88 ANNELIDES—CRUSTACEA. 



numerous, and sometimes, as in Cymbulia, collected 

 into a mulberry-like group. 



In many cases the auditory sac rests directly on the 

 gauglion. 



The actual mode of termination of the nerves is still 

 uncertain. I have already mentioned that vibrations, if 

 fewer than thirty in a second, do not produce on us the 

 effect of sound. But it is possible that these organs in 

 the lower animals are intended quite as much to record 

 movements in the water as for hearing properly so called. 



The Organs of Hearing in Crustacea. 



Fig. 57. — Base 

 right antenuule of 

 lobster {Astacus 

 marinus) ; after 

 Farre. a, Orifice ; 



' '' ■ Fig. 58. — Interior of auditory sac of lobster (after 



Farre). a, Orifice ; b, auditory hairs. 



It was long supposed that the auditory organ of the 

 Crustacea was situated in the basal segment of the 

 outer antenna. The true auditory organ was, indeed, 

 discovered by Eosenthal in 1811,* who, however, re- 



* HeiVs Arch, fur Phys.,lSn. 



