2i 4 MANUAL OF ZOOLOGY SECT. 



pregnated with carbonate of lime, so as to be hard and 

 but slightly elastic. 



The abdomen is made up of seven segments : the first six 

 (XIV-XIX) of these are metameres in the strict sense of 

 the word, and have 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 an- 

 terior and posterior portions. All seven segments are calci- 

 fied, and are united to one another by chitinous articular 

 membranes : the first segment is similarly joined to the 

 thorax. 



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 incapa- 

 ble of movement from side to side. This is due to the fact 

 that, while adjacent segments are connected dorsally and 

 ventrally by flexible articular membranes, they present at 

 each side a hinge placed at the junction of the tergum 

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

 one segment fitting into a depression or socket in the 

 other. A line drawn between the right and left hinges 

 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 

 is immovable, and shows no distinction into segments either 

 on its dorsal (tergal), or lateral (pleural) aspect. But on the 

 ventral surface the sterna of the thoracic segments are clearly 

 marked off by transverse grooves, and the hindmost of them 

 is slightly movable. Altogether eight thoracic segments can 

 be counted. 



