ARTHROPODS. CLASS CRUSTACEA 105 
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. 
105. 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- 
Fic. 63.—Isopod or pill-bug (Porcellio laevis). 
zontally, thus forming on the under side of the body a 
brood-pouch (Fig. 63) 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. 
