What does "trapdoor" mean?

Trapdoor: (Mining) A door in a level for regulating the ventilating current; -- called also weather door. Raymond. Trapdoor spider (Zoöl.), any one of several species of large spiders which make a nest consisting of a vertical hole in the earth, lined with a hinged lid, like a trapdoor. Most of the species belong to the genus Cteniza, as the California species (C. Californica).