4 8 



CRUSTACEA. 



reduced to a small head-shield. The appendages have 

 Undergone less modification than those of the lobster, 

 and are therefore simpler in structure. The swimmer- 

 ets are under the abdomen, and the first pair forms a 

 cover for the others. The skills are the two leaves of 



Fig. 20. 



hd 



th 



ai 



Jdvhtu irroratu Say. 



(After Emerton.) 

 The line shows natural si?e of species. 



the swimmerets attached to the bases of the posterior 



pairs. The seven pairs of walking-legs are very simi- 

 lar to each other, and this is one of the distinguishing 

 characteristics of the order, as might be inferred from 

 its name of Isopoda, or equal-footed Crustacea. The 



