PHILADELPHIA — Near the entrance to the Philadelphia Zoo, a new, long tubular cage hangs about 20 feet above the ground, twisting through trees and around buildings as it leads into the zoo’s densely packed campus.

If they are lucky, visitors will see monkeys or lemurs that have decided to leave their regular quarters to explore other areas using the stainless-steel mesh structure.

The installation is part of the zoo’s expanding trail system, designed to provide more variety and stimulation for the animals and bring them closer to visitors. It is the start of a multiyear plan that will extend throughout the zoo’s 42 acres, allowing a variety of species, even bears and lions, to wander from place to place (though at different times) and rotating them through habitats.

While other zoos, like Denver’s and the National Zoo in Washington, have adopted trail systems in limited areas, the Philadelphia Zoo is the world’s first with a plan to rotate animals throughout its grounds.