ii8 The Animal Mind 



§ 32. 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 statoUths, 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 ^ (294). In 

 1899 he was still inclined to believe that the latter can 

 serve no other than an auditory function (295). 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- 

 Hke 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 Uttle sig- 

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

 tion (38) . To sounds made by tapping the wall of the aqua- 

 rium Palsemonetes reacted by leaping away from the wall 

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



' This observation is sometimes incorrectly quoted as if the hairs con- 

 cerned were actually the statocyst hairs. C}., for example, Morgan, 504, 

 p. 266. 



