fcHAj. XTV The Simplest Animals ail 



thinner than thread and lither than eels : both of these may be very 

 small, but closer examination shows that they have parts and organs, 

 that they are many-celled not single-celled animals. 



Vary the observations by taking water in which hay 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 make 

 the earth a habitable dwelling-place. 



2. Survey of Protozoa. — Three great types of unicellular 

 animals or Protozoa have been recognised in almost every classi- 

 fication. 



(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 [Paramecium) is covered with rows of lash- 

 ing 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 soi;newhat um-shaped cell. 



{b) In marked contrast to these are the parasitic Protozoa, the 

 Gregarines, which infest most backboneless animals, notably the 

 male reproductive organs of the earthworm or the gut of lobster and 

 cockroach. Sluggishness and the absence of all locomotor pro- 

 cesses are their characteristics. 



[c) Between these two extremes of activity and passivity there 

 is a third type well represented by the much-talked-of Avmba which 

 glides about on the mud of the pond, by the sun -animalcules 

 {Actinospharium) which float in the clear water of brooks, by the 

 limy-shelled, chalk-forming Foraminifera which move slowly on 

 seaweeds or at the bottom of shallow water, or in some cases float 

 at the surface of the sea, and by the flinty-shelled Radiolarians 

 which live in the open ocean. In all these the living matter 

 spreads out in thick or thin, stiff or plastic, free or interlacing pro- 

 cesses, which often admit of a slow gliding motion, and are still 



