Sometimes dissolution is gradual, with the dissolved material immediately put back in place as a more stable arrangement of calcium carbonate, namely calcite. This process of recrystallization produces no strong chemical change, and is therefore most readily detected by the obliteration of fine skeletal details. Complete dissolution of skeletal material in full, without any precipitation of new or recrystallized minerals, so as to leave a void in the sediment, produces molds. If sediment or new mineral matter come to fill the void in the sediment left by the dissolution of the organism, the resulting replica of the orginal material is termed a cast . Filter feeders pull in current and feed upon tiny planktonic organisms and larvae.
Detrital feeders eat organic-rich muds.
Although symmetry is correlated with life mode, exceptions occur as the result of past evolutionary history.