498 ZOOLOGY SECT. 



I. The Fresh-water Crayfish (Astacus fluviatilis). 



Astacus fluvicdilis is common in streams and rivers in England 

 and the continent of Europe; allied species occur in Asia and 

 North America; and fresh- water Crayfishes belonging to other 

 genera, but agreeing with Astacus in all essential features, are 

 found in America, Australia, and New Zealand. 



External Characters. The body of the Crayfish (Fig. 396) is 

 divided into two regions an anterior, the ccphalothorax (cth.), 

 which is unjointed, and is covered by a cri><iec resembling that of 

 Apus, but of smaller proportional size ; and a posterior, the aW<>- 

 men(al>), which is divided into distinct segments, movable upon 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. 404). 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 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 

 terguin, a narrow ventral region or sternum, and downwardly 

 directed lateral processes, the pleura the latter quite unrepre- 

 sented in Apus. The seventh division of the abdomen is the tcUon : 

 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 membranes : 

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

 exoskeleton of Astacus resembles that of Apus in being a con- 

 tinuous cuticular structure, but differs from it in being discon- 

 tinuously 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 dorsal ly and ventrally by flexible articular membranes, 

 they present at each side a hinge (Fig. 400, h\ placed at the 



