8 CALIFORNIA ACADEMY OF SCIENCES. 



and the thorax of eight, some or all of which may fuse 

 with the head forming a more or less complete cephalo- 

 thorax. The abdomen is sharply marked off from the 

 thorax and consists of seven (in Nehalia of eight) seg- 

 ments, all of which, except the last, the telson, may 

 bear articulated appendages. 



The Malacostraca may be divided as follows:— 



Body composed of 21 segments and furnished with a bivalve shell. 



Leptostraca. 

 Body composed of 20 segments and not furnished with a well defined 



carapace. Eyes sessile Arihrostraca.. 



Body composed of 20 segments and furnished with a well developed 



carapace which is never bivalved Thoracostraca.. 



Synopsis of the Thoracostraca. 



Eyes sessile Ciimacea\. 



Eyes stalked. 



Carapace rather small, not covering all of the thoracic segments. The 

 thoracic feet dissimilar, the anterior ones devoid of exopods . 



Stomatopoda , 

 Carapace covering some or all of the thorax but not joined to the 

 posterior segments. Thoracic feet usually similar, biramous. 



Schizopoda.. 



Carapace covering and joined to all of the thoracic segments. The 



three anterior thoracic feet generally biramous and modified into- 



month-parts Decapoda. 



As it is with the Decapoda that we are here mainly con-^ 

 cerned, those parts of the structure of these animals 

 which are most used for the purpose of classification 

 will be briefly described. The exoskeleton of a Decapod 

 crustacean may be considered as a series of rings, each 

 of which, with the exception of the last, is typically 

 furnished with a pair of articulated appendages. These 

 rings and their appendages are modified in various 

 ways in different regions of the body in relation to the- 

 different functions they are called upon to perform. 



