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TEXTBOOK OF ZOOLOGY 



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

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

 particularly in the sheep of the Southwest. 



Since shells are easily fossilized they serve as excellent guides to 

 the geologists in determining the type of rock formation and relative 

 age of the strata, 



CLASSIFICATION 



Classification of this phylum is based on the nature of the foot, 

 and respiratory organs; shape and structure of the shell; arrange- 

 ment and structure of the nervous and reproductive systems. 



Class I. Amphineura 



Includes the Chitons, which are found abundantly on rocks between 

 tide marks along the Atlantic and Pacific Coasts. This class ap- 

 pears to be the most primitive in the phylum, and its members 

 have departed least from the ancestral condition. Bilaterally 

 symmetrical body; tentaculess head, eyes absent; shell, if present, 

 consists of eight overlapping plates. Most species have a flattened 

 foot but other species are slender and wormlike Ischnochiton con- 

 spicuus. 



Class II. Pelecypoda 



Includes the bivalve moUusks, such as the oysters, clams, scallops, 

 and cockles. More than ten thousand species have been described, 

 of which approximately four-fifths live in the ocean. Division of 

 the class into orders is based on giU characters. 

 Order 1. Protobranchiata 



Marine species; gills consist of short, flattened leaflets; dis- 

 tribution along the Atlantic and Pacific Coasts. 

 Order 2. Filibranchiata 



Marine species; gills composed of long filaments which hang 

 down into the mantle cavity. The edible scallops and the sea 

 mussel, Mytilus, exemplify this order. 



Order 3. Eulamellibranchiata 



Fresh-water and marine species; with two platelike gills 

 which hang down into the mantle cavity on each side of the 

 foot. 



Family 1. Unionidae 



Fresh-water clams or mussels; shell large or relatively 

 large; valves equal and umbo anterior to center. 



Family 2u Sphaeriidae 



Fresh-water species. Shell small; umbo median or pos- 

 terior to middle of shell. 



