X PHYLUM ARTHROPOD A 215 



(kd) on each side, separated from the actual body-wall by 

 a narrow space in which the gills are contained. The cara- 

 pace is made of chitin, strongly impregnated 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 reduced 

 in size, flattened horizontally, and divided by a transverse 

 groove into anterior and posterior portions. All seven seg- 

 ments are calcified, and are united to one another by chiti- 

 nous 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 seg- 

 ment 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. 



The ventral and lateral regions of the thoracic exoskeleton 

 are produced into the interior of the body in the form of a 

 segmental series of calcified plates, so arranged as to form a 

 row of lateral chambers in which the muscles of the limbs 

 lie, and a median tunnel-like passage or sternal canal, con- 



