Wordnet 3.0
NOUN (1)
1. 
 one of the often brightly colored and branching hornlike structures on the back of the nudibranch (and other related mollusks) that serve as gills; 
The Collaborative International Dictionary of English v.0.48:
ceras \ceras\ n.
   one of the often brightly colored and branching hornlike
   structures on the backs of nudibranchs and other related
   mollusks that serve as gills.
   [WordNet 1.5]
WordNet (r) 3.0 (2006):
ceras
    n 1: one of the often brightly colored and branching hornlike
         structures on the back of the nudibranch (and other related
         mollusks) that serve as gills