232 



GENERAL BIOLOGY 



modified to serve reproductive or respiratory functions. 



The eight segments following the head constitute the thorax 



and the seven last segments (counting the rudimentary 2oth 



segment) , the abdomen. 



The typical crustacean appendage consists of a single solid 



basal piece (basipodite) and two jointed branches arising 



therefrom, one on 

 the outer side (exo- 

 podite) and one on 

 the inner (endopo- 

 dite). This typical 

 structure is best 

 shown by the swim- 

 merets of the abdo- 

 men. Crustaceans 

 being primitively 

 free-swimming 

 aquatic animals, it 

 is their swimming 

 appendages that 

 are least altered by 

 adaptation. The 

 legs are the stoutest 

 of the appendages, 

 and these offer but 

 one branch arising 

 from the basal 

 piece, and that 

 composed of a re- 

 duced number of 

 highly differentia- 

 ted segments. A comparison of a leg with the last maxilli- 

 pede in the crawfish will show which appendage has been 

 lost and which preserved and specialized. The best clues 



FIG. 143. A common crawfish. 



