1. Arms
   1. Left arm III hectocotylized in males with an elongate ligula. Third right arm sexually dimorphic with 1-3 greatly enlarged suckers in males.
      

      Figure. Male B. pygmaea. Left - View of right side showing enlarged sucker. Drawing from Chun, 1910. Right - Side view of the right arm and oral view of the left arm tip showing ligula. Drawings from Thore (1949).
      

2. Eyes
   1. Eyes well removed from brain on long optic stalks but within head.
   2. Eyes small compared to Japetella of comprable size.
      

Comments

The distinctive hectocotylus makes identification of nearly mature males easy. Unfortunately these males are rarely seen. As a result, separation of this species from Japetella diaphana depends largely on the differences in eye size and length of the optic stalks.