Ciliated sensory cells and associated neurons in the lip of Octopus joubini Robson

The lip of Octopus joubini is a fleshy fold around the beak that is subdivided distally into finger-like papillae and overlayed by an uninterrupted noncellular cuticle. The muscular core of the lip has a high proportion of nervous tissue. The simple epithelium contains numerous ciliated sensory cells, especially in the papillae. In many of these cells the cilia lie deep within the cytoplasm and usually appear to extend toward the surface. Receptors with intracellular cilia also lie below the epithelium and send dendrites bearing cilia to the surface. Large unipolar interneurons that may receive synapses from the ciliated receptors lie in the musculature near the papillae. The sensory system of the octopus lip is more advanced than that of the squid, and it is very similar to that of Sepia. The relationship of these findings to the phylogeny and ecology of cephalopods is discussed.