INVERTEBRATA. 



307 



Fig. 151. AN ARTICULATION IN 

 THE CRAYFISH. 



The soft cut icle is shaded. 

 Flexion, left ; Extension, right. 



By the presence of these two peg-and-socket articulations, motion of 

 the two sclerites relative to one another can only take place about 

 one axis that which passes through 

 both articulations. Thus the range 

 of motion at any one articulation is 

 much less than at one of the articula- 

 tions of the human arm ; but this is 

 made up for by the greater number 

 of articulations, so that after all a 

 crayfish can grasp its food with its 

 pincers quite as quickly and certainly 

 as a human being. If we examine 

 the two areas of thin cuticle we 

 shall see in the centre of each a 

 narrow transverse groove : this marks 

 the point where an invagination of 

 the cuticle (and the epidermis with 

 it) occurs, for the purpose of giving 

 a larger surface of attachment to the 

 muscles. Ingrowths of the exoskeleton for muscular attachment 

 are characteristic of the class Arthropoda, to which our crayfish 

 belongs. When, as in this case, they are thin, flat, flexible plates 

 they are called tendons (they are of course analogous to, but not 

 in the least homologous with, the tendons of a Vertebrate, which 

 are mesoblastic connective tisssue, not epiblastic cuticle). There 

 are thus two tendons at each articulation (fig. 152). If one is 

 pulled by the contraction of the muscles attached to it the 

 corresponding soft area is wrinkled up and the two sclerites are 

 brought nearer one another on that side. When the other tendon 

 is pulled, they are brought nearer on the other side. The articula- 

 tions are so arranged that bending (flexion) can take place on one 



side only : one set ox muscles 



I {flexors) acting on their tendon 



I cause the flexion ; while the 



^^* contraction of the other set 



V / j^ ^^ (extensors) pulls the limb 



r ilf\~~' straight again (extension). In 



I 4-1 \--r<.fxoA ) the chelate joints (see 6) 



jrr/vflov-j the extensors become adductors 



TV "| fin (bringing the movable claw 



into contact with the fixed 

 one), and the flexors, abductors. 

 The motions of the abdomen 

 are of just the same kind, 

 only here the terga overlap or 



telescope when extension occurs, while the sterna are brought 

 nearer together and the soft areas between them wrinkled, when 

 flexion takes place and there are no tendons. The arrangement 

 of the extensor and flexor muscles is shown in fig. 153, and a 



Fig. 152. SECTION THROUGH AN 

 ARTICULATION. 



Flexion, right ; Extension, left. 



