Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER II BACTERIA AND PROTOZOA BACTERIA N the border line between the plant and the animal worlds are many forms which possess some of the characteristics of both. Indeed when an attempt is made to separate all known organisms into two groups one is immediately confronted with difficulties. In looking over the text-books of Botany we will find that certain low forms are discussed there as belonging with the plants, and on turning to the manuals of Zoology we will find that the same organisms are placed among the lowest forms of animals. The question is of course of little actual importance from certain points of view. It serves, however, to show the close relation of all forms of life, and from a medical standpoint it may be of very great importance owing to the difference in the life-habits, methods of reproduction and methods of transmission of many of the forms that cause disease. We have already seen that none of the diseases thatare caused by animal parasites is contagious, while many of those caused by bacteria are both contagious and infectious. Just over on the plant side of this indefinite border line are the minute organisms known as bacteria. Their numbers are infinite and they are found everywhere. The majority of them are beneficial to mankind in one way or another, but some of them cause certain of the diseases that we will have to discuss later so attention may be called here to a few of the important facts in regard to their organization and life-history in order that we may better understand how they may be so easily transferred from one host to another. Although these bacilli are so extremely minute (Fig. 7), some of them so small that they cannot be seen wifh the most powerful microscopes, they differ in size, shape, methods of division and spore-...