392 ESSENTIALS OF ZOOLOGY 



shipworm, Teredo navalis, attacks the wood of ships and pilings, 

 making extensive excavations. Certain species of snails serve as the 

 intermediate host of parasitic flatworms or flukes. The liver fluke 

 (Fasciola hepatica) whose intermediate host is the small fresh-water 

 snail, Lymnaea bulimoides, causes the disease, liver rot in livestock, 

 particularly in the sheep. 



Since shells are easil}^ fossilized they serve as excellent guides to 

 the geologists in determining the type of rock formation and relative 

 age of the strata. 



References 



Baker, F. C. : The Fresh Water Mollusca of Wisconsin, Madison, 1928, Wisconsin 



Geol. & Nat. Hist. Survey. 

 Morris, Percy A.: A Field Guide to the Shells of Our Atlantic and Gulf Coasts, 



Boston, 1947, Houghton Mifflin Co. 

 Tryon, G. W., and Pilsbry, H. A.: Manual of Conchology, 1885 to date, Acad. 



Nat. History, Philadelphia. 

 Ward, H. B., and Whipple, Geo. C. : Fresb-Water Biology, New York, 1918, John 



Wiley & Sons, Inc. 



