430 EVOLUTION OF ANIMALS Part V 



protozoan of man, is the cause of amebic dysentery in temperate as well as 

 tropical climates and is estimated to inhabit 10 per cent of the world's popula- 

 tion. All protozoan parasites of the blood and intestines live completely im- 

 mersed in fluid food (Fig. 21.3). 



Place of Protozoans in the Food Supply. Protozoans feed upon bacteria and 

 unicellular algae, mainly diatoms and desmids. They are important food, in 

 some places almost the sole food, of multitudes of minute animals, crustaceans, 

 rotifers, larval fishes, and in salt waters the ciliated swimming young of jelly 

 fishes and other invertebrates. This floating population (plankton) is the food 

 of larger animals, of medium-sized fishes that in their turn furnish food to still 

 larger ones. The bluefish and the cod would die in infancy if it were not for 

 the protozoans, and the bacteria and algae which support the protozoans. 



Locomotion. Protozoans move about by means of flagella, by the flowing of 

 protoplasm in pseudopodia, or by cilia. All of them have one or the other of 

 these structures through some period of their lives, except the sporozoans 



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 f 



Fig. 21.4. Swimming motions of Euglena. The blunt end containing the reddish 

 eye spot is forward. The flagellum lashes sidewise and backward, pushing the body 

 forward in a spiral path and turning it over as it goes. Euglena swims toward the 

 light except when too strong. (Data from Jennings.) 



which have no locomotor organs. The classes of Protozoa are arranged on the 

 basis of their ways of locomotion. 



Flagella. The flagellum is a whip-like extension from the cell, with a con- 

 tractile core. Its simplest motion is like that of a swimming eel or a snake that 

 glides through the grass, bending its body from side to side in one plane. In 

 most flagellates the flagellum moves in a spiral that turns the body obliquely, 

 at the same time rotating it as in Euglena (Fig. 21.4). 



Pseudopodia. The flowing of protoplasm is the most primitive means of 

 animal locomotion. It is caused by the changing states of protoplasm from 

 mobile watery plasmasol to the firmer plasmagel and vice versa (Fig. 2.11). 

 Such changes occur in response to those in the animal's surroundings and to 

 conditions within its body. A pseudopodium looks like a spreading spatter of 

 egg white. Its significance appears when the ameba moves in a definite direc- 

 tion, only after several small pseudopodia have been overcome by larger ones 

 (Fig. 21.12). 



Cilia. The ciliates are the fastest, most versatile swimmers of all protozoans. 



