xx. CRA YFISHES. 297 



stream than their parents. Their offspring would in turn 

 be swept yet further seawards ; and the constant continu- 

 ance of this process through a series of generations must 

 have resulted in the whole race of crayfishes being carried 

 out to sea and perishing. This fate has been avoided by 

 the crayfish through the late hatching of the young and 

 the habit they have, even when hatched, of clinging to the 

 swimmerets of the mother. 



