Watson, Journ. R. Mic. Soc. 1890, p. 685; see also Dalyell, loc. cit. ii. p. 195.
Schmiedeberg, Mt. Zool. Stat. Neapel, iii. 1882, p. 373.
For pelagic forms, see Camille Viguier, Arch. de Zool. Expér. (ser. 2) iv. 1886, p. 347; also Reibisch, Die pelag. Phyllodociden u. Typhloscoleciden d. Plankton Exped. 1895.
Lankester, Journ. Anat. and Physiol. 1868, p. 114; and 1870, p. 119; see also MacMunn, "On the Chromatology of the Blood in some Invertebrates," Quart. J. Micr. Sci. xxv. 1885, p. 469.
For coloured pictures of worms consult Schmarda, "Neue wirbellose Thiere," 2nd part, 1861; Milne Edwards in Cuvier's "Règne Animal" (Ed. Disciples de Cuvier).