io8 The Animal Mind 



34. Hearing in Crustacea 



In the Crustacea the function of the statocyst organs has 

 been the subject of much dispute. They are in this group of 

 animals sometimes closed sacs with statoliths, sometimes open 

 sacs containing grains of sand. Most commonly the organs 

 are situated in the basal segment of the small antennae. 

 There is usually inside the sac a projection bearing several 

 ridges of hairs, graded in size, which tempt to the hypothesis 

 that they respond to vibrations of different wave lengths, as 

 the fibres of the basilar membrane of the human cochlea are 

 supposed by the Helmholz theory to do. Hensen, indeed, 

 placing under the microscope the tail of a small shrimp, 

 Mysis, whose statocyst is situated in that region, observed 

 that the long hairs of the tail vibrated in response to musical 

 tones, from which he infers that the statocyst hairs may do so * 

 (163). In 1899 he was still inclined to believe that the latter 

 can serve no other than an auditory function (164). Never- 

 theless the weight of authority is in favor of regarding the 

 "sac" in Crustacea as a static rather than an auditory organ. 

 The only evidence of sound reaction in two shrimp-like 

 forms, Palaemon and Palaemonetes, was a " flight reflex" 

 given by some individuals when sounds were produced very 

 near them in the water; and although this response ceased 

 when the statocysts were destroyed, the fact is of little sig- 

 nificance, as other reflexes also were abolished by the opera- 

 tion (19). To sounds made by tapping the wall of the aqua- 

 rium Palasmonetes reacted by leaping away from the wall 

 nearest to it, even though the leap was made toward the sound. 

 When both statocysts were removed, the reactions were still 

 made, but not so markedly nor at so great a distance from the 



* 



1 This observation is sometimes incorrectly quoted as if the hairs concerned 

 were actually the statocyst hairs. Cf., for example, Morgan, 279, p. 266. 



