Not being able to sleep, Iwan sat up in bed peering into the darkness. Suddenly he felt a current of cold air. He jumped out of bed. However, having forgotten the motion of the ship, he was instantly thrown violently across the room. Hardly he got up and groped towards the window which, to his surprise, he found wide open, for he was certain that he had fastened it securely before he went to bed. Suddenly, as he stood by the window, he distinctly heard something moving behind him and, a moment afterwards, as he turned to look, he heard a faint groan. He sprang across the room and jumped back into bed, only to discover to his horror that someone else was lying there.

    Stretching out his hand, he laid hold of something that had the shape of a man's arm but was smooth, wet and icy cold. He pulled, but the creature, whatever it was, tore itself out of his grasp, ran across the room, opened the door and rushed out. Quickly recovering from his fright, he started to chase at top speed but he was too late. The passage outside his room was empty.

Why did Iwan Jump and try to get to the window?