When Bill is very young, he loves picture. His mother often draws some for him on old pieces of paper. She is very bad at drawing, but bill likes her pictures and always wants more. Then, when he is a little older, Bill’s mother gives him some pencils and a drawing book, and he begins drawing pictures, too. But they are never good .When Bill is five years old, his mother gives him a small blackboard and some pieces of a baby on the blackboard, he draws lines(线条) and rubs(擦) them out too for ten minutes , but when he looks at his picture , he is not happy. “Well”, he says at last(最后) to his mother, “I’ll put a tail(尾巴) on it and make it a monkey.”

下列不具有细胞结构的生物是（　　）