140 



ARTHROPODA 



pearance as to lack all features characteristic of the 

 class. 



The barnades found only in salt water show how organs 



originally intended for 

 one purpose may be 

 adapted to another. The 

 acorn-shell barnacle (Bal- 

 anus balanoides) lives in 

 calcareous shells at- 

 tached to the bottoms 

 of ships, wharves, or 

 other objects in the 

 water. The legs no longer 

 needed for locomotion, 

 as in its ancestors, are 

 extended through the 

 open shell to secure food. 

 On account of disuse the 

 eyes have been lost. 

 The goose barnacle (Lepas 



anatifera) is often found attached to floating objects by 

 a stem two or three inches in length. Between the two 

 pairs of calcareous plates 

 protecting the body are 

 thrust appendages, form- 

 erly used as legs, to 

 catch food. 



The terrestrial crusta- 

 ceans number but a few 

 species. The common 

 ones known as pill bugs, 



FIG. 163. Goose barnacles with tb e right shell 

 removed from the one on the left. Natural 



FIG. 164. Sow bug (Oniscus asellus). After 

 Paulmier. 



