138 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



with black paper, except for a vertical slit one-half inch wide along one side of 

 the jar. Place both. jars side by side in a light place, with the slit toward the 

 light. After twenty-four hours remove paper and note quickly the distribution 

 of the snails in both jars. The one jar which is not covered serves as a control, 

 that is, it gives us the distribution of snails on the basis of chance. If the. snails 

 in the other jar, which is the experiment, are not so distributed, then we may 

 conclude that light, the one factor which is different in the two cases, must be 

 responsible for the different distribution of the snails in the two jars. This is 

 the way in which every experiment must be conducted to be convincing. What 

 is the reaction of snails to light? Do they go to the bright light, i.e., are they 

 found collected on the slit, or do they remain in the dark, or are they to be found 

 in weak light? How would this affect their distribution in ponds? The condi- 

 tion which an animal selects when put in a gradient of a particular factor is 

 called its optimum. It could similarly be determined which temperature is 

 optimum for the snails by putting them in a trough of water which was warm 

 at one end and very cold at the other; and any other factor can be determined 

 in the same way. 



7. Growth and composition of the shell. Observe the parallel lines upon the 

 shell. They are lines of growth. The shell is secreted by the mantle at its 

 margin and thus grows continually larger. Do small snails have as many spiral 

 turns as large ones? The distance between successive lines of growth records 

 the rate of growth. Can you find places on the shell where the lines of growth 

 are so close together as to make one deep mark on the shell? How do you explain 

 this? Could you determine in this way how old a snail is? Put an empty shell 

 in hydrochloric acid and observe what happens. What does this indicate as to 

 the composition of the shell? Is the shell entirely soluble in acid? What part 

 is insoluble and why? Does this explain why the shells do not dissolve in the 

 water in which the animal lives? Pond water often becomes acid through the 

 decay of dead organisms in it and the carbonic acid gas produced by the living 

 organisms. Cut out small pieces of shell in living snails, replace in the aquarium 

 and observe whether regeneration occurs. 



8. Reproduction. Egg masses of snails will be commonly found in jars 

 where snails are kept. Where are they placed by the snails? Why should the 

 snail not simply drop them to the bottom of the pond in which it lives? Observe 

 the process of development by removing eggs from time to time and studying 

 under the microscope. In what condition are the young snails when they emerge? 

 Are they completely formed? 



