Recent work (Roller 1925; Perkins 1928) on crustacean colour change has provided suggestive evidence of humoral co-ordination. Before it can be accepted as conclusive, it is necessary to examine more carefully the normal reactions, the relevant stimuli and the receptive mechanisms involved. The present communication is mainly concerned with one aspect of the latter. That Ligia oceanica responds by colour change to “background” , i. e. light scattered from its immediate surroundings, and that the response depends on the eye has been recorded in a note by Tait (1910). This animal has been selected for further investigations for the following reasons: ( а ) The eye is sessile. Hence the field of vision can be controlled more readily than is possible in decapod and in schizopod species on which most previous work dealing with crustacean colour change has been based.