Some species of bacteria have the power of active motion, and may be seen darting rapidly to and fro in the liquid in which they are growing. This motion is produced by flagella which protrude from the body. These flagella (Fig. 15) arise from a membrane surrounding the bacterium, but have an intimate connection with the protoplasmic content. Their distribution is different in different species of bacteria. Some species have a single flagellum at one end (Fig. 15 a). Others have one at each end (Fig. 15 b). Others, again, have, at least just before dividing, a bunch at one or both ends (Fig. 15 c and d), while others, again, have many flagella distributed all over the body in dense profusion (Fig. 15 e). These flagella keep up a lashing to and fro in the liquid, and the lashing serves to propel the bacteria through the liquid. INTERNAL STRUCTURE. It is hardly possible to say much about the structure of the bacteria beyond the description of their external forms. With all the variations in detail mentioned, they are extraordinarily simple, and about all that can be seen is their external shape. Of course, they have some internal structure, but we know very little in regard to it. Some microscopists have described certain appearances which they think indicate internal structure. Fig. 16 shows some of these appearances. The matter is as yet very obscure, however. The bacteria appear to have a membranous covering which sometimes is of a cellulose nature. Within it is protoplasm which shows various uncertain appearances. Some microscopists have thought they could find a nucleus, and have regarded bacteria as cells with inclosed nucleii (Figs. 10 a and 15 f). Others have regarded the whole bacterium as a nucleus without any protoplasm, while others, again, have concluded that the discerned internal structure is nothing except an appearance presented by the physical arrangement of the protoplasm. While we may believe that they have some internal structure, we must recognise that as yet microscopists have not been able to make it out. In short, the bacteria after two centuries of study appear to us about as they did at first. They must still be described as minute spheres, rods, or spirals, with no further discernible structure, sometimes motile and sometimes stationary, sometimes producing spores and sometimes not, and multiplying universally by binary fission. With all the development of the modern microscope we can hardly say more than this. Our advance in knowledge of bacteria is connected almost wholly with their methods of growth and the effects they produce in Nature. ANIMALS OR PLANTS?