A motile cilium found in dinoflagellates. It trails the cell and acts as a steering rudder. It is often partially contained in a furrow called the sulcus, and emerges from a flagellar pore located in the sulcus.
Source:
Wikipedia:Dinoflagellate#Morphology,
ISBN:0632009152,
GOC:at,
http://tolweb.org/Dinoflagellates/2445,
http://www.sms.si.edu/irlspec/Phyl_Dinofl_Glossary.htm
Comment
Note that we deem cilium and microtubule-based flagellum to be equivalent. In this case community usage refers to 'flagellum' rather than 'cilium', hence the primary term name, but the cilium parentage is deliberate.