PHYSIOLOGY 



265 



Movements of Naked Protoplasts and Single Cells 



The creeping movements of naked protoplasts, such as are shown 

 by an amoeba or plasmodium, in the protrusion, from one or more 

 sides, of protuberances which ultimately draw after them the whole 

 protoplasmic body, or are themselves again drawn in (Fig. 219), are 

 distinguished as AMOEBOID MOVEMENTS. These movements resemble, 

 externally, the motion of a drop of some viscid fluid on a surface to 

 which it does not adhere, and are chiefly due to surface tension, 

 which the protoplasm can at different points increase or diminish, by 

 means of its quality of irritability. By means of irregular changes 

 of surface-tension similar amoeboid move- 

 ments are also exhibited by drops of life- 

 less fluids, such as drops of oil in soap 

 solution ; or drops of mercury in 20 

 per cent solution of potassium nitrate in 

 contact with crystals of potassium bi- 

 chromate. 



In the SWIMMING MOVEMENTS BY 

 MEANS OF CILIA, on the contrary, the 

 whole protoplasmic body is not involved, 

 but it possesses special organs of motion 

 in the form of whip-like FLAGELLA or 

 CILIA. These may be one, two, four, or 

 more in number, and arranged in various 

 ways (Figs. 97, 98). They move very 

 rapidly in the water and impart con- 

 siderable velocity to the protoplast, often 

 giving it at the same time a rotary move- 

 ment. The minute swarm -spores of 

 Fuligo varians traverse 1 mm. (sixty 

 times their own length) in a second, those of Ulw, O'lo mm., while 

 others move more slowly. The Vibrio of Cholera, one of the most 

 rapidly moving bacilli, takes 22 seconds to traverse a millimetre. 

 The ciliary movement is so regulated as to propel the protoplast in 

 a definite direction. 



Gravity and light, certain substances in solution, and mechanical hindrances 

 are the principal influences which regulate the movements of free-swimming proto- 

 plasmic bodies and cells. The direction of the movements of the swarm-spores of 

 Algae are chiefly determined by the light. So long as they remain in darkness 

 they move through the water in all directions ; but as soon as they are illuminated 

 from one side only, a definite direction in their movements is perceptible. They 

 move either straight towards the light or turn directly away from its source. 

 Their retrogressive movements from the light occur either in case of too intense 

 illumination, or at a certain age, or through some unknown disturbing irritation. 

 The advantage of such HELIOTACTIC MOVEMENTS (phototactic) is at once apparent 



FIG. 211>. AiiKtboid movement. The 

 arrows indicate the direction and 

 energy of the movement ; the 

 crosses, the points at rest. At the 

 time being the principal movement 

 is from H to V, but at any moment 

 it may be towards R or L, and so 

 changei the direction of the course 

 taken by the amoeba. 



