148 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
divisible, correspond with one another, not only 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 facades 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 type of appendage is comparable to that pro- 
duced by adding « 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; 
