How does the anime for Excel Saga differ from the manga? More specifically, does the plot in the Excel Saga anime depart significantly from that in the manga?

I've read that some characters only appear in the anime. What are notable differences between the characters who appear in the manga and those who appear in the anime? Are there any other differences? Is the structure of the anime (i.e. one episode spoofs of different genres) different from that of the manga or are there similarities?