Ciliated Sensory Cells in Amphioxus (Branchiostoma)

Abstract

Various workers during the past century have figured sensory cells bearing cilia amongst the ordinary non-ciliated epidermal cells of the body surface. Similar sensory cells were also found on the buccal cirrhi, and on the velar tentacles at the entry to the atrium. This earlier work at the light microscope level was summarized by Franz (1923) in his excellent review. More recently Schulte & Riehl (1977) have re-examined the innervation of the oral region and buccal cirrhi at the ultrastructural level, and observed two types of sensory cell in these regions, distinguished by their apical structure. They suggest that these are both secondary sensory neurons (as Franz had previously surmised), but their figures hardly make this suggestion convincing. In this note we show that there is probably only a single type of sensory cell amongst the epidermal cells of the body surface and buccal cirrhi; that it synapses at its base with incoming axons of central cells, and that a different type is indeed found in amphioxus, but only in the velar tentacles.