METAZOA CRUSTACEA. 343 



Interesting adaptive characters are found in Alpheus 

 heterochelis Say (PI. 841; No. 842). It is a burrower and 

 one of its arms has an immense claw (No. 842) which is 

 fully as large as the cephalothorax. The wonder is that 

 so small a body and so delicate a basal portion of the 

 appendage can control the movements of such a large 

 organ. That this organ is an adaptation of the adult 

 animal to the life it leads is proved by the fact that in 

 the young Alpheus (PI. 841, fig. i, greatly enlarged) the 

 claws are not developed. In the more advanced stage, 

 however (fig. 2, enlarged), they are of considerable size. 

 Fig. i gives the color of the young Alpheus which changes 

 with growth (fig. 2). 



In Evatya crassus Smith (No. 843), the abdomen tapers 

 and the last segment is small. The swimmerets of the 

 forward segments (excepting those of the first segment) 

 are unusually large, broad, and hairy organs. The small 

 thoracic legs have horny spikes at their ends. Then 

 comes a pair of enormously developed legs covered with 

 dark horny knobs ; the terminal hooks are still darker 

 and everything indicates that these organs perform hard 

 work. 



The eyes of this species are inconspicuous but the 

 second pair of antennae are extremely long and slender. 



The shrimp (Crangon franciscorum Stimp.) (No. 844) 

 has the segments of the abdomen tapering to a long, nar- 

 row telson. The cephalothorax is small and the rostrum 

 weak. The swimmerets are long, slender organs and in 

 one of the specimens (No. 844) they hold the mass of 

 eggs in place. The last pair of swimmerets are large 

 and efficient swimming organs. The last two pairs of 

 legs are the strongest and these are used in digging holes 

 when the shrimp buries itself in the sand. The second 

 and third thoracic legs are slender ; they are not provided 

 with claws but the terminal section of the arms bends 

 upon the next lower section like the blade of a knife 

 upon the handle. The antennae have large basal scales 



