278 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



crab is, in all fundamental respects, the same as that of the 

 crayfish. The hody 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 

 antennae have vanished, and not even epipodites re- 

 main to represent the podobranchise 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 antennae, 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 



