32 ZOOLOGY UIEECTIONS 



7. 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. 3Iucus 

 threads may usually be demonstrated when the animal is crawl- 

 ing 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 

 between 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. 



11. 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. 



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

 Vol. I, No. 4. 



