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 diiferent 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 



