The Matryomin is a theremin-like electronic instrument encased (for reasons unknown) in a wooden Russian doll. From the description of the Matryomin on inventor and theremin player Masami Takeuchi‘s website:
Matryomin is the unique, original erectronic musical instrument invented by Masami Takeuchi in 1999. It is a type of theremin – oldest electro-musical instrument invented in Russia – shaped Russian traditional wooden doll, Matrioshka. It hold form of Matrioshka perfectly, moreover, performing five octaves range. The distance of 1 octave at Low-Middle range is equal to Etherwavetheremin of Moog Music Inc. If you have acquired the basic technique to play theremin by Etherwavetheremin, you can enjoy playing Matryomin by same way. Matryomin is only pitch controlled theremin. Mandarin Electron, a company directed by Masami Takeuchi, started manufacturing Matryomin on a commercial basis in 2003. Now, Matryomin is going on 2nd generation model. Selled over 1,600 till now in Japan. (mandarinelectron.com)
via Infocult