XVlll 



INTRODUCTION. 



The thorax in the Decapods in general is externally 



only visible under- 

 neath, the upper part 

 being covered by the 

 carapace, and being in 

 that part incomplete. 

 The number of obvi- 

 ous segments in these 

 higher forms is five, 

 and as each segment 

 bears its proper pair 

 of appendages, which 

 here are true ambula- 

 tory legs, the character of Decapods is thus produced. 

 The superior surface of the thoracic 

 segments is limited to the epimera, 

 the tergum being absolutely wanting- 

 Upon this upper surface on each side 

 lie the branchiae, or gills. In this 

 brief sketch it is only nJcessary to 

 refer to the apoderaata as consti- 

 tuting the large cells of the thorax, formed by a dupli- 



