THE ARTHROPODS 



121 



some for known reasons, but more often not readily under- 

 stood. And not only does the form vary, but the external 

 surface may be sculptured or beset with spines or tubercles 

 which frequently render the animal inconspicuous amid its 

 natural surroundings. Such an effect is heightened by the 

 presence of sponges, hydroids, and various seaweeds which 

 the crab often permits to gather upon its body. 



115. Pill-bugs and sandhoppers. Finally there remain the 

 groups of the pill- or sow-bugs (Isopods) and the sand-fleas 

 or sandhoppers (Amphipods). In the first of these the 

 body is usually small and compressed, the thorax more or 

 less plainly segmented, and the seven walking (thoracic) 

 legs are similar. In the female each leg bears at its base a 

 thin membranous plate which extends inward and hori- 



FIG. 09. Isopocl or pill-bug (Porcdlio laevis). 



zontally, thus forming on the under side of the body a 

 brood-pouch (Fig. 69) in which the young develop. As 

 one may readily discover in any of the common species, 

 the abdominal segments are more or less fused, and bear 

 appendages adapted for respiration and, in the aquatic 

 forms, for swimming. 

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