230 



SYSTEMATIC ZOOLOGY. 



There are two subdivisions of Tetradecapods: Isopoda 



and Amphipoda. 



In the Isopods (fig. 57) the 

 body is depressed, as in the 

 sow-bug, and the gills are borne 

 under the abdomen. Most of 

 the Isopoda feed upon decay- 

 ing matter, but some have 

 become parasites upon other 

 animals, and have consequently 

 so changed their appearance 

 that one knowing only the 

 adult would never regard them 

 as Isopods at all. But the young 

 settle the question, since before 

 they begin their parasitic life 

 they are regular Isopods. 



In the Amphipods (fig. 58) 

 the body is compressed from side to side, and the gills are 

 borne on the thoracic region between the legs. These 

 forms are familiar to all visitors to the shore under the 



FIG. 57. Marine Isopod (Idotea 

 irrorata). After Harger. 



FIG. 58. Beach-flea (Gammarus ornatus). From Smith. 



common name of ' beach-fleas, ' a name which those 

 forms living under dried seaweed, etc., have won for 

 themselves through their leaping powers. Others live 



