  The effects of line tension on the morphology of a lens-shaped droplet and
bubble placed on the inner wall of a spherical cavity are studied. The contact
angle between the lens-shaped droplet and the concave spherical substrate is
expressed by the generalized Young's formula. The equator of the spherical
substrate is found to play a crucial role. Neither a droplet with its contact
line on the upper hemisphere of the substrate nor one with its contact line on
the lower hemisphere can transform into each other continuously. On a
hydrophobic substrate, the contact angle jumps discontinuously to
$180^{\circ}$, and the droplet is detached from the substrate to form a
spherical droplet when the line tension is positive and large. This is similar
to the drying transition on a flat substrate. On the other hand, on a
hydrophilic substrate, the contact angle jumps discontinuously to $0^{\circ}$
when the line tension is positive and large. Then, the droplet spread over the
whole inner wall to leave a spherical bubble. Therefore, not only the drying
transition but also the wetting transition is induced by positive line tension
on a concave spherical substrate. There also exist stable as well as metastable
droplets, whose phase diagrams can be complex. When the line tension is
negative and its magnitude increases, the contact line approaches the equator
infinitesimally from either above or below. However, it cannot cross the
equator of a spherical cavity continuously. The droplet with a contact line
that coincides with the equator is a singular droplet. The contact line is
pinned and cannot move, irrespective of the magnitude of the line tension.
