174 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
In addition, therefore, to their adaptation to the pur- 
poses which they subserve, the parts of the skeleton 
of the crayfish show a unity in diversity, such as, if 
the animal were a piece of human workmanship, would 
lead us to suppose that the artificer was under an obliga- 
tion not merely to make a machine capable of doing cer- 
tain kinds of work, but to subordinate the nature and 
arrangement of the mechanism to certain fixed architec- 
tural conditions. 
The lesson thus taught by the skeletal organs is re- 
iterated and enforced by the study of the nervous and the 
muscular systems. As the skeleton of the whole body is 
capable of resolution into the skeletons of twenty separate 
metameres, variously modified and combined; so is the 
entire ganglionic chain resolvable into twenty pairs of 
ganglia various in size, distant in this region and 
approximated in that; and so is the muscular system 
of the trunk conceivable as the sum of twenty 
myotomes or segments of the muscular system appro- 
priate to a metamere, variously modified according to 
the degree of mobility of the different regions of the 
organism. 
The building up of the body by the repetition and 
the modification of a few similar parts, which is so ob- 
vious from the study of the general form of the somites 
and of their appendages, is still more remarkably illus- 
trated, if we pursue our investigations further, and trace 
