By T.L. Jahn, this is an older book, it is a very valuable reference. Published in 1949, you’ll have to dig this up from out of print dealers to get an inexpensive copy. You can get a new copy on Amazon here. The definition of protozoan early in the book is a reason for having this book in itself. There is an eloquent discussion of what is plant and what is animal, and how the microscopic world doesn’t make this distinction in many cases. The author talks about size estimations, staining, and slowing down these fast critters. There is a wonderfully detailed explanation of reproduction and movement. Finally, the meat of the book which classifies and describes most of the species you would find, goes into tremendous detail about each type of creature. The drawings are excellent, and the keyed descriptions superb. One is able to understand in detail the various parts and workings of hundreds of organisms. This book is a MUST.