They have a tympanum on each side of their heads which is involved in hearing and , in some species , is covered by skin .
Frogs have three eyelid membranes : one is transparent to protect the eyes underwater , and two vary from translucent to opaque .
two have a tympanum on each side of two 's heads which is involved in hearing and , in some species , is covered by skin .
Frogs have three eyelid membranes : one is transparent to protect the eyes underwater , and two vary from translucent to opaque .