The coelotrich: form and function of an unusual sensillum in Lowrya (Ostracoda: Myodocopina: Cypridinidae).

This study reports a previously undescribed and unique sensillum. Some species of Lowrya Parker 1998 (Ostracoda: Cypridinidae) possess sensillae of an unusual type arranged in a single row parallel to, and near, the anterior, ventral and posterior margins of the external surface of each carapace valve. These sensilla are here termed coelotrichs, and are each centred within an almost spherical cavity in the carapace. The cavity forms at the base of a depression, and the depression in turn lies within a sunken channel on the exterior carapace surface. The opening of the coelotrich cavity bears a 'plug', through which only the widened, divided base of a seta passes. The seta is very fine and stiff throughout most of its length, and exhibits no pores. For most of its length the seta is orientated parallel to the carapace surface, shielded within the sunken channel. The coelotrich is probably a velocity detector, acted upon by the viscous drag of the surrounding fluid. Its function is probably to detect vibrations in the substrate, such as acoustic motion in the surrounding liquid, or steady fluid drainage motion.