseudopodia food cup form to engulf active prey like Paramecium.
• Circumfluence: Amoeba rolls over the inactive prey.
• Import: Food particles like algal filaments passively sink into the body; when it comes in contact.
• Invagination: A sticky and toxic substance is secreted by pseudopodia, which kills the prey and then taken by invagination
• Pinocytosis: Certain channels present on the body surface of amoeba to ingest liquid food.

Digestion of food in Unicellular Organism (Amoeba): Ingested food remains in food vacuole. The food vacuole is then transported deeper into the cytoplasm where they fuse with lysosome that contains amylase and certain proteinases. After digestion solid food is converted into liquid diffusible form and readily absorbed by cytoplasm through diffusion process. The undigested food is egested by exocytosis. The complete steps of nutrition are shown in Figure 11.1.

Mechanism of Digestion in multi-cellular organism (such as Hydra)
Hydra is a fresh water diploblastic animal. Its body composed of two layer. Outer layer is protective and sensory epithelium and inner layer is gastrodermis act as a nutritive epithelium. The central body cavity known as coelenteron act as digestive tract.