174 THE LIFE OF CRUSTACEA 



carried in a brood-pouch, and do not seem to be in 

 any way protected against drought. It is no doubt 

 in consequence of this that the fresh-water species 

 and genera of both Amphipoda and Isopoda, though 

 widely distributed, do not have the world - wide 

 range of many of the more minute Crustacea 

 described above. 



The common Crayfish, Astacus (or Potamobius) 

 pallipes, is the only truly fresh-water Decapod found 

 in England, although a small Prawn, Pal&monetes 

 varians, which usually inhabits brackish water, may 

 occasionally be found in places where the water is 

 practically fresh. The structure of the Crayfish is 

 very similar to that of the Lobster, but, as already 

 mentioned, it differs in its mode of development, 

 having no free-swimming larval stage. From its 

 size, and from the fact that the eggs are carried by 

 the female, the Crayfish cannot be transported from 

 one locality to another by the agencies which dis- 

 tribute the smaller fresh-water Crustacea. On the 

 other hand, the adult animals can live out of the 

 water for days, or even weeks, if they are kept moist, 

 and the English species is stated to leave the water 

 occasionally, and to make short excursions on land. 

 Many species found in foreign countries are still 

 more truly amphibious in their habits. It is clear, 

 however, that the means of dispersal of the Cray- 

 fishes are very limited, and on this account the 

 problems connected with their geographical distribu- 



