TABLE 1.

Table summarizing the principal findings on the tunicate secondary sensory cells.

Taxon	Species	Location	Proposed function	Behavioural test	Hair bundles	Cytoplasm of sensory cell	Radial filament connecting the cilia	Supporting cells	Accessory secretory cells	References
Pleurogona stolidobranchia	Botryllus schlosseri	tentacles of oral siphon	Sensitivity to contact of inflowing particles	Tentacle stimulation test	Single cilium and stereovilli	Accessory centriole	Loose fibrillar matrix generally present among microvilli and cilia	Supporting cells form a wall or crest		Burighel et al. (2003)
	Botrylloides leachi, B. violaceus	tentacles of oral siphon	Sensitivity to contact of inflowing particles		Single cilium and stereovilli	Accessory centriole	Loose fibrillar matrix generally present among microvilli and cilia	Supporting cells form a wall or crest		Burighel et al. (2003), Burighel et al. (2008)
	Molgula socialis	tentacles of oral siphon	Sensitivity to contact of inflowing particles		Three types (type 1,2 and 3). Stereovilli	Electron dense granules, accessory centriole in sensory cells	Extracellular radial filaments connecting the cilium or cilia to the surrounding stereovilli	Supporting cells form a wall or crest		Caicci et al. (2007)
	Pyura stolonifera, P.haustor	tentacles of oral siphon	Sensitivity to contact of inflowing particles		A pair of cilia surrounded by a crescent ring of stereovilli graded in length	Accessory centriole in sensory cells, electron dense granules	Extracellular radial filaments connecting the cilium or cilia to the surrounding stereovilli	Supporting cells form a wall or crest		Caicci et al. (2010a)
	Styela plicata. S. montereyensis, S. gibsi	tentacles of oral siphon	Sensitivity to contact of inflowing particles		A pair of cilia surrounded by a crescent ring of stereovilli graded in length	Accessory centriole in sensory cells, electron dense granules	Extracellular radial filaments connecting the cilium or cilia to the surrounding stereovilli	Supporting cells form a wall or crest		Manni et al. (2004), Caicci et al. (2010a)
	Polyandrocarpa zorritensis	tentacles of oral siphon	Sensitivity to contact of inflowing particles		A pair of cilia surrounded by a crescent ring of stereovilli graded in length	Accessory centriole in sensory cells, electron dense granules	Extracellular radial filaments connecting the cilium or cilia to the surrounding stereovilli	Supporting cells form a wall or crest		Caicci et al. (2010a)
Enterogona aplousobranchia	Clavelina lepadiformis	tentacles of oral siphon	Sensitivity to contact of inflowing particles		More than two cilia of same length that constitute an oriented rows parallel to coronal organ; microvilli			Supporting cells form a wall or crest		Manni et al. (2006)
	Diplosoma listerianum	tentacles of oral siphon	Sensitivity to contact of inflowing particles		More than two cilia of same length that constitute an oriented rows parallel to coronal organ; microvilli			Supporting cells form a wall or crest		Manni et al. (2006)
Enterogona phlebobranchia	Ciona robusta	tentacles of oral siphon	Sensitivity to contact of inflowing particles	Tentacle stimulation test	More than two cilia of same length that constitute an oriented rows parallel to coronal organ; microvilli	Accessory centriole in sensory cells				Mackie et al. (2006), Manni et al. (2006)
	Ascidiella aspersa	tentacles of oral siphon	Sensitivity to contact of inflowing particles		More than two cilia of same length that constitute an oriented rows parallel to coronal organ; microvilli				Accessory secretory cells	Mackie et al. (2006), Manni et al. (2006)
	Phallusia mammillata	tentacles of oral siphon	Sensitivity to contact of inflowing particles		More than two cilia of same length that constitute an oriented rows parallel to coronal organ, no microvilli/stereovilli	Electron dense granules in sensory cells				Mackie et al. (2006), Manni et al. (2006)
	Chelyosoma productum	tentacles of oral siphon	Sensitivity to contact of inflowing particles		More than two cilia of same length that constitute an oriented rows parallel to coronal organ; microvilli				Accessory secretory cells	Mackie et al. (2006), Manni et al. (2006)
	Corella inflata, C. willmeriana	tentacles of oral siphon	Sensitivity to contact of inflowing particles	Tentacle stimulation test	More than two cilia of same length that constitute an oriented rows parallel to coronal organ; no microvilli/stereovilli				Accessory secretory cells	Mackie et al. (2006), Manni et al. (2006)
Appendicularia	Okopleura dioica, O. albicans	lower lip and pharynx	Monitoring particle flow into pharynx		More than two cilia different in lengths and shorter toward the cell edges; microvilli			Supporting cells form a wall or crest		Bone, 1998; Rigon et al. (2013)
Thaliacea	Pyrosoma atlanticum	flaps and a single ventral tentacle	Sensitivity to contact of inflowing particles		Single cilium, stereovilli					Caicci et al. (2013)
	Doliolum nationalis	flaps	Sensitivity to contact of inflowing particles		Single cilium, stereovilli					Caicci et al. (2013)
Salpe	Thalia democratica	absent								Rigon et al. (2013)