1. The causes of extreme toxicity of heavy metal ions as well as of impurities in water and glass on Paramecium aurelia;Ludwig;Zeitschrift für vergleichende Physiologie,1927
2. The basal metabolism of Paramecium caudatum Ehrbg. A contribution to the question of the function of the contractile vacuoles;Ludwig;Archiv für Protistenkunde,1928
3. Permeability and water exchange in Noctiluca miliaris;Ludwig;Zoologischer Anzeiger,1928
4. On the functional relationship between population density, nutrient supply, and division rate in Protista and on increase in population size in general;Ludwig;Biologia,1928
5. A comparative study of growth laws;Ludwig;Biologisches Zentralblatt,1929