We propose Janus, a C++ template library of container classes and communication primitives for parallel dynamic mesh applications. The paper focuses on two-phase containers that are a central component of the Janus framework. These containers are quasi-constant, i.e., they have an extended initialization phase after which they provide read-only access to their elements. Two-phase containers are useful for the efficient and easy-to-use representation of finite element meshes and generating sparse matrices. Using such containers makes it easy to encapsulate irregular communication patterns that occur when running finite element programs in parallel.