In what way is Anna like "a burned leech?"

The narrator describes her as covered with sand and pine needles, like the leeches she and her brother used to throw into the fire. They would try to crawl to safety, but they, like Anna, would be caught and thrown back into the fire. Anna is continually thrown into the "fire" of her marriage, despite any futile efforts to stand up for herself, because she cannot change her behavior or her attitudes.