184 THE FRESH- WATER MUSSEL [CH. VI 



otolith. There is no evidence that Anodonta is sensitive 

 to sound as we understand it 1 : it is far more probable 

 that the otocysts are. equilibrating organs. Possibly they 

 are concerned in detecting vibrations of the water or of 

 the pond-bottom. The animals are certainly extremely 

 sensitive to vibrations when in captivity : unless the 

 aquarium or dish in which they are kept be placed upon 

 a table which is not affected by vibrations of the floor, 

 e.g. a slate balance-table affixed to the walls direct, it is 

 extremely difficult to approach them without causing 

 every specimen to retract its foot and close the shell. 



Reproduction and Development*. In all the Unionidce 

 the sexes are normally separate, though occasional instances 

 of hermaphrodite Anodonta have been found. It is often 

 stated 3 that the shell of the female Anodonta or Unio can 

 be distinguished from that of the male by its greater 

 convexity and increased transverse diameter. As a matter 

 of fact there is no criterion by which the shell of either 

 sex can be distinguished. Out of nineteen specimens 

 selected as males from my stock by persons professing to 

 be able to distinguish the sexes, only one proved, on 

 dissection, to be of that sex ; and a small U. pictorum 

 set aside as "undoubtedly female" turned out to be a 

 male. I have always found males far less common than 

 females. 



1 Cf. Baudon, Rtv. Mag. Zool. 1852. 



2 von Jhering, Zeit. wiss. Zool. xxix. ; Schierholz, ibid. xxxi. 1878; 

 and Denkshr. Acad. Wiss. Wien, LV. 1888-9; Goethe, Zeit. wiss. Zool. 

 LII. 1891; Latter, Proc. Zool. Soc. 1891; Faussek, Biol. Centralbl. 

 xv. 1895; and 5th Intcrnat. Zoologic. Congress (Berlin). 1901. 



3 Bronn, Klas. u. Ord. in. 1; Hazay, Malacozool. Bldt. in. 1881. 



