Table 3.

Ciliary mechanosensory cells in posture and orientation.

animal group	species	stage	cell name (s)	evidence of mechanosensory modality	molecular dataa	sensory neuron (SN) or secondary sensory cell (SSC)b	cilium features	actin or other structures	location of cells
Cnidaria	Halistaura cellularia [179], Aegina citrea [179], Solmissus marshalli [179], Cunina lativentris [182], Rhopalonema velatum [182], Geryonia proboscidalis [182], Aurelia aurita [183], Aglantha digitale [128]	medusa	type A–C sensory cells [179], type I receptor [183], ciliated sensory cell	removal of statocyst leads to loss of righting behaviour [180]	—	SN	— uniciliated — 10–15 µm long non-motile cilium — 9+2 axoneme — no rootlet (type A, B) to strong rootlet (type C)	— no microvilli (type A) — unorganized microvilli (type C) — collar of 7 to 15 stereovilli	— statocyst
Ctenophora	Pleurobrachia pileus [184,185], Bolinopsis sp. [184]	adult	balancer ciliated cell	deflection of a balancer increases or decreases beating frequency [184,186]	—	—	— uniciliated — 50–100 µm long cilium — 9+2 axoneme — dense striated rootlet — cilia tips embedded in folds of lithocytes — central pair oriented at right angles to beating direction	— no microvilli	— statocyst (aboral sense organ)
Mollusca	Bivalves Pecten sp. [187,188] Gastropods Pomacea paludosa [189], Aplysia californica [190,191], Pterotrachea sp. [187], Lymnaea stagnalis [192], Biomphalaria glabrata [193] Nudibranchs Rostanga pulchra [194], Hermissenda crassicornis [195] Cephalopods Octopus vulgaris [196], Sepia officinalis, Loligo vulgaris [197]	adult	hair cell	generator potentials recorded in statocyst cells upon mechanical displacement or statocyst rotation (nudibranchs [198]; gastropods [199,200] cephalopods [201])	—	SN/SSC	— multiciliated — 9+2 axoneme — basal feet in a single cell point in the same direction (cephalopods and some other molluscs [187,189,194,196,197]) — non-motile cilia (cephalopods) — motile cilia (at least some gastropods, and nudibranchs) — basal feet radially arranged (i.e. no hair cell polarization [187,188,190,192–195]) — striated rootlet opposite basal foot	— ring of microvilli around each cilium [187,189,194,196] — few dispersed microvilli [187,188,192,197] — no microvilli [193]	— statocyst — in cephalopods also found in neck [202] and lateral line [203]
Tunicata	Ascidiacea Diplosoma macdonaldi [204], Ciona spp. [205], Botryllus schlosseri [206] Larvacea Oikopleura dioica [207]	swimming larva (ascidian)	statocyst-associated cells, antenna cells, S1 cells (B. sch.)	statolith-ablated larva (Ciona) fails to show gravitaxis [208]	—	SN	— uniciliated — cilium filled with vesicles (O. dioica) — cilium contacts statolith (O. dioica) — striated rootlet	— several microvilli (D. mac.) — laminar microvillar extensions (B. sch.)	— sensory vesicle
Cephalochordata	Branchiostoma floridae [209]	12.5-day larva	ciliary bulb cell	—	—	SN	— uniciliated — enlarged distally and filled with a dense matrix — rootlet	—	— infundibular region of cerebral vesicle

^aOnly molecular data related to the expression of mechanotransduction-related components is included.

^bSecondary sensory cells lack a neurite projection (e.g. vertebrate hair cells).