Sentence:  "Among the daughters of the air," answered one of them. "A mermaid has not an immortal <WSD>soul</WSD>, nor can she obtain one unless she wins the love of a human being. On the power of another hangs her eternal destiny. But the daughters of the air, although they do not possess an immortal <WSD>soul</WSD>, can, by their good deeds, procure one for themselves. 
Meaning of the word marked with <WSD> tags:  The spirit or essence of a person usually thought to consist of one's thoughts and personality. Often believed to live on after the person's death.