176 THE LIFE OF CRUSTACEA 



tion are of great interest. An admirable discussion 

 of the subject will be found in Professor Huxley's 

 book on the Crayfish, and the conclusions reached 

 by him have hardly been modified by thirty years 

 of subsequent research. Only a very brief outline 

 can be attempted here. 



Crayfishes are found in the fresh waters of the 

 Northern and Southern Hemispheres (Fig. 61), but in 

 each case they are practically confined to the tem- 

 perate regions, and are absent from a broad inter- 

 vening tropical zone. The Northern Crayfishes, 

 forming the family Astacidas (or Potamobiidse) are 

 distinguished, among other characters, by having a 

 pair of appendages on the first abdominal somite, 

 at least in the male sex ; the Southern Crayfishes 

 have no appendages on that somite, and for this and 

 other reasons are regarded as constituting a distinct 

 family Parastacidae. There is thus a general corre- 

 spondence between the geographical distribution of 

 the Crayfishes and the more important structural 

 differences expressed in their classification. There 

 can be no doubt that the two families have been 

 derived from a common stock of marine lobster-like 

 animals, and it is reasonable to suppose that two 

 branches of this stock became independently adapted 

 to a fresh-water habitat in the North and in the 

 South, giving rise to the Astacidae and the Par- 

 astacidae respectively. 



The distribution of the individual genera is, how- 



