(Close): Stocks on Wall Street fell sharply on Thursday as worries about global economic growth continued to hit markets around the world.
The Dow Jones closed down 358.04 points, or 2.1%, at 16,990.69.
The S&P 500 index ended 43.88 points lower at 2,035.73, while the Nasdaq dropped 141.56 points to 4,877.49.
Twitter shares dipped below their IPO price of $26 during the day, hitting $25.92 at one point, but closed at exactly $26, down 5.8%.
Shares in Twitter were listed on the market in November 2013, and hit highs above $70. However they have fallen out of favour recently on worries over the firm's growth.
Last month, Twitter's shares fell sharply after it said the number of average monthly active users had grown to 316 million worldwide, up from 308 million in the previous quarter. Twitter co-founder Jack Dorsey said at the time has was "not satisfied" with the user growth.
Walt Disney was the biggest faller on the Dow, dropping 6% after a downgrade from analysts at broker Bernstein Research.
Other media stocks also fell, with Netflix down 7.8% and 21st Century Fox 4.2% lower.
Apple shares fell more than 2% following a report that smartphone sales in China - a key market for Apple - fell in the second quarter, the first time they have done so.
