Melusina: A French Folk Tale
Melusina is a popular French folk tale. After Hans Christian Anderson's "The Little Mermaid" it is probably the best known mermaid story in the world except maybe for Undine. There are many variations, but the bare bones are these:
Melusina, a fairy, is cursed to take on the form or a mermaid for one day a week. Further, during her "mermaid period" she cannot be seen by her husband or they will be parted forever. To make matters still worse, she cannot explain to him why he cannot see her. This sends Melusina on a quest for a trusting husband. She finds one who agrees to her terms and they live happily for many years. Of course, one day he starts to become jealous and decides that he must know what she is doing on her special day. He spies on her. Inevitably, Melusina finds out about it and that ends their relationship.
Some of the more common variations of this French folk tale are: Melusina becomes not a mermaid but a snake from the waist down (basically the same thing). Her husband is made jealous by someone else. Melusina's curse is explained to come from her mother (who was the victim of a similar curse) when Melusina attacked her father for not trusting her mother. The husband, when he sees Melusina's other form, may be horrified or ashamed for not trusting her. Melusina is often spelled Melusine (probably the original French version). Many more details may be given attributing the story to specific places, persons or times.