Flabellifera are a type of isopod. Their bodies end in a tail fan, made by the last pair of appendages and the telson (end piece). This species is parasitic, sharing the title of ‘fish lice’ with other certain parasitic copepods.
John H. Finley ed. Nelson's Perpetual Loose-Leaf Encyclopaedia (vol. 5) (New York, NY: Thomas Nelson and Sons, 1917)