THE CRAYFISH AND ITS ALLIES 105 



along our coast, and even penetrate into rivers. Two river 

 shrimps l are found in the United States east of the Missis- 

 sippi River. These Crustacea are able to maintain their 

 enormous numbers only by virtue of their great repro- 

 ductive capacity, necessitated by the circumstance that 

 they furnish almost the entire food supply for many kinds 

 of fishes and other foes. Even in the principal shrimping 

 grounds of the United States- -such as San Francisco and 



FIG. 97. Gebla affinis, right side. Two-thirds natural size. 

 ' Photo, by W. H. C. P. 



New Orleans there is said to be no diminution in the 

 numbers of shrimps. 



The Thalassinidae 2 include certain crayfish-like species 

 which live on our coast, burrowing in mud-flats, where 

 they live concealed during the day. On account of their 

 being of only medium size and difficult to obtain, they are 

 commonly little known. Our Eastern species are Grebia 

 affinis (Fig. 97) and Callianassa stimpsoui. 



The hermit crabs (Pagurida3 3 ) occupy a position inter- 

 mediate between the long and short tailed Decapods in 



1 Palwmon Ohionis and Pal eomonetes exilipes. 



2 0a\d(Tcrivos, color of the sea. 3 irdyovpos, a kind of crab. 



