CHAPTEE V. 



COMPARATIVE MORPHOLOGY OF THE CRAYFISU.- 

 STRUCTURE AND THE DEVELOPMENT OF THE CRAY- 

 FISH COMPARED WITH THOSE OF OTHER LIVING 

 BEINGS. 



UP to this point, our attention has been directed 

 almost exclusively to the common English crayfish. 

 Except in so far as the crayfish is dependent for its 

 maintenance upon other animals, or upon plants, we 

 might have ignored the existence of all living things 

 except crayfishes. But, it is hardly necessary to observe, 

 that innumerable hosts of other forms of life not only 

 tenant the waters and the dry land, but throng the air ; 

 and that all the cra} T fishes in the world constitute a hardly 

 appreciable fraction of its total living population. 



Common observation leads us to see that these multi- 

 tudinous living beings differ from not-living things in 

 many ways ; and when the analysis of these differences 

 is pushed as far as we are at present able to carry it, it 

 shews us that all living beings agree with the crayfish 

 and differ from not-living things in the same particulars. 

 Like the crayfish, they are constantly wasting away by 



