CHAPTER IV. 



THE CRAYFISH {ASTACUS FLUVIATILIS). 



The Crayfish is common in many streams of this country, as 

 well as on the Continent. 



A very slight examination of a specimen will show that it is 

 an animal made up, like the earthworm, of a series of segments. 

 But it will soon be noticed that this segmentation is clearest in 

 the posterior (abdominal) region of the body, and is less 

 obvious anteriorly. Furthermore, the number of segments 

 into which the body is divisible is much less than in any earth- 

 worm, and the number is absolutely constant. Each segment, 

 too, is provided with a pair of jointed limbs, or appendages. It 

 is from this character that the na,me of the great group to which 

 the crayfish belongs (Arthropoda) is derived. 



The body is covered with a hard shell externally, which 

 corresponds to the cuticle of the worm or to the shell of the 

 anodon. Like the latter, it is indurated by the abundant 

 deposition of calcareous salts. Were the whole exoskeleton to 

 be thus permeated with carbonate of lime, locomotion would 

 be clearly impossible ; so we find that the skeleton is ringed, 

 denser tracts alternating with softer tracts ; upon the latter the 

 former move. When the abdomen, for instance, is straightened, 

 the softer tracts of the exoskeleton are seen to disappear 

 beneath the calcified plates. In the . anterior region of the 

 body, however — the cephaloiho7'ax—\he entire back and sides 

 of the animal are covered by a continuous hard plate. This 

 represents a number of the separate hard plates of the abdomi- 

 nal region fused together. The term carapace is applied to this 

 anterior plate ; it may be distinguished by an oblique (cervical) 



