- Left arm III hectocotylized in males with an elongate ligula. Third right arm sexually dimorphic with 1-3 greatly enlarged suckers in males.
- Eyes well removed from brain on long optic stalks but within head.
- Eyes small compared to Japetella of comprable size.
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.
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