22G THE MORPHOLOGY OF THE CRAYFISH. 



behind the liver. Rathke states that the sexual apertures 

 are not visible until the young cra3'fisli has attained the 

 lengtli of an inch ; and that the first pair of abdominal 

 appendages of the male appear still later in the form of 

 two papillae, which gradually elongate and take on their 

 characteristic forms. 



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