xi PHYLUM ARTHROPODA 541 



one another in a vertical plane. The cephalothorax is again divided 

 into two regions an anterior, the head ; and a posterior, the thorax 

 by a transverse depression, the cervical groove. The divisions of 

 the body are thus the same as in Apus, but the abdomen alone is 

 movably segmented, owing to the fact that the carapace, instead 

 of being a purely cephalic structure continued backwards as a 

 loose fold over the thorax, is developed from the dorsal and 

 lateral regions of both head and thorax, and is free only at the 

 sides of the thorax, where it forms a flap or gill-cover (kd) on each 

 side, separated from the actual body- wall by a narrow space in 

 which the gills are contained (Fig. 436). The carapace 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 six segments and a tail-piece or 

 telson: the six segments (XIV-XIX) 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 latter quite unrepresented in Apus. The telson is flattened 

 horizontally, and divided by a transverse groove into anterior and 

 posterior portions. All the segments and the telson are calcified, 

 and are united to one another by chitinous articular membranes : 

 the first segment is similarly joined to the thorax. Thus the exo- 

 skeleton of Astacus resembles that of Apus in being a continuous 

 cuticular structure, but differs from it in being 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 cephalo- 

 thorax : the segments are incapable 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 (Fig. 434, h}, 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 con- 

 stitutes 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. 



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 



