278 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 
crab is, in all fandamental respects, the same as that of the 
crayfish. The body is made up of the same number of 
somites. The appendages of the head and of the thorax 
are identical in number, in function; and even in the 
general pattern of their structure. But two pairs of 
abdominal appendages in the female, and four pairs in 
the male, have disappeared. The exopodites of the 
antenne have vanished, and not even epipodites re- 
main to represent the podobranchie of the posterior five 
pairs of thoracic limbs. The exceedingly elongated eye- 
stalks are turned backwards and outwards, above the 
bases of the antennules and the antenne, and the bases 
of the latter have become united with the edges of the 
carapace in front of them. In this manner the extra- 
ordinary face, or metope (fig. 72, B) of the crab results 
from a simple modification of the arrangement of parts, 
every one of which exists in the crayfish, The same 
common plan serves for both. 
The foregoing illustrations are taken from a few of our 
commonest and most easily obtainable Crustacea ; but they 
amply suffice to exemplify the manner in which the con- 
ception of a plan of organization, common to a multitude 
of animals of extremely diverse outward forms and habits, 
is forced upon us by mere comparative anatomy. 
Nothing would be easier, were the occasion fitting, than 
to extend this method of comparison to the whole of the 
several thousand species of crab-like, crayfish-like, or 
