218 



HOW DO WE CLASSIFY ANIMALS? 



Amoeba. The simplest of all animals is the amoeba, a tiny cell, 

 which changes its shape as it moves about in the water. It has 

 no organs of locomotion, nor of sensation, yet it is aware of the 

 nearness of food. It has no mouth, but takes in food at any point 

 in its body. It is able to change this food into living matter, 

 - . ^ f° r it grows, and 



when it reaches a 

 certain size, divides 

 to form two amoebas. 

 Several theories 

 have been advanced 

 to account for its 

 locomotion: such as 

 the flowing of the 

 ^4-lL|ood. Vacuole protoplasm, a rolling 



motion of the cell, 

 and most recently 

 the belief that the 

 amoeba moves by a 

 series of body con- 

 tractions. 



Although but a 

 single cell, the 

 amoeba appears to 

 be aware of the ex- 

 istence of food when 

 it is near at hand. 

 Food may be taken 

 into the body at any point, the semifluid protoplasm surrounds 

 and takes in the bit of food, with a little water. Thus a food 

 vacuole is formed. Digestion takes place in the vacuole by means 

 of enzymes. As the food vacuole is circulated by the constant 

 streaming of the protoplasm within the cell, the digested nutrients 

 pass, by osmosis, from the vacuole into the protoplasm. Parts of 

 the food material that cannot be used, such as the shells or outer 

 coverings of tiny plants, are passed out of the food vacuole. Waste 

 products, other than carbon dioxide, resulting from oxidation col- 



pseucCopocCium 



.ectoplasm. 

 ^^pJ^v. - - encCoplasm. 



-JLJ.Contndctile/ 

 Vccotjeole^ 



nucleus 



State the use of each part that is labeled. 



