Phagocytella Theory

Mechnikov proposed the 'parenchymella' theory (later 'phagocytella'), according to which multicellular animals descended from an extinct ancestor — an organism consisting of two parts: an outer layer of cells and an inner mass of cells capable of capturing and digesting food particles. Mechnikov contrasted this theory with Ernst Haeckel's 'gastraea theory'. The phagocytella theory became an important milestone in understanding the evolution of multicellular organisms.