CHAPTER I

IMMUNITY IN UNICELLULAR ORGANISMS 11 Infective diseases of the unicellular organisms.—Intracellular digestion in the Protozoa.—Amoebo-diastase.—Part played by digestion in the defence of the Protozoa against infective parasites.—Defences of the _Paramaecia_ against micro-organisms.—Part played by irritability in defence in the lower organisms. Immunity of unicellular organisms to toxins.—Acclimatisation of bacteria to toxic substances.—Protective secretion of membranes by bacteria. Adaptation of the Protozoa to saline solutions—of yeasts to poisons—of yeasts to milk-sugar. Irritability of unicellular organisms and Weber-Fechner’s psycho-physical law.