278 ESSENTIALS OF ZOOLOGY 



ment are spoken of as gregaloid. The difference between these 

 colonial Protozoa and simple Metazoa is a difference in the relation- 

 ship of single cells to the group as a whole and not a simple difference 

 in numbers of cells. In the colony each cell is an independent or 

 almost independent individual so far as the functions of living are 

 concerned. In metazoa, the cells are specialized and distributed, so 

 that certain groups carry out a definite portion of the entire metab- 

 olism. They are classified into general body (somatic) cells and 

 reproductive (germ) cells. Certain of the spheroid colonies, such 

 as Volvox, have a rather striking resemblance to the blastula stage 

 in the early development of metazoans. Both are spherical organi- 

 zations 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 currents, 

 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 



