Innervation of ascidian siphons and their responses to stimulation

The distribution of sensory cells and nerves was studied in the siphons of Corella inflata Huntsman, 1912 and Corella willmeriana Herdman, 1898 by immunohistology and electron microscopy. Each siphon has about 8000 primary sensory neurons. A coronal organ of the compound type is present on the oral tentacles. Convergence in the afferent pathway is estimated at >10:1. A new category of cells associated with the velar sphincter muscle is described at the tentacle bases. Responses to stimulation were recorded using flow meters. Both siphons are sensitive to touch and near-field vibrations. Removal of the oral tentacles did not diminish vibration sensitivity. Gentle stimulation of the oral siphon evokes crossed responses in which the atrial siphon closes and the velar sphincter contracts. Stronger stimulation produces squirts with closure of both siphons and branchial ciliary arrest. Experiments with polystyrene beads show that the oral tentacles are sensitive to contact with inflowing particles. Beads of 500–600 μm diameter evoked rejection responses 88% of the time, 355–425 μm beads 61%, and beads <125 μm less than 8%. These responses, attributed to the coronal organ, were lost after amputation of the tentacles. Electrophysiology confirmed that crossed responses and squirting are centrally mediated reflexes, but local conduction pathways also exist and survive deganglionation.