"Challenger" Reports, vol. xii. 1885, "Polychaeta," p. 198; and Oka, Zoolog. Centralbl. ii. 1895, p. 591.
Two new heads have been observed in Typosyllis variegata by Langerhans, and two new tails in another Syllis.
Dalyell, The Powers of the Creator revealed, etc., vol. ii. 1853, p. 225 et seq.
von Kennel, Arb. Zool. Instit. Würzburg, vi. 1883, p. 259.
Leidy, Proc. Acad. Nat. Hist. Philadelphia, 1883, p. 204.