Family: Frontal cirri differ in size and/or pattern from ventral ones of files;
"collar" extends short distance along right edge of body; oral area not U-shaped
(Illustrated Guide, 1985).
Genus: Elongate body drawn out into a tail-like portion; 3 frontals; 2-4 rows of ventral cirri; marginals; no transverse cirri; sometimes rose- or violet-colored (Kudo, 1966). Frontventral cirri in zig-zag files; rear half narrowed, tail-like (Illustrated Guide, 1985).