50 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



true trunk muscles which are necessary only when the segments move 

 individually. (See figs. 15 and 16.) 



The muscles of the last five thoracic segments are separated inter- 

 nally by a series of irregularly shaped partitions. Each of these par- 

 titions consists of two thin plates, formed by the anterior wall of one 

 segment closely applied to the posterior wall of the preceding segment. 



Fig. 15. — Dorsal view of the blue crab. 



Fig. 16. — Ventral view of the blue crab. 



The lower half of each partition is formed by a pair of the plates 

 arising from the sternal borders of neighboring segments and is called 

 an endosternite. The upper half of each partition is similarly formed 

 by a pair of the plates which originate on the pleural walls of neigh- 

 boring segments and is called an endopleurite. Each endosternite 

 coalesces with its corresponding endopleurite, and it is at this line of 

 coalition that the break occurs during ecdysis to allow the crab to 

 molt completely. (See figs. 17 and 18.) 



