98 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



marked off by a transverse groove from the rest of the 

 tergum as an articular facet. The front edge of the 

 articular facet is continued into a sheet of flexible cuti- 

 cula, which turns back, and passes as a loose fold to the 

 hinder edge of the overlapping tergum (fig. 21). This 

 tergal interarticular membrane allows the terga to move as 

 far as they can go in flexion ; wdiilst, in extreme exten- 

 sion, they are but slightly stretched. But, even if the in- 

 tersternal membranes presented no obstacle to excessive 

 extension of the abdomen, the posterior free edge of each 

 tergum fits into the groove behind the facet in the next 

 in such a manner, that the abdomen cannot be made more 

 than very slightly concave upwards without breaking. 



Thus the limits of motion of the abdomen, in the 

 vertical direction, are from the position in which it is 

 straight, or has even a very slight upward concavit}^, to 

 that in which it is completely bent upon itself, the telson 

 being brought under the bases of the hinder thoracic 

 limbs. No lateral movement between the somites of the 

 abdomen is possible in any of its positions. For, when 

 it is straight, lateral movement is hindered not only by 

 the extensive overlapping of the terga, but also by the 

 manner in which the hinder edges of the pleura of each 

 of the four middle somites overlap the front edges of 

 their successors. The pleura of the second somite are 

 much larger than any of the others, and their front edges 

 overlap the small pleura of the first abdominal somite ; 

 and when the abdomen is much flexed, these pleura even 



