4 ESSENTIALS OF ZOOLOGY 



It will be observed that the granules are in constant motion, 

 streaming in currents along the cell in the direction of movement, 

 and when a lobe of the ectoplasm is being protruded they 

 suddenly pour into it and distend it. These lobes of the cyto- 

 plasm are called pseudopodia, and are produced freely in the 

 water or in contact with a surface, so that in the latter case 

 a sort of creeping movement results amoeboid movement. 

 In contact with a surface the Amoeba is thus translated in a 

 given direction. Not merely the pseudopodia but the whole 

 Amoeba are rotated in the direction of progression. A point 

 on the upper surface is carried to the front end, and when it 

 turns over to reach the substratum it adheres and is stationary 

 until it is released by the passage of the rest of the cell over it. 

 Then it is gradually carried over the posterior end to reach the 

 upper surface again. The pseudopodia are by the movement 

 brought into an ineffective position and are retracted, becoming 

 crenulated in the process at the posterior end. The rolling 

 movements and the formation of pseudopodia can be imitated 

 exactly by drops of oil and even water in contact with fluids 

 with which they form a surface film. Like the Amoeba, also, 

 such can be made to change direction by suitable stimuli. 

 The movements of the Amoeba are thus apparently produced 

 by changes in the surface tension of the ectoplasm. But in 

 the Amoeba the ectoplasm is contracted and expanded in a 

 purposive manner, and pseudopodia are protruded without 

 contact with a surface. Their near allies the Foraminifera 

 produce long filamentous pseudopodia, extending far into the 

 water as sensitive feeding organs. If the Amoeba be floating 

 freely the contact of a pseudopodium with an object results 

 in the Amoeba being pulled to the surface by the pseudopodium, 

 which becomes sticky or thrombocytic, and this is followed by 

 amoeboid movement along the object. The cytoplasm thus 

 behaves as a colloid in a fluid state, and is limited externally 

 in contact with the water as a film in surface tension, and it is 

 occupied by vacuoles of watery fluid, also in a state of tension. 



The pseudopodium is able to secrete a sticky substance 

 which enables it to adhere to certain structures. This aids 

 the movement, and it also helps in the ingestion of food, although 

 experiment has shown that the food is not always fixed in this 



