ROLL CXLL 97 



PHYLUM XI — MOLLUSCA — Snails, clams, and oysters. 



Characteristics : Unsegmented, bilatorally synunotiical, triijloblastic ani- 

 mals bearing a shell, muscular foot, and mantle; four main pairs of nerv- 

 ous ganglia ; 70,000 species. 



Class I — Amphineura — Chitons (Chaetopleura, Ischnochiton) . 



Characteristics: Bilaterally symmetrical; shell typically composed of 

 eight transverse calcareous plates with many pairs of gill filaments. 



Class II — Gastropoda — Snails, slugs, whelks {Umax, Physa, Helix, Lymnaea, 

 Campelotna, Busy con). 

 Characteristics : Asymmetrical animals with well-developed head ; spi- 

 rally-coiled shell. 



Class III — Scaphopoda — Elephant's-tusk shells (Dentalium, Siphonodenta- 

 lium). 

 Characteristics: Both shell and mantle tubular; protrusible foot ; rudi- 

 mentary head. 



Class IV — Pelecypoda — Clams, mussels, oysters, and scallops {Ensis, Ano- 

 donta, Venus, Teredo, Ostrea, Pecten). 

 Characteristics: Usually bivalved shells with two-lobed mantle; no 

 head ; body laterally compressed ; bilaterally symmetrical. 



Class V — Cephalopoda — Squids, cuttlefishes, octopus, nautilus (Loligo, 

 Polypus, Dosidieus). 

 Characteristics: Bilaterally symmetrical; with foot divided into siphon 

 and arms provided with suckers; well-developed nervous system con- 

 centrated in head; mouth possesses strong jaws. 



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