`` well , then , silkies are strange mystical beings that are seals in the daytime but can change into half men at night , and when they do , they stand on great strong legs and have huge hairy chests and great beards all twined with seaweed , and sometimes one of them takes it into his mind to suddenly appear in the middle of the night at the foot of the bed of a comely maiden , and when he leaves , she ai n't a maiden no more , oh , no , she ai n't .