The body of a kidnapped 12-year-old boy whose finger was cut off and sent to his family in December to pressure them into sending ransom money was found Sunday afternoon wrapped in plastic bags off the Hutchinson River Parkway in the Bronx, the police said yesterday.

The body of William Porter of 155 West 132d Street in Manhattan was found on a bicycle path near the parkway’s City Island exit by a homeless man looking for cans, Lieut. Raymond O’Donnell, a police spokesman, said. It was less than a mile from where the body of his older brother, Richard, who the police said was a crack dealer the kidnappers wanted to pay the ransom, was found shot to death on Jan. 4.

William’s body was inside 14 black plastic garbage bags, stuffed one inside the other. It was clothed in clean white sneakers, blue jeans and a white shirt. The body was badly decomposed but not cut up or dismembered, Lieutenant O’Donnell said.

William was kidnapped the morning of Dec. 5 as he walked the four blocks from his home to Public School 92. For 48 hours his abductors tried to extort as much as $500,000 from his 25-year-old brother, Richard Thomas Porter, the police said. On the second day the kidnappers cut off the boy’s right index finger and left it in a coffee cup in a bathroom with a minute long tape of the child pleading with his family to pay the ransom.

‘Get the Money’

”They cutted my finger off,” William’s voice said, shaking, on the tape, a police official said. ”Please help me. Get the money. I love you, Mommy.” On Jan. 4, Richard Porter was found shot to death near Orchard Beach Park, less than a mile from where William’s body was found Sunday. He was shot several times in the head and chest and the police found $2,239 in his pocket.

Detectives have speculated that William was kidnapped because of his brother’s drug dealing and that Mr. Porter was trying to negotiate without the help of the police when he was killed. They said Mr. Porter sold about $50,000 worth of crack a week. He was convicted of drug possession in April 1984 and of weapons possession in November 1984. The investigation is focusing on rival drug dealers and those who extort from them.

On Jan. 16, a drug-dealing associate of Mr. Porter’s was found shot to death at 149 West 132d Street. Stanley Harvey, 24 years old, of 716 St. Nicholas Avenue, was found dead of several bullet wounds on the fifth floor of an abandoned building. Investigators believe the three killings may be connected.

Investigators say they are looking for a 1989 two-door black Nissan with the New York license plate number 7HL209, in which Richard Porter was seen in Harlem the evening before his body was found.

Pathologists will try to determine whether William was killed before or after his brother was.

Crying on the Phone

William’s mother, Velma Porter, 44, discovered her son was missing at 4 P.M. on Dec. 5, when he did not come home from school. At 5 P.M., she received the first of seven telephone calls, with William crying on the phone. A caller believed to be male asked for $500,000 in ransom, but Mrs. Porter said they could not afford the ransom.

The next day, a caller told the family to go to the nearby McDonald’s restaurant, and a family friend found two rings there that belonged to William, a cassette tape and a two-inch piece of an index finger.

The last contact with the family that the police know of was made on Dec. 10, police officials said, when a woman handed a note to a child in upper Manhattan and told him to deliver it to William’s aunt. The note said the boy was in pain and needed medical attention.”