258 ZOOLOGY 



quently the larger crustaceans, such as the crayfish, 

 which are easy to examine, are commonly used to illus- 



Photosraptt by W. P. Bay 



FIG. 71. A "lady crab" (Ovalipes ocellatus'), about half natural size. 

 Found in the sea along the Atlantic coast. 



trate an important evolutionary principle, the modifica- 

 tion for various functions of a series of originally similar 

 Modifica- parts. It is not difficult to see that the two pairs of 

 antenn3e > tne mouth appendages and the feet, are built 

 upon the same general plan, but are greatly altered in 

 detail to serve different purposes. The same principle 

 is illustrated in the mammalian teeth, which are vari- 

 ously modified for grinding and cutting, or in the hands 

 and feet of man. The typical crustacean appendage is 

 said to be biramose (two-branched) ; that is to say, it 

 has a basal part and two terminal parts, like a hand 

 with two fingers. The outer of these terminal parts 

 is called the exopodite (outer leg part) and the inner the 

 endopodite (inner leg part). In this respect the Crus- 

 tacea differ from the terrestrial group of arthropods, 

 but the legs of terrestrial Crustacea (Isopoda), as 

 well as of many aquatic forms, are without the ter- 



