By T. J. Burrill, Ph. D., Springfield, Illinois, 1882. This is a monograph reprinted from the report of the Illinois Industrial University, and to those who have compound microscopes will be found especially interesting. The class of organisms to which it relates have become famous since naturalists have ventured to grapple with the great problem of the origin of life. Prof. Burrill says that mankind could not continue to exist; could never have existed, but for these minute creatures, and on the other hand many of our diseases would not be but for them.