SEA-FISHERIES LABORATORY. 363 



extensor pulling the coxopodite backward and an anterior 

 flexor pulling it forward. 



The extensor is situated in the outer and posterior 

 region of the fourth thoracic pleural muscle chamber. It 

 arises from the anterior and posterior walls of the latter 

 and passes forward and downward. Its insertion is on a 

 long narrow tendon arising from the posterior side of the 

 coxopodite (fig. 21, ex. C, Text fig. 10, d.). 



The flexor is a much larger muscle than the extensor, 

 and lies in the fourth thoracic sternal muscle chamber. 

 It arises from three parts of the endophragmal system — 

 (1) from the posterior face of the third thoracic 

 endosternite ; (2) from the inner side of the third endo- 

 pleurite ; and (3) from the anterior face of the fourth 

 thoracic endosternite. It passes downward and forward, 

 and is inserted on an extremely broad tendon on the 

 anterior side of the coxopodite (fig. 21, /. C, Text fig. 

 10, c). 



Basi-ischiopodite. There are two extensors and three 

 flexors inserted on the proximal region of this segment. 



The anterior extensor is situated in the anterior and 

 ventral portion of the fourth thoracic pleural muscle 

 chamber. It arises from the ventral part of the anterior 

 and inner walls of the chamber. It runs outward, and is 

 inserted on a long and narrow tendon situated immediately 

 above the anterior hinge (fig. 21, a. ex. B, Text fig. 10, /.). 



The posterior extensor is a small muscle situated 

 almost entirely in the base of the coxopodite. It is 

 attached to the ventral part of the wall of the fourth 

 pleural muscle chamber. It passes outward, and is 

 inserted on a small tendon which is immediately above 

 that of the anterior extensor (fig. 21, f. ex. B., Text fig. 

 10, e.) 



The anterior flexor lies in the ventral region of the 



