Whelks are mollusks. The shells aren't soft, but the animal that lives inside is. Not all mollusks have shells. Think of the octopus, squid and even the slug. Conversely, all animals with shells are not mollusks, such as crabs, barnacles and shrimp.
All mollusks have soft bodies.
The Lightning Whelk is also called the Left-handed Whelk and has been scientifically named Busycon sinistrum. Sinistrum is Latin for left. It grows to about one foot in length though 15 inches has been recorded. After the shell reaches eight to nine inches, the colors fade to an overall grayish white. Females are generally larger than males, so the record setters are probably of the former gender.