MOLLUSKS 



265 



oxygen from the water above. The clam industries of the eastern 

 coast aggregate over $1,000,000 a year. 



Scallop. — The scallop, another highly esteemed mollusk, forms 

 an important fishery. The scallop rests on one valve on the 

 bottom in shallow water and if 

 disturbed swims away by clapping 

 the valves rapidly together. The 

 single adductor muscle is eaten, 

 whereas in the clam the soft parts 

 of the body are used as food. 



i''ulgur, a univalve rnollusk common in 

 Long Island Sound, which does much 

 harm by boring into the shells of edible 

 moUusks. 



A Univalve Shell. — Any large uni- 

 valve shell, as Fulgur, may be used in 

 the laboratory.^ The shell, which is 

 one piece, is called a univalve. Notice 

 the spiral arrangement of the shell. 

 How- many turns does it make? The 

 lines of growth run parallel, as in the clam, to the edge of the shell. Hold 

 the openmg toward you. The opening is known as the aperture. When 

 the animal is alive, part of the body is protruded through this. 

 Draw the shell twice natural size, showing all above parts. 



Living Snail. — (Use the pond snail 

 Limnea or Physa.) Watch the move- 

 ment of living snails in the aquarium. 

 The large fleshy mass which protrudes 

 from the shell is called the foot. Try 

 to decide how locomotion takes place 

 as the animal moves along the side 

 of the acfuarium. 



Watch the animal as it feeds. What 

 kind of food docs this snail eat? 

 Notice the position of the mouth. Is 

 there any distinct head? These ani- 

 mals are called gastropodss (stomach- 

 footed). Do you see any reason for 

 this name? P'ind the two tentacles 

 or horns. Touch them with a pencil. 

 What happens? Look for the dark 

 eye-spots at the base of th.e tentacles. 

 Make any experiments you can to see 

 if the snail can distinguish between 

 light and darkness. (Cover part of 

 the dish and Ica^e it for some minutes 

 undisturbed.) Do you find any other 

 structures protruding from the edge of the foot ? Two siphons, one for tak- 

 ing in water, the other for sending it out, may be found. 



Gastropods. — Snails, whelks, slugs, and the like arc called gastropods 

 because the foot occupies so much space that most of the organs of the 



1 See Hunter and Valentine, Manual, pages 143 antl 145. 



Strophia.s from different localities at An- 

 dros and New Providence, Bahamas. 

 From photograph loaned by the Ameri- 

 can Museum of Natural History. 



