An entire change in the construction of the Catacombs now took place. They became obviously designed for purposes of safety and concealment. The new galleries were less wide and lofty, and the _loculi_ more crowded on account of the greater difficulty of removing the excavated 101 material. At this time, too, many of the lower _piani_ were made for additional graves and greater secrecy. The main entrances were blocked up and the stairways demolished. Sometimes entire galleries were filled with earth, the removal of which is the chief obstacle to modern exploration, or were built up with masonry to obstruct pursuit; and means of escape were provided, in case of forcible invasion of these retreats. A striking example of this occurs in the Catacomb of Callixtus. The ancient stairway was partially destroyed, the entrance completely obstructed, and some of the galleries walled up. Narrow passages for escape were made connecting with an adjacent _arenarium_, and a very narrow secret stairway constructed from the roof of the latter to the surface of the ground, as shown in the section above [Transcriber’s Note: Fig. 22, below], which stairway could only be reached by a movable ladder connecting it with the floor.[147] [Illustration: Fig. 22.--Secret stairway into Arenarium.] It is impossible that the mass of the Christian community, or even any 102 considerable proportion of it, could ever have taken refuge in these subterraneous crypts. Their vast extent and the number of chambers would indeed permit a great multitude to remain concealed for a time in their depths; but the difficulty of procuring a regular supply of food, the confined atmosphere, and the probable exhalation of noxious gases from the graves--especially on the opening of a _bisomus_, or double tomb, for its second inmate--seem insuperable obstacles. As it was the religious leaders of the Christian community who were especially obnoxious to those in power, they would be the most likely to seek concealment in the Catacombs, not from inferiority of courage, but, like the afterward martyred Cyprian, that they might the better guide and govern the persecuted church. Hence the examples before given of bishops and other ecclesiastics lying hidden, some for years, in these depths, and visited by the faithful for instruction or for the celebration of worship.[148] There is evidence, however, that during the exacerbations of persecution private Christians sought safety in these recesses, and, burrowing in their depths, evaded the pursuit of their enemies. Tertullian speaks of “a lady, unaccustomed to privation, trembling in a vault, apprehensive of the capture of her maid, upon whom she depends for her daily food.” The heads of Christian families, and those most obnoxious to the pagan authorities, would be especially likely to leave the fellowship of the living in order to live in security among the dead. Father Marchi conjectures