URCHINS 



GRAVEL 



2.0 



1 — ' — f 

 0.125 O0625 mm 



MUD - 



Figure 2. --Size-frequency curves for the sand fraction of sediment from alimentary 

 tracts of uncaged urchins (upper), from urchins and rocks in bucket enclosures 

 (middle), and from rocks without urchins in bucket enclosures (bottom). Data 

 for the gravel and mud fractions are not given; scale of the y-axis is identical 

 for all sediments. 157 



