W123

I've got a 1977 240D with a starter that won't turn (or do much of anything) when the key is turned. It will, however, start right up when I bridge the two screw terminals on the smaller cylinder on the starter assembly (solenoid?). I'm not too clear how exactly the starter motor works, so I'm looking for some help on what to look at/for. More, possibly relevant, info: Drove about 150 miles through 100+ degree heat. When I stopped for lunch, car exhibited above symptoms. Banged on large starter cylinder, car started with minimal fuss. After sitting in a driveway overnight and most of the next day, starter wouldn't turn at all (or even "click"). My friend suggested bridging the two screw terminals on the solenoid with a screw driver, and this started the engine right up.