222 



HOW DO WE CLASSIFY ANIMALS? 



material forming the micronuclei of each cell pass over and unite 

 with material forming the nuclei of the opposite cell. After this 

 mutual exchange of nuclear material the cells separate. It is be- 

 lieved that this stage of reproduction, as in the plants, is a sexual 

 stage. 



Practical Exercise 1. Compare an amoeba and Paramecium as to size, 

 shape, method of locomotion, method of taking in food, digesting food, excret- 

 ing waste, and reproducing. 



The cell as a unit. In the daily life of a one-celled animal we 

 find the single cell performing all the vital activities which we 

 shall later find that the many-celled animal is able to perform. 

 In the amoeba no definite parts of the cell appear to be set off to 

 perform certain functions ; but any part of the cell can take in 

 food, can absorb oxygen, can change the food into protoplasm, and 

 excrete the waste material. The single cell is, in fact, an organism 

 able to carry on the business of living as effectually as a very 

 complex animal. 



Practical Exercise 2. Draw a cell and label all its parts. Give the use of 

 each part. How do cells move about? What do we mean by conjugation? 

 Why is the cell called a "unit of structure"? Why is a cell called an 

 organism ? 



T^njjopocCcc 



Amoeba 



Infusoria 



VortiCelliX 



,wgrl&Vidc 



Plcc^rraootitcm 



The principal classes of Protozoa, examples of which we may have seen or read about, are — 



Class I. Rhizopoda (root-footed). Having no fixed form, with pseudopodia. Either naked 

 as Amoeba or building limy (Foraminifera) or glasslike skeletons (Radiolaria) . 



Class II. Mastigoph'ora. They move by means of long whiplash threads of protoplasm, called 

 flagella. Examples are Euglena and Monosiga. 



Class III. Infuso'ria (in infusions). Usually active ciliated Protozoa. Examples, Parame- 

 cium, Vorticella. 



Class IV. Sporozo'a (spore animals). Parasitic and usually non-active. Example, Plas- 

 modium malariae. 



