Morgan Stanley, the last of the big U.S. banks to release third-quarter earnings, said on Monday its trading revenue fell 17.2 percent to $2.03 billion in the period, contributing to a 42.4 percent drop in profit attributable to shareholders. Morgan Stanley joins arch rival Goldman Sachs Group Inc (GS.N) as well as Citigroup Inc (C.N), Bank of America Corp (BAC.N) and JPMorgan Chase & Co (JPM.N) in reporting grim results from trading, a business many of them are trying to back away from.