320 THE CRAYFISH LESS. 



or appendages, structures which we have not hitherto met 

 with. Both trunk and appendages are covered with a sort 

 of shell, formed of a substance called chitin, strongly im- 

 pregnated with carbonate of .''me so as to be hard and but 

 slightly elastic. 



The abdomen is made up of seven segments : the first 

 six of these (Fig. 81, xiv-xix) are to be considered as meta- 

 meres in the sense in which the word is used in the case of 

 Polygordius. Each has a ring-like form, presenting a broad 

 dorsal region or tergum : a narrow ventral region or sternum , 

 and downwardly directed lateral processes, the pleura. The 

 seventh division of the abdomen is the telson : it is flattened 

 horizontally and divided by a transverse groove into anterior 

 and posterior portions. All seven segments are calcified, 

 and are united to one another by chitinous articular mem- 

 branes : the first segment is similarly joined to the thorax. 

 Thus the exoskeleton of the Crayfish is a continuous 

 structure, but is discontinuously calcified so as to have the 

 character of a hard jointed armour. 



It has been stated that the abdominal segments are 

 movable upon one another in a vertical plane, i.e., the whole 

 abdomen can be extended or straightened, and flexed or bent 

 under the cephalothorax : the segments are incapable of 

 movement from side to side. This is due to the fact that, 

 while adjacent segments are connected dorsally and ven- 

 trally by flexible articular membranes, they present at each 

 side a joint, placed at the junction of the tergum and 

 pleuron, and formed by a little peg-like process of one seg- 

 ment fitting into a depression or socket in the other. A 

 line drawn between the right and left joints constitutes the 

 axis of articulation, and the only possible movement is in a 

 plane at right angles to this axis. 



Owing to the presence of the carapace the thoracic region 



