220 THE STUDY OF ANIMAL LIFE CHAP. 



stems or other parts of dusty dead plants have been 

 steeped for a few days, and even with the unaided eye 

 you will see a thick crowd of the mobile whitish motes 

 which, from their frequent occurrence in such infusions, 

 are usually called Infusorians. Or if a piece of flesh be 

 allowed to rot in an open vessel of water, the fluid becomes 

 cloudy and a thin flaky scum gathers on the surface. If 

 a drop of this turbid liquid be examined with a high 

 power of the microscope, you will see small colourless 

 rods and spheres, quivering together or rapidly moving 

 in almost incalculable numbers. These, though without 

 green colour, are the minutest forms of plant life ; they 

 are Bacteria or Bacilli, the practically omnipresent 

 microbes, some of which, as disease germs, thin our human 

 population, while others, as cleansers, help to keep the 

 earth habitable. 



Three great types of unicellular animals or Protozoa 

 have been recognised in almost every classification. 



(a) The Infusorians, so abundant in stagnant water, 

 have a common character of activity expressed in the 

 possession of actively mobile lashes of living matter known 

 as cilia or flagella. Thus the slipper-animalcule (Para- 

 meci.um) is covered with rows of lashing cilia, while 

 smaller, equally common forms, generally known as 

 Monads, are borne along by the undulatory movement of 

 one or two long whips or flagella. The bell-animalcules 

 (Vorticella) which live in crowds, a white fringe on the 

 water weeds, are generally fixed by stalks, but are 

 crowned with active cilia at the upper end of the some- 

 what urn-shaped cell. 



(b) In marked contrast to these are the parasitic 

 Gregarines, or Sporozoa, which infest many animals, and 

 cause many diseases, such as the pebrine of silkworms and 

 the malaria of man. They tend to be very sluggish, and 

 they multiply chiefly by forming within the parent cell 

 numerous minute units or spores. 



(c) Between these two extremes of activity and passivity 

 there is a third type well represented by the much-talked- 

 of Amoeba which glides about on the mud of the pond, by 

 the sun-animalcules (Actino splicer ium) which float in the 



