222 



THE SENSES OF ANIMALS 



where there is a gravity-receptor and three rows of sensory 

 cells set in planes at right angles to each other which in 

 function strongly resemble the three semicircular canals of 

 the vertebrates. In many invertebrates the sensory cells and 

 the calcareous grains are contained in a small bag, as in the 

 jelly-fish described on page 133. They appear to combine the 

 functions of gravity-receptors with angular-acceleration- 

 receptors and perhaps also respond to vibrations that we 

 perceive as sounds. 



In many of the Crustacea these otocysts, or more correctly 

 statocysts, are open to the exterior and filled with water. In 

 the crayfish they lie at the bases of the second pair of feelers ; 

 every time the animal moults their linings are shed with the 

 rest of the shell, and with the lining the contained granules 

 are also lost. As the new shell hardens the animal picks up 



AntenmHe 



STATOCSST 



Fig. 19. (a) A freshwater crayfish showing the second pair of feelers 

 (antennules) . 



(b) Enlarged view of an antennule showing the entrance to the 

 statocyst. 



{c) The arrangement of hair-like sensory cells and sand granules 

 inside the statocyst (highly magnified) . 



minute sand grains and places them in the statocyst where 

 they serve the same function as the calcareous granules in 

 other animals where they are secreted by the tissues of the 

 body. If a moulting crayfish is supplied with very fine iron 



