A mother has been left horrified after squeezing a sea snail from a giant growth on her son’s leg.

Rachael Franklin, from Orange County, California, became concerned when her seven-year-old son, Paul’s, knee grew to the “size of an orange”.

Rachael Franklin and her son Paul Animal Planet/ YouTube (Animal Planet/ YouTube)

Ms Franklin told CBS LA: “I came home and his knee was the size of an orange and hot to touch and he’s limping.”

Taking her son to a doctor, Paul was diagnosed with a staph infection, prescribed antibiotics and warned not to pop the huge swelling.

However, when the lump began to turn black and ooze pus, Ms Franklin decided to defy doctor's orders and popped the wound.

When she did, Ms Franklin was horrified to find an odd, rock-like object inside the wound.

“I’m peering at this object and it has a strange look about it,” she said during an episode of Animal Planet’s Monster Inside Me, which will air next week.

“I realised it wasn't a rock. It has whirls on it,” she said.

The mollusc had been growing in the tissue above Paul’s knee, while the pus and black-looking tissue developed as Paul’s immune system rejected the alien object.

The snail is known as a Littorina scutulata and lives on inland tidal areas along the West coast of America, the biologist Dan Ruskin explained to the programme.

The snail had been growing in the tissue above Paul’s knee Animal Planet/ Youtube (Animal Planet/ YouTube)

The species can survive a vast range of temperatures and waters, he said. “It can even live outside of water for several weeks by retreating into its shell."

Ken, Paul’s father, told Detroit News Time, he believes the snail made its way into Paul’s knee after he fell onto a nest of snail’s eggs at a beach in Spooner’s Cove, California.

While Ms Franklin was left aghast at the sight of the snail, Paul appeared to have a much calmer reaction to the incident.