Shares of internet service provider Prodigy Communications Corp soared on their first day of trading Thursday, closing at $28.125, an 88% rise from the offering price of $15 a share. The shares traded as high as $32 in the session. Heavy demand for the issue led to a volume of 25.8 million shares, allowing the company to eclipse the likes of Dell Computer Corp and Cisco Systems Inc atop the list of the most active shares of the day. Meanwhile, in other IPOs Thursday, business-to-business e- commerce venture VerticalNet Inc closed at $45.375, up 183.5% from its offering price of $16. And Healtheon Corp, the web-based healthcare start-up backed by Netscape cofounder Jim Clark, shot from the offering price of $8 to $33.25 almost as soon as it began delayed trading at about 12:45 pm. The shares closed at $31.375, up 292% on the day and the biggest gainer on the market. VerticalNet took second place on the most advanced board, while Prodigy held the fourth spot. The three debuts and a strong earnings report from MCI WorldCom Inc – as well as talk on the Street that Applied Materials Inc will post stronger-than- expected results next week – led the Nasdaq composite index to its largest one-day point gain ever. The index surged 96.05 points, to close at 2,405.55. Overall volume of Nasdaq stocks was 966 million shares and advancing issues led decliners 2,306 to 1,601.