ABSTRACT
The ultrastructure of the sensory ‘hook-like’ structures on the arm spines and tentacle scales of the brittlestar Ophiocomina nigra are described. Sensory hooks are shown to be cuticle covered epidermal projections structurally composed of two monociliated sensory cells surrounded by support cells. These hooks are located exclusively in the proximal third of the spines and on the tentacle scales. It is suggested that these hooks may be responsible for monitoring the depth of mucus on the spines and initiating a spine cleaning reaction in the podia. This possible function is considered within the context of the spines being responsible for mucus production for both mucus net feeding and as part of a defensive reaction.
