the nervous system
when the atom has as many electons as it does protons, either through transfer (in reaction) or bonding. ie, O2(the type of oxygen we breathe) is neutral because oxygen by itself usually has 2minus electrons, however by combining two oxygens--a bond is formed and so both the elergy levels(shells/orbits) of the electrons are now completed, thus equating to an evenness between the proton and electron count for each atom. another example is of a chemical reaction, one may look to salt(NaCl). by itself, Na(sodium) is a metal which is not stable (1 electon in excess) and Cl(calcium) is a poisonous gas by itself. when combined, they make for the harmless table salt we all know and love.
A horseshoe crab eats plankton.
One of the things it is used for is to smell things.
They store honey to use as food when the weather is too poor for them to leave the hive to forage for nectar.