Muscular thickening in blood vessels whose valves control the direction of blood flow. Earthworms have several (commonly 5 pairs) of these blood vessels which connect the dorsal to the ventral blood vessels.

Dense metal such as cadmium, lead, copper, and zinc which can be toxic in small concentrations. Build up of heavy metals in garden soil should be avoided.

Iron containing compound in blood responsible for its oxygen carrying capacity.

Complex, highly stable material formed during breakdown of organic matter.