1 6 HUMAN PHYSIOLOGY 



protozoa movements take place along some definite line, 

 and the reticulum is arranged more or less parallel to the 

 line of movement. Such contractile processes, from their 

 resemblance to muscles, have been termed myoids. In 

 other protozoa the pseudopodia may manifest a to-and-fro 

 rhythmic waving movement, which may cause the cell to 

 be moved along, or may cause the adjacent fluid to move 

 over the cell. Such mobile processes when permanent have 

 been called cilia. 



These movements are modified by the various STIMULI 

 which modify the activity of the chemical changes in the 

 protoplasm (p. 8). Thus cooling diminishes, and finally 

 stops them. Gentle heat increases them, but when a certain 

 temperature is reached, they are stopped. Drying and 

 various drugs, such as chloroform, quinine, &c., also arrest 

 the movements. 



Changes in the surroundings may cause either contraction 

 or expansion, may repel or attract. When a repelling or 

 attracting influence, a positive or negative stimulus, acts at 

 one side of the cell unilateral stimulation it may lead to 

 movement of the cell away from it or towards it. Movements 

 are produced by various chemical substances (chemiotaxis), 

 or by light (photo taxis) or by electricity (galvanotaxis). If 

 the action is towards the stimulus, it is said to be positive, 

 if away from it negative. 



Chemiotaxis is the attraction or repulsion produced by one- 

 sided application of chemical stimuli. This is well seen in 

 the plasmodial masses of aethalium septicum which grow on 

 tan. Oxygen and water both attract it towards them, and 

 exercise a positive chemiotaxis. It is also seen in the stream- 

 ing of the white cells of the blood to disintegrating tissues, 

 or to various micro-organisms which have to be destroyed to 

 prevent their poisoning the organism, and in the attraction 

 exercised by the ovum upon the male element in repro- 

 duction. 



Barotaxis is the effect of unilateral pressure or mechanical 

 stimulation. Many protozoa appear quite unable to leave 

 the solid substance e.g. the microscope slide with which 

 they are in contact, the unilateral pressure seeming to cause 

 a positive attraction in that direction. 



