MOLLUSCA 



Unsegmented. Usually provided with a calcareous pro- 

 tecting shell and a ventral foot. 



Class 1. Pelecypoda. (Lamellibranchiata.) 



Bivalve shell. Gills adapted for gathering 

 food as well as for respiration. Foot usually 

 adapted for burrowing. No hard mouth parts. 



Order 1. Protobranchia. 



Gills composed of a series of transverse plates. 

 Foot apparently split at the end. Two adduc- 

 tor muscles, posterior frequently the smaller. 

 (Nucula, Yoldia.) 



Order 2. Filibranchia. 



Gills lamelliform. Filaments united by modi- 

 fied cilia. Anterior adductor muscle, fre- 

 quently greatly reduced. (Mytilus, Modio- 

 lus.) 



Order 3. Pseudolamellibranchia. 



Gills lamelliform. Interfilamentar junctions 

 usually not very extensive, may be either cili- 

 ary or vascular. Only one adductor muscle. 

 (Pecten, Ostrea.) 



Order 4. Eulamellibranchia. 



Gills lamelliform. Interfilamentar junctions 

 extensive and vascular. Adductor muscles of 

 nearly equal size. (Venus, Unio, Mya.) 



Order 5. Septibranchia. 



Gills reduced to a horizontal partition. Two 

 adductor muscles. Deep sea forms. (Silenia, 

 Cuspidaria.) 

 Class 2. Amphineura. 



Bilaterally symmetrical, elongated. Nervous 

 system not concentrated. Radula sometimes 

 present. Shell, when present, composed of 

 eight transverse pieces. 



Order 1. Placophora. 



Dorsal shell, composed of eight transverse 

 pieces. Foot broad. Gills simple, lateral. 

 (Chiton, Chaetopleura, Trachydermon.) 

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