Apart from the core orchestral complement, various other instruments are called for occasionally. These include the flugelhorn and cornet. Saxophones and classical guitars, for example, appear in some 19th- through 21st-century scores. While appearing only as featured solo instruments in some works, for example Maurice Ravel's orchestration of Modest Mussorgsky's "Pictures at an Exhibition" and Sergei Rachmaninoff's "Symphonic Dances", the saxophone is included in other works, such as Ravel's "Boléro", Sergei Prokofiev's Romeo and Juliet Suites 1 and 2, Vaughan Williams' Symphonies No. 6 and 9 and William Walton's "Belshazzar's Feast", and many other works as a member of the orchestral ensemble. The euphonium is featured in a few late Romantic and 20th-century works, usually playing parts marked "tenor tuba", including Gustav Holst's "The Planets", and Richard Strauss's "Ein Heldenleben". The Wagner tuba, a modified member of the horn family, appears in Richard Wagner's cycle "Der Ring des Nibelungen" and several other works by Strauss, Béla Bartók, and others; it has a prominent role in Anton Bruckner's Symphony No. 7 in E Major. Cornets appear in Pyotr Ilyich Tchaikovsky's ballet "Swan Lake", Claude Debussy's "La Mer", and several orchestral works by Hector Berlioz. Unless these instruments are played by members "doubling" on another instrument (for example, a trombone player changing to euphonium or a bassoon player switching to contrabassoon for a certain passage), orchestras typically hire freelance musicians to augment their regular ensemble.