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]