Metamerism is the external and internal division of animal body into segments with a serial repetition of at least some organs. e.g., annelids (earthworm).
Segmentation refers to the division of animal's body into a series of repetitive segments. It is external in arthropods, and internal in vertebrates.
Metagenesis is the phenomenon in which one generation of certain animals and plants reproduce asexually, followed by a sexually reproducing generation, i.e., alternation of generation (e.g. Obelia).
Metamorphosis is the developmental process in an organism through which it changes from one life form to another.