A 一区画 of 洞くつ壁 opens like a 口 without 歯(複), throat, or anything usable by a 体. So perhaps we should say that a 一区画 of 洞くつ壁 opens like the 口 on a flexible cloth puppet handled by an invisible expert 失われた in a 部屋 so 暗い that only the glow-in-the 暗い cloth of that 口 is visible, as well as the strange 暗い rough texture of the cloth visible beneath the glow.