Lombricine is a phosphagen that is unique to earthworms. Structurally, it is a phosphodiester of the guanidine, 2-guanidinoethanol (IUPAC name: 2-carbamimidamidoethanol), and D-serine (not the usual L-serine), which is then further phosphorylated by lombricine kinase to phospholombricine.