An autotroph is an organism capable of self-nourishment using inorganic materials as nutrients and photosynthesis or chemosynthesis as an energy source, including most plants and certain bacteria. A heterotroph requires organic compounds for its principal food source. A detritivore feeds on and breaks down dead plant or animal matter, returning nutrients to its ecosystem. Radial symmetry means an organism can be divided into similar halves by a plane through any central axis.