PROBLEM I. The Kinds of AiinuaU 



water which takes on a brown tint. Many 

 kinds of microscopic creatures will soon 

 be swarming in the infusion. 



When pond \\ater is lacking, there- 

 fore, you may turn to the hay infusion 

 for your first look at the world of micro- 

 scopic living things. If you are able to 

 get the use of a microscope you can look 

 forward to many happy hours of dis- 

 covery. 



A giant among microscopic animals. 

 One of the commonest inhabitants of 

 the hay infusion is an enormous niicro- 

 organisj/1 (microscopic organism), which 

 is just visible to the naked eye as a white 

 speck darting about in the water. You 

 may have heard its name, Farmjiechmi 

 (par-a-mee'see-um). Paramecium is easy 

 to raise and with a microscope fun to 

 study. Do Exercise i i . 



It is not easy to examine a lively para- 

 mecium with the microscope; it moves 

 too fast. But it is possible to catch it in 

 the fibers of cotton or even to thicken 

 the water so that the paramecium pushes 

 its way through with difficulty. Either 

 one of these tricks will slow the animal 

 enough for you to see that the little sub- 

 marine-shaped Paramecium is covered 

 with tiny hairlike parts or cilia (siPee-a). 

 The singular is ciliinn. These cilia beat 

 vigorously and thus push the paramecium 

 rapidly through the water. By lashing 

 the cilia hard in the opposite direction 

 the animal can go into reverse. The cilia 

 are arranged diagonally in rows so that 

 as they beat they make the paramecium 

 roll over and over like a barrel at the 

 same time that it moves forward or back- 

 ward. 



As the paramecium rolls over, one can 

 see that on one side there is a groove as 



oj the Earth 63 



though part of its cigar-shaped body had 

 been scooped out. This depression leads 

 to a spot, the ynonth. Longer cilia line 

 the depression; their beat is inward so 

 that any smaller microorganism caught 

 by the current is swept to the mouth and 

 into the paramecium. 



The microorganism of ever-changing 

 shape. Exercise 12 gives you directions 

 for studying this animal: Aineba. Because 

 of its habit of clinging to some solid 

 base and because it is almost transparent, 

 it is difficult to find. Ameba is not trim 

 and compact like paramecium, but 

 spreading and shapeless. Its body is soft 

 and jellylike — just a blob of living mat- 

 ter. Some of the living material flows 

 for a while in one direction and forms 

 a projection called a false foot or pseu- 

 dopod (siu'doe-pod). It is a temporary 

 foot which can form on any part of the 

 body; in fact, ordinarily an ameba has 

 several pseudopods at the same time 

 sticking out in difi"erent directions. Some- 

 times, however, the material keeps on 

 oozing in one direction; in this way the 

 ameba, by ever changing its shape, crawls 

 along over the surface of some leaf or 

 stem under water. 



The pseudopods are used for feeding 

 too. If some smaller microorganism or 

 other particle of food lies in the ameba's 

 path, false feet flow out above, below, 

 and on all sides of it and join together on 

 the other side. The food particle is then 

 inside the ameba, or, more correctly, the 

 food particle is inside a little drop of 

 water which is inside the ameba; for 

 when the pseudopods join together they 

 enclose a little water too. If the animal 

 picks up some w^orthless particle like a 

 orain of sand, it simply drops it behind as 



