PHYLUM PROTOZOA IN GENERAL 77 



distributed, so that certain groups carry out a definite portion of the 

 entire metabolism. They are classified into general body (somatic) 

 cells and reproductive (germ) cells. Certain of the spheroid proto- 

 zoan colonies, such as Volvox, have a rather striking resemblance to 

 the blastula stage in the early development of metazoans. Both are 

 spherical organizations of cells. 



Tropisms and Animal Reaction 



Organisms, whether plant or animal, of all degrees of complexity 

 respond to various kinds of stimuli. The important stimuli which 

 call out immediate or direct response by the animal are light, bodily 

 contact, chemical change, temperature, gravity, mechanical cur- 

 rents, and electric currents. The response to a stimulus may be 

 either positive or negative. Tropism, which means turning, refers 

 to the reaction of an organism to a stimulus. Taxis may also be used 

 here if the response involves the movement of the organism as a 

 whole. Tropisms are named with respect to the stimulating agent, 

 and the common ones usually recognized are : 



a. Phototropism, response to light 



b. Thigmotropism, response to contact 



c. Chemotropism, response to chemical changes 



d. Thermotropism, response to temperature 



e. Geotropism, response to gravity 



f. Rheotropism, response to mechanical currents 



g. Electrotropism or galvanotropism, response to electric currents 



If the animal is attracted to the source of the stimulation and turns 

 toward it, the response is said to be positive. If the organism is 

 repelled by the stimulus, the response is negative. It has not been 

 thoroughly determined why an animal responds to a specific stimulus 

 in a certain way. The minimum strength of stimulus which is neces- 

 sary to get a response is known as the threshold. The simpler ani- 

 mals under a given set of conditions respond to these stimuli in a 

 certain way not because of power of choice, but because they cannot 

 behave in any other way. The Protozoa are controlled in their be- 

 havior largely by tropisms. 



Economic Relations of Protozoa 



Man has not yet found a way or need to eat Protozoa directly as 

 food material, although he does draw on it indirectly by a food 



