78 



XIV. Fourth walking leg (PL III, tig. 23). 



In the coxopodite and basi-ischium there are the 

 same muscles as in the other walking legs. There is, 

 however, an additional extensor of the coxopodite. This 

 ventral extensor is inserted on a small narrow tendon 

 (v. ex. C.} immediately below the insertion of the dorsal 

 extensor. 



The muscle chamber in this somite is not divided into 

 pleura! and sternal regions. Hence it may be designated 

 the pleuro-sternal muscle chamber (see section on 

 Endophragrnal System). All the muscles of the coxa and 

 the basi-ischium arise from this chamber. 



The dorsal extensor of the coxopodite arises from the 

 anterior wall of the chamber ; the ventral extensor from 

 the median plate ; the flexor of the coxopodite from the 

 anterior end of this muscle chamber. The extensor of the 

 basi-ischium arises from the anterior and inner corner of 

 the chamber, and the flexor has its origin on the median 

 plate. 



The muscles of the remaining parts of this appendage 

 are similar to those of the other walking legs. 



In the chela the coxa swings horizontally. In the 

 first walking leg the coxa is slightly tilted, so that it 

 swings forward and slightly downward. In each of the 

 succeeding walking legs the corresponding part is more 

 tilted, and in the last walking leg the ventral hinge of the 

 coxa is more posterior and the dorsal hinge anterior. So 

 that, instead of swinging horizontally as in the chela, the 

 coxa swings in a plane inclined at a considerable angle to 

 the vertical, and during the movement of extension the 

 limb is capable of being turned almost on to the dorsal 

 side of the carapace. The presence of the additional 

 extensor muscle undoubtedly aids such a movement. 



This freedom of movement is probably not of much 





