136 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



their environment? How does it change in the various seasons of the year? 

 During the day? Is the top water of a pond different from the bottom as to 

 light, temperature, content of oxygen, carbon dioxide, etc.? What chemical sub- 

 stances do you think are present in pond water? Do the snails occur where 

 there is vegetation? How does vegetation change the water chemically? How 

 would it affect the penetration of sunlight into the pond? What is found on the 

 bottom of the pond and what role does vegetation play in its production? 



2. Structures of the pond snail which are related to the environment. How 

 does the animal move about? The broad flat surface on which it crawls is the 

 ventral surface of the body and is called the foot. Is the movement of the foot 

 a muscular one or not? Can you see muscular waves passing along it? Con- 

 sidering the method of locomotion of the pond snail where would they occur 

 in a pond? Can they swim free in the water? How would they reach the surface 

 of the water? Can the animal go up and down in the water, without climbing 

 on objects? If you see it doing so it is traveling on a mucus thread secreted by 

 the foot. Remove a pond snail and throw it forcibly back into the water. Does 

 it sink? Does it let out bubbles of air? The pond snails do in fact possess a 

 chamber filled with air. By changing the amount of air in this chamber, could 

 they rise and sink in the water at will? 



The rounded projection in front of the foot is the head. It bears a pair of 

 tentacles at the bases of which is a pair of eyes. Touch the tentacles. What 

 do you think is their function? On a snail which is crawling along the sides of 

 the vessel so that you can see the underside, observe that a fold of the body, the 

 mantle, is fastened to the inside of the shell. In the center of the ventral side of 

 the head is the mouth opening. 



Pick up a snail out of the water. How far can it withdraw into the shell? 

 What is the purpose of this reaction? What is therefore the advantage of the 

 shell? What disadvantages does its possession entail? What does a snail do 

 when it is violently disturbed? Is this reaction ample to protect it from enemies? 



Observe the mud in the bottom of the jars where snails are kept, or else put 

 some snails in a jar of water with mud in the bottom. Note the trails left by 

 the snails as they crawl. Stir up a trail and observe that the particles of dirt 

 stick together. How does the snail accomplish this? Is this of advantage in 

 crawling over soft mud or slippery surfaces? 



3. Food taking. As the animal crawls along on the side of the aquarium 

 observe a wedge-shaped object protruding from the mouth at regular intervals. 

 It is the radula. The radula is a horny ribbon covered with teeth, used by the 

 snail for feeding. It is worked by a complicated muscular apparatus back and 

 forth over the end of a hard radular cartilage, like a rope over a pulley, and thus 

 exerts a rasping action on the food, reducing it to minute bits which are then 

 sucked into the esophagus. What does a snail get by rasping the glass of the 

 aquarium? Do you see evidence that the snails have been feeding on the plants 



