Darwin, Vegetable Mould and Earthworms, p. 121.
"An Attempt to classify Earthworms," Quart. J. Micr. Sci. xxxi. 1890, p. 201.
Oxford, 1895.
See especially Vejdovsky, Syst. u. Morph. Olig. Prag, 1884.
Vejdovsky, Monographie der Enchytraeiden, Prag, 1879. Michaelsen, "Synopsis der Enchytraiden," Abh. Ver. Hamburg, xi. 1889, p. 1.