204 ELEMENTARY BIOLOGY. [CHAP. 



P. The integument ; largely composed of branched 

 nucleated granular cells : the outermost giving 

 off a large number of short processes which pene- 

 trate a short way into the exoskeleton. 



Prolongations of it extend into the setae and 

 into all skeletal outgrowths. 



The ischiopodite of the third maxillipede may 

 be preferably utilized, as, in it, all gradations from 

 seta to spine and from spine to tooth can be 

 readily followed. 



C. The muscular system. 



1. Detach one of the great chelae from the body and lay 

 bare its interior. The enlarged base of the propodite 

 will be found to lodge two muscles, which pass be- 

 tween it and the dactylopodite ; they are 



a. A larger adductor, arising from the outer side. 



b. A smaller abductor, arising from the inner side. 



2. Remove the superficial portions of the two muscles ; 

 there will be found lying within each a central tendon 

 a plate-like ingrowth of the exoskeleton, towards which 

 the muscular fasciculi converge. Remove all the soft 

 parts and examine 



a. The tendons in relation to the dactylopodite; 

 they are ingrowths of the opposite sides of its 

 base. 



fi. The articular facets and interaxticular membranes; 

 so arranged as only to admit of motion in one 

 plane (abduction and adduction.). 



3. Examine the remaining segments of the appendage. 

 Note that 



