66 



III. Second antenna. The basal region of 

 this appendage is fused with the carapace, and the muscles 

 have degenerated. The operculum, which probably 

 represents the coxopodite, is still freely movable, but its 

 extensor and flexor muscles have now another function in 

 connection with the raising and closing of the operculum. 

 The whole question of the homology of the opercular 

 muscles has been fully discussed by Marchal.* The 

 flagellum has not much movement, and its muscles are 

 very small. 



IV. Ma n d i b 1 e (fig. 31). There are two sets of 

 muscles — the adductors for closing the mandibles and the 

 abductors for opening the mandibles. 



External adductor muscle (e.a. md.). Arises as a broad 

 band from the anterior and outer portion of the' sub- 

 hepatic region of the carapace. It passes inwards and 

 upwards, and is inserted on a long tendon attached to the 

 outer part of the mandibular apophysis. 



Internal adductor muscle {i.a. md.). Arises from the 

 urogastric region of the carapace. It passes downwards 

 and forwards as a short broad muscle, and is inserted on 

 an extremely long narrow tendon attached to the posterior 

 margin of the mandible. 



External abductor muscle (e.b. md.). Arises from the 

 posterior and inner corner of the hepatic region of the 

 carapace. It passes directly downwards, and is inserted on 

 a narrow tendon attached to the posterior side of the 

 apophysis. This muscle is comparatively small. 



Internal abductor muscle [i.b. md.). Arises from the 

 top of the vertical rod-like portion of the first post-oral 

 endopleurite. It passes outwards and forwards, and is 

 attached to the outer part of the apophysis. 



* Marchal. " Appareil excreteur des Crustaces Decapodes." 

 Archives Zool. exp. et gen. (Ser. 2), T. X, 1892. 



