232 



GENERAL BIOLOGY 



modified to sen^e reproductive or respiratory functions. 

 The eight segments following the head constitute the thorax 

 and the seven last segments (counting the rudimentary 20th 

 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 

 f r e e-swimmin g 

 aqua+'O animals, it 

 is their swimming 

 appendages that 

 are least altered by 

 adaptation. The 

 ' egs 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. (Cambarus). 



