POND SNAILS 27 



6. Observe the method of feeding by looking through the 

 side of the aquarium at the ventral side of the animal. Can you 

 determine the nature of the food? Notes required. 



/. When snails are not disturbed in what part of the 

 aquarium do they accumulate? Notes required. 



8. Describe the method of crawling. Observe a snail crawl- 

 ing on the lower side of the surface film of water. The lateral 

 stretch or pull of surface tension supports the weight, just as an 

 oiled needle may float upon the upper surface of the film. Mucus 

 threads may usually be demonstrated when the animal is crawling 

 upon the surface film by drawing a dissecting needle across the 

 path of the animal just a short distance behind the posterior end 

 of the body. Such threads are frequently found extending be- 

 tween the bottom of the aquarium and the surface film thereby 

 providing a path along which the snails crawl back and forth. 



9. Have you observed other animals moving upon the sur- 

 face film? If so, what kinds? Notes required. 



10. Describe the masses of snail eggs. Examine them with 

 the microscope and describe their general appearance. 



n. The growth of the shell takes place in what direction? 

 Can you find evidence of periods of growth and rest? Notes 

 required. 



12. Observe carefully the lung opening, and determine by 

 your watch how frequently and under what conditions air is 

 taken in. RECORD IN YOUR NOTES six trials. What is the average? 



REFERENCES 



Walter, H. E., 1906. The Behavior of the Pond Snail. Lymnaeus 

 eleodes Say. Cold Spring Harbor Monographs VI. 



Davvson, J., 1911. The Biology of Physa. Behavior Monographs. 

 Vol. I, No. 4. 



