Ceratosoma gracillimum is characterised by the complete absence of the mantle
edge between the head and the well-developed lateral lobes.
Pretty similar are Ceratosoma trilobatum
which also has 3 lateral lobes, but a mantle edge between the head and the lobes and
Ceratosoma tenue which has 5 mantle
lobes.
More informations on Ceratosoma gracillimum are at
Bill Rudman's Sea-Slug Forum!
