Emily had only a moment to relax in her saltwater bath before she felt the changes begin.
Her legs and feet twitched and spasmed. It wasn't painful, just strange - like nervous squirming and shivering at the same time. Her legs rubbed against each other for a moment before seeming to stick together like glue; Emily tried but could not seperate them.
Her toes felt funny and she leaned forward to get a better look at them. They were webbed now and seemed to be lengthening. Her heels stuck together and her feet began to slowly twist. Emily realized they were becoming a tailfin. Her feet-fins stretched as wide as the bottom of the bathtub. Emily wiggled her new tailfin. She leaned forward to touch it. It seemed farther away than her toes would have been - was her tail growing longer than her legs? But she was able to curl her tailfin back towards her so she could grab it. At first it just felt like her normal skin but as she held her fin it began to feel different, more like a fish. The color began to change, taking on a lighter shade of green.
Her ankles and heels were gone now; her tailfin gave way to a tail. She ran her hand from her tailfin slowly up what had once been her legs. There was a slow change working its way up her tail: her legs loosing what semblance remained of seperate limbs and becoming a single limb. Then her skin developing a scaly fish-like texture which then turned a light shade like her tailfin before shifting to a darker shade of green.
The changes worked their way up her legs, her tail steadily growing wider the closer it go to her torso. She soon had nothing resembling knees but found she could bend her tail at any point. As the changes approached her waist she wondered what would happen to her groin and how far up the changes would go.
Emily soon had her answers. The last of her thighs' definition was lost in her tail, but a slit remained on the front of her tail where her vagina was even after the scales spread past it.
Her tail fully formed, the scales reached up past her hips then faded out.
She was a mermaid!