MOLLUSCA, 



Unsegmented. Usually provided with a calcareous pro- 

 tecting shell and a ventral foot. 



Class 1. 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 adductor 

 muscles, posterior frequently the smaller. (Nu- 

 cula, Yoldia.) 



Order 2. Filibranchia. 



Gills lamelliform. Filaments united by modified 

 cilia. Anterior adductor muscle frequently 

 greatly reduced. (I\Iytilus, ^lodiola.) 



Order 3. Pseudo-lamellibranchia. 



Gills lamelliform. Inter-filamentar junctions 

 usually not very extensive, may be either ciliary 

 or vascular. Only one adductor muscle. (Pec- 

 ten, Ostrea.) 



Order 4. Eulamellibranchia. 



Gills lamelliform. Inter-filamentar junctions 

 extensive and vascular. Adductor muscles of 

 nearly equal size. (Venus, Unio, ]\Iya.) 



Order 5. Septibranchia. 



Gills reduced to a horizontal partition. Two ad- 

 ductor 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. 

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