148 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



divisible, correspond witli one another, not onl}^ in form, 

 but in their relations to the general plan of the whole 

 abdomen. Or, in other words, a diagrammatic plan of 

 one somite will serve for all the three somites, with 

 insignificant variations in detail. The assertion that 

 these somites are constructed upon the same plan, in- 

 volves no more hypothesis than the statement of an 

 architect, that three houses are built upon the same plan, 

 though the fa9ades and the internal decorations may 

 differ more or less. 



In the language of morphology, such conformity in the 

 plan of organisation is termed homology. Hence, the 

 several metameres in question and their appendages, are 

 homologous with one another ; while the regions of the 

 somites, and the parts of their appendages, are also 

 homologues. 



When the comparison is extended to the sixth meta- 

 mere, the homology of the different parts with those of the 

 other metameres, is undeniable, notwithstanding the great 

 differences which they present. To recur to a previous 

 comparison, the ground plan of the building is the same, 

 though the proportions are varied. So with regard to 

 the first and second metameres. In the second pair 

 of appendages of the male, the difference from the 

 ordinary tj'pe of appendage is comparable to that pro- 

 duced by adding a portico or a turret to the building ; 

 while, in the first pair of appendages of the female, 

 it is as if one wing of the edifice were left unbuilt ; 



