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; whilst, 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 concavity, 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 
