CHAPTER V. 
THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.—THE 
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 crayfishes 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 
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