174 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



In addition, therefore, to their adajitation to the pur- 

 l^oses 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 woi'kmanship, 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 b}' the skeletal organs is re- 

 iterated and enforced by the study of the nervous and the 

 muscular systems. As the skeleton of the whole bod}' is 

 callable 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 tliis 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 ai)pro- 

 priate to a metamere, variously modified according to 

 the degree of mobility of the difi'erent regions of tlie 

 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 



