ONE-CELLED ANIMALS 217 



the other hand, we often find that organs which do not have the 

 same structure or origin are used for similar purposes. Such are 

 the wings of a bird and the wings of a butterfly. Such structures 

 are analogous. Analogy is likeness in function, regardless of origin. 



In our study of biology so far we have attempted to get some 

 notion of the various factors which act upon and interact with 

 living things. We have examined a number of forms of plants 

 and have found all grades of complexity from the one-celled plant, 

 Pleurococcus, to the complicated flowering plants of considerable 

 size and with many organs. So in animal life the forms we study 

 will show a constant change, and the change is toward greater 

 complexity of structure and of function. A worm is simpler in 

 structure than an insect, and shows by its sluggish actions that 

 it is not so high in the scale of life as its more lively neighbor. 



We are probably aware of the fact that we are better equipped 

 for the battle for life than lower animals, for we are thinking 

 creatures and can change our surroundings if they are unfavorable, 

 while the lower forms of animals are largely controlled by stimuli 

 which come from without, such as temperature, moisture, light, 

 and the presence or absence of food. 



There are a great many ways of arranging animals in groups. 

 One way is to put all animals that have no backbones into a large 

 group called the invertebrates as opposed to those which have a ver- 

 tebral column, the vertebrates. 



PROBLEM I. WHAT ARE THE CHARACTERISTICS OF ONE- 

 CELLED ANIMALS? 



Habitat of Protozoa. Protozoa, or one-celled animals, are found 

 in water, seemingly never at any great depth. They appear to 

 be attracted to the surface by light and by the supply of oxygen. 

 Every fresh- water lake swarms with them; the ocean contains 

 countless myriads of many different forms. 



Demonstration 1. To show a living amoeba. To obtain amoebas, 

 crush some water plants and let the mass remain undisturbed for a week 

 or so. Living animals will usually be found in the scum that forms. 

 Mount a bit of the scum and observe it under a compound microscope. 

 Describe the amoeba as to shape and size. 



