Quart. J. Micr. Sci. xli. 1901, p. 477.
Loisel, J. de l'Anat. et Phys. xxxiv. 1898, p. 1.
R. v. Lendenfeld, Acta Ac. German. lxix. 1896, p. 22.
Challenger Report, lix. 1887, p. 214.
Topsent, Zoologie Descriptive, i.; also Cotte, C. R. Soc. Biol. Paris, 1902, pp. 638-639.