PHYLUM MOLLUSCA 



257 



Order 4. Pseudolamellibranchiata 



Marine species; gills plaited into vertical folds; shell fre- 

 quently inequivalve. The oyster (Ostrea) and Pecten illus- 

 trate this order. 



ischnochiton 



Dental ium 



Loliqo brevipennis Polypus bimaculatus (Octopus ) 



Fig. 146. — Repre-sentatives of three classes of mollusks. Class Amphineura, 

 Ischnochiton; Class Scaphopoda, Dentalium ; Class Cephalopoda, Loligo brevipennis 

 (squid) and Polypus bimaculatus (octopus). 



Class III. Gastropoda 



Includes the snails and slugs. Approximately fifty-five thousand 

 species have been discovered and described. Shell, if present, uni- 

 valve. 



