94 ANIMAL BIOLOGY 



but the most familiar ones, our common sow bugs and pill 

 bugs, live in damp situations upon land and have become 

 adapted to breathing air. These terrestrial forms are 

 often found under logs and stones, in damp cellars and 

 around old buildings. They are mostly vegetarians, but 

 do not disdain a little meat occasionally. For the most 

 part, however, they are content to fill themselves up on 

 such apparently unattractive pabulum as partially de- 

 cayed wood. While they sometimes attack tender young 

 plants they are for the most part harmless creatures and 



may even be of benefit in a small 

 way as scavengers. 



The nearest allies of the isopods 

 are the amphipods which agree 

 with the isopods in having sessile 

 eyes, no carapace and the typical 

 number of fourteen legs. The 



FlG ' rato^^Sf flea> g ills > however > are attached to the 

 thorax. Most amphipods are 



marine, but there are many fresh water species, and a 

 few terrestrial ones called sand fleas commonly found on 

 sandy sea beaches. 



At a first glance no one would classify the barnacles with 

 the Crustacea and up to less than a century ago even 

 zoologists classed them, along with clams and snails, 

 among the Mollusca. This was doubtless done on ac- 

 count of the hard shell with which the body of most bar- 

 nacles is surrounded. It was later found that barnacles 

 hatch from the egg as a nauplius, a common larval form 

 in other groups of Crustacea. The nauplius is a free swim- 

 ming larva with a median eye and three pairs of append- 

 ages. The barnacle nauplius as it grows undergoes a 

 series of molts accompanied by considerable changes of 

 form, and finally settles down and attaches itself by its 



