MECHANORECEPTION 73 



the second abdominal sternum. Each consists of a shallow ovoid 

 depression in which the usual plastron hairs are replaced by large 

 hydrofuge hairs (± 60,000 per sq. mm.), among which are delicate 

 innervated sensilla trichodea (Fig. 53). It is assumed that increases of 



53. A. Ventral view of the left 

 spiracular 'rosette' on the second 

 abdominal segment of Aphelo- 

 cheinis with the specific organ of 

 pressure sense and the col- 

 lapsible air sac. B. Section 

 through the edge of the organ 

 of pressure sense of Aphelo- 

 cheirus showing one sensory hair 

 and its bipolar neuron and the 

 plastron hair pile. (Redrawn 

 from Thorpe and Crisp, 1947.) 



pressure due to water currents and turbulence, depth, or the animal's 

 position with respect to gravity are registered because the large slant- 

 ing hairs press down upon the sensilla (Thorpe and Crisp, 1947). 

 Unilateral damage causes the animal to swim in spirals, but this is only 

 indirect proof of their static function (Larsen, 1955). 

 The air space around the pressure organs is continuous with tracheal 



