226 



ELEMENTARY ZOOLOGY. 



in the ocean by man. Snails sometimes attack foliage 

 in such numbers as to work injury. 



A large portion of the limestone of the earth's crust 

 has been extracted by these animals from its solution 

 in sea-water and deposited at the bottom, when they die, 



FIG. 73. Pecten irradians, a scallop. From Binney's Gould. 



Questions on the Figure. Is this an internal or an external view? 

 Where is the urnbo? What is peculiar about the hinge in this 

 case? What is the significance of the lines nearly concentric with 

 the margin? Of the radial lines? 



as solid shells. Here it is cemented together by the action 

 of the water and finally compressed into the familiar and 

 useful stone. 



It is believed that the oyster is a frequent spreader of 

 certain germ diseases, as typhoid fever, owing to the fact 

 that many of the oyster fisheries are located on inlets of 

 the sea into which the sewage from great cities passes. 



