368 



THE MAINTENANCE OF THE INDIVIDUAL 



swim backwards, while in the former case it can only crawl forward. In 

 both cases the change in position is caused by the action of the current 

 on the flexor and extensor muscles, which in one case are contracted 

 and the other case extended, thus causing the animal to assume the po- 

 sitions mentioned. Experiments 

 such as these give rise to the theory 

 of tropisms, which is simply another 

 term for a series of responses of an 

 organism to the various factors of 

 its environment. Tropisms may 

 be briefly classified as phototro- 

 pisms, or responses to light ; geo- 

 tropisms, or responses to gravity ; 

 hydrotropisms, or responses to 

 water ; chcmotropisms, or responses 

 to chemical substances ; thermotro- 

 pisms, or responses to temperature 

 changes ; galvanotropisms, or re- 

 sponses to electricity; thigmotro- 

 pisms, or responses to contact ; 

 rheotropisms, or responses to water 

 currents ; and aneinotropisms, or 

 responses to air currents. 



A tropism is a kind of directional 

 urge. It represents a condition 

 within an organism, resulting from 

 the interaction between its struc- 

 ture (nervous) and the stimuli of the environment. Loeb explained 

 tropisms as specific irritabilities or sensitivities to stimuli at the 

 surface of the body, and in terms of body symmetry, since corre- 

 sponding parts on two sides of the body would show the same sen- 

 sitivities. Noncorresponding parts, according to this theory, would 

 show unequal sensitivities, resulting in directive movements. 



Loeb explained his famous example of the reversal of tropisms in a 

 caterpillar by showing that the caterpillar moves toward light when 

 hungry and is irresponsive to light when satisfied. The result is 

 most useful to a caterpillar, because as it leaves its nest when hun- 

 gry, it is near the surface of the ground and is drawn by light to the 

 tips of the branches where young edible leaves are sprouting, returning 

 to the lower branches when nonresponsive to light. 



Position taken by lejjs of shrimp 

 when current goes laterally through 

 animal, from left to right. (After Loeb 

 and Maxwell.) 



Which direction would the animal 

 be forced to take in movement .3 



