EXERCISE ON ECOLOGY 137 



in the aquarium? What would they get besides plant tissue by scraping the 

 surface of vegetation? Scrape the surfaces of various aquatic plants and examine 

 the scrapings with a microscope. What do you see (-4)? Would you expect 

 to find snails in bodies of water where vegetation is scanty or is composed of 

 harsh, tough plants? This gives us a clue as to where to look for pond snails 

 and as to one of the factors which limit their distribution. Examine a slide of 

 the radula and note the teeth upon its surface. 



4. Respiration. Watch the snails for some time and observe that they 

 eventually come to the surface and assume a characteristic attitude. (How 

 can a heavy animal like a snail keep on the surface?) Observe between the head 

 and the edge of the shell, which is covered by the mantle, a small conical pro- 

 jection. This is the respiratory or pulmonary sac. Observe that the tip of the 

 sac is thrust above the surface film, and is then opened to the air. The animal 

 remains in this attitude a little while taking air into the sac. It then closes the 

 sac and descends. The common pond snails therefore breathe air, but there are 

 some fresh-water snails which breathe by means of gills and do not have to come 

 to the surface. By changing the amount of air in the pulmonary sac, snails 

 can rise and sink in the water. 



What stimulus do you suppose drives the snail to the surface for air? What 

 factors would determine how often it would need to come to the surface? Try 

 difference in rate of taking air between snails kept at ordinary temperatures and 

 those in ice water. How does the snail know in which direction to go for air? 

 To determine this, take a wide-mouthed bottle, put some snails in it, fill it com- 

 pletely with water, and cork tightly so that no air bubbles are included. Turn 

 it upside down and watch in which direction the snails go to seek air. Does this 

 answer the question? 



How long will snails live without access to oxygen? Take three bottles of 

 equal size, place an equal number of snails in each and some vegetation for food. 

 Fill two bottles completely with water and stopper tightly. Place one in the ice 

 box, leave the other at room temperature. Fill the third bottle partly with 

 water and leave open in the room. How long do the snails survive in the stop- 

 pered bottles? Does temperature make a difference? Why? How would this 

 apply to the living conditions of snails in winter, when the ponds are covered with 

 ice and they cannot get to the surface? 



5. Desiccation experiment. Take a glass jar, put about an inch of water 

 in it, and several snails. Let stand until the water is completely dried up. What 

 do the snails do? Can they carry on activities in the absence of water? Pick 

 one up and examine. What do you find across the mouth of the shell? Are 

 the snails dead? Put some water in the jar and note results. How does this 

 apply to the possibility of ponds drying up in hot weather? 



6. Reaction to light. Prepare two jars exactly alike, filled with water and 

 containing several snails and some vegetation. Cover one of them completely 



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