Rés. sci. Expéd. Travailleur et Talisman, "Échinodermes," 1894, pp. 10-15.
Schiemenz (reference on p. [440] n.).
This fact was discovered by Dr. E. J. Allen, Director of the Plymouth Biological Station, who pointed it out to the author during the latter's sojourn at the station in 1899.
This figure does not show the animal's attitude during forward progression quite correctly. The tips of the two anterior arms should be bent outwards, not inwards as in the figure.
In the more primitive Ophiuroidea (Streptophiurae) it persists all over the body; in Cladophiurae it is found on the central part of the disc.