BOSTON — A former cocaine dealer who sought to protect his criminal empire by aligning with reputed mobster James “Whitey” Bulger testified on Monday that even as he worked with Bulger’s closest associates he knew better than to utter the boss’s name.

Joseph Tower, who said he used to run a “pretty big empire” selling cocaine and marijuana in the Boston area, said everyone in the underworld knew not to mention the widely feared Bulger or even his nicknames — “Whitey” and “Boots” — in connection with any sort of criminal activity.

As he made the rounds collecting his cut of drug profits, Bulger was referred to as “the other guy,” Tower said.

His testimony came at the end of a day devoted mostly to a sharp-edged cross-examination of former FBI Agent John Morris, who has admitted to taking bribes from gangsters and tipping them off about federal investigations during his years at the helm of the FBI’s organized crime squad in Boston.

Bulger, 83, is on trial for killing or ordering the murders of 19 people while at the helm of the Winter Hill Gang, which authorities say ran bloody extortion and gambling rackets in Boston during the 1970s and ’80s.

He has pleaded not guilty to all charges, though his defense attorney acknowledged in opening statements that Bulger ran an “unbelievably lucrative criminal enterprise.”

At one point, Morris — who ran the FBI’s Boston organized crime squad before leaving the bureau in 1995 — made an emotional apology to the family of one of the innocent bystanders allegedly killed by Bulger.

“I don’t want to ask for your forgiveness, but I do want to express my sincere apology for things that I may have done and things that I didn’t do,” Morris said, his face reddening and his eyes tearing as he addressed the family of Michael Donahue.

Donahue was killed in a hail of gunfire as he and Edward Brian Halloran left a restaurant in May 1982. Shortly before the murder, Morris had told another corrupt FBI agent, John Connolly, that Halloran had become an FBI informant. Connolly turned that information over to Bulger.

Morris said he hadn’t meant to leak the news that Halloran had turned informant but “somehow it happened.”

Defense attorney Hank Brennan asked: “You knew you were signing Mr. Halloran’s death warrant?”

“No,” Morris said, explaining that he believed Halloran to be in a witness protection program. “I thought he was going to be safe.”

Morris, his voice cracking, turned to Donahue’s widow and sons, who were sitting in court.

“Not a day in my life has gone by that I haven’t thought about this,” he said. “I do not ask your forgiveness. That’s too much. But I do acknowledge it publicly.”

During an aggressive cross-examination, Brennan aimed to discredit Morris by leading him through a litany of his corrupt acts during his tenure at the FBI, including taking money and gifts from Bulger and other informants, obstructing justice, tipping off organized crime about FBI investigations, carrying on an affair with his secretary and lying repeatedly — under oath — to cover up those actions.

Morris, who was granted immunity for his testimony about FBI misconduct in 1998, now works as a part-time wine consultant and continues to receive a government pension. Connolly is serving a 40-year prison term for murder and racketeering.

After Morris stepped down, Tower took the witness stand, speaking so rapidly and with such animation that the judge had to ask him to slow down.

Asked to identify Bulger in the courtroom, Tower paused dramatically, looked around for a moment and then pointed his finger at the defense table. “How’re you doing, Jim,” he said with a smile and a nod.

Tower said Bulger urged him to work with his associates and provided him protection for his cocaine business. Bulger, he said, regularly came around to collect a share of the proceeds.

“Mr. Bulger would have to be paid because he was the protection, he was the OK,” Tower said.

The defense will cross-examine Tower on Tuesday.