Complex sentence: The ligament is usually brown to black in color and connects the two bivalve shells at the hinge line and functions like a spring to open the valves when the adductor muscles relax.
Split into simpler sentences: The ligament is usually brown to black in color. As well as connecting the two bivalve shells together at the hinge line, it also functions like a spring to open the valves when the adductor muscle or muscles relax.