Interstitial cells are reserve cells which can differentiate into any type of cells. Nematocytes are stinging cells used for offence and defence. Gastrodermal cells line the gastrodermis and intracellular digestion takes place inside these cells. All these cells are found in cnidarians. Choanocytes are found in sponges; they are specialised flagellated cells that line spongocoel and canals.