': BIOLOGY OF PHYSA 61 



activity and darkness to have a quieting effect. These facts 

 may be shown by observations upon snails kept in the laborator}^ 

 by comparing them in the day time and at night when they 

 are placed in a well lighted jDOsition. It is noticed in the field 

 that the snails do not become very active until late morning, 

 reaching their maximum in the middle of the afternoon and then 

 continuing active during the early part of the evening. Tem- 

 perature and light both undoubtedly play an important part 

 in this behavior. An increase in the temperature of the water 

 always causes an increase in the activity of Physa even to the 

 thermal death point. 



Walter (1906) has shown by an experiment that Physa' s 

 movements are accelerated in higher temperatures, its rate 

 of movement being as high as an inch in 18 seconds in bright 

 sunlight and as low as an inch in 98 seconds in diffused light. 

 This fact is also brought out on cold mornings and evenings, 

 and during cold weather in spring and fall and in winter hiber- 

 nation. Spinning is at its maximum in the warmer months. 

 Even in the laboratory it does not go on at a uniform rate during 

 the year. After the snail has been kept in an aquarium for 

 several months, the general tonus of the animal is lowered and 

 its spinning is greatly lessened. 



3. Habitats affect spinning. It may be readily seen that the 

 habitat in which the snail is living affects the amount of spinning. 

 Physa living on stones in such swiftly flowing water as is found 

 in the creek-bed habitat and those living on Elodea submerged 

 under quite rapidly flowing water were never observed to spin 

 threads. It is evident that if they did so it would not take 

 long to deplete the habitats of their members, since they would 

 be swept away by the currents as soon as they become loosened 

 from the substratum. These snails, however, move about and 

 eat as do those in still water habitats. If placed in still water 

 they spin as readily as any of the snails of their species, yet 

 in situations where great danger attends the process, no spinning 

 occurs. It can be readily shown that the snail hugs the sub- 

 stratum more closely when the water is agitated than when 

 it is not, i. e., the animal exerts muscular energy at times when 

 needed to keep from being swept away. This may be demon- 

 strated by throwing a stream of water upon a moving sub- 

 merged snail ; it stops and hugs the substratum and if the water 



