Non-motile sensory cilia and neuromuscular junctions in a ctenophore independent effector organ

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Abstract
Non-motile cilia of the (9 + 2) pattern, having a specialized onion-like root structure, act as sensitive receptors of water displacement and thereby detect vibrations of small objects in the water nearby. These receptors are situated on sensory nerve cells on finger-like processes up to 1 cm long, on the surface of the ctenophore Leucothea (= Eucharis) multicornis. In response to vibration a single finger can shoot outwards as an independent effector by an extension of its mesogloeal hydrostatic skeleton, acted on by circular and transverse muscle fibres which run mainly through the mesogloea. A copepod which may be hit is immobilized, presumably by a poisonous secretion. Retraction is brought about by longitudinal ectodermal fibres. The neuromuscular junctions have presynaptic vesicles of 30 to 50 nm diameter, a cleft of 15 to 20 nm wide, and occur at discrete points far from each other on the muscle cells, suggesting that excitation is propagated along the muscle fibres. No direct connexion has been traced between a sensory ciliated cell and a muscle fibre, but sensory cells connect with nerve net neurons and these form synapses with each other and with muscle cells. There are numerous nerve fibres in the epithelium and synapses with vesicles on one side of a cleft 12 to 15 nm wide occur between them sufficiently closely for spatial summation to be possible. The separate co-ordination of movements of extension, retraction and bending requires that certain types of sensory cells be connected specifically, if in directly, with muscle fibres of a particular directionality. This provides a primitive example of specificity of connexions which must imply two overlapping nerve nets.

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