THE LOBSTER. 149 



water, and one side of this sac is pushed inward to form 

 a sort of fold, or pocket, in which a nerve that comes 

 from the brain, or head ganglia, spreads itself all out 

 (Fig. 128). The side of the pocket toward the water is 



Fig. 128. 



The Lobster's Ear. 



covered with fine hairs, and these hairs touch against 

 little bits of sand which get into the water through the 

 outside slit (Figs. 128, 129). These particles of sand 

 are like the tiny stones, or otoliths, you found in the 

 mussel's ear-sac, and they likewise help to increase the 

 sound. The lobster's ear is made on much the same 

 plan as your own. The sac is really a fold of the 



