PHYLUM MOLLUSC A 201 



1. Order Placophora. — Shell dorsal, of eight transverse plates; 

 foot broad; gills lateral, simple, mantle folds. Chito n, Crypto- 

 chiton, Trachydermon, Amicula, Chaetopleura, Lepidochitona, 

 Ischnochiton. 



2. Order Aplacophora. — ^Elongate; covered bj^ a mantle; spicules 

 instead of shell; gills posterior. Neotnenia, Dondersia, Chaeto- 

 derma. 



n. Class Acephala. — No head; shell of two valves; aquatic. 



1. Order Protobranchia. — One pair plumelike gills; Yoldia, 

 Nucidn, Lcda, Solenomya. 



2. Order Filibranchia. — Gills platelike, filaments V-shaped; 

 anterior adductor reduced. Mytilus . Modiola, Area, Anomia, 

 Trigonia. 



3. Order Pseudolamellibranchia. — Gills platelike, in vertical 

 folds; but one adductor. Pccicn, Ostrea, Meleagrina. 



4. Order Eulamellibranchia. — Gill filaments grown together to 

 form a continuous sheet; adductors approximately equal. Mya, 

 Venus, Cardiitni, Ensatella, Teredo, Unto, Anodonta, Lampsilis, 

 Qiiadrula, Sphaerium, Pisidium, Pholas, Barnea. 



5. Order Septibranchia. — Gills a horizontal partition in mantle 

 cavity. Silenia, Cuspidaria. 



III. Class Scaphopoda. — Shell tubular; open at each end. Dentalium, 



Cadulus. 



rV. Class Gastropoda. — Shell single, usually spiral; radula. 



a. Subclass Prosobranchia. — Nervous system twisted in figiire 

 eight; sexes separate. 



1. Order Aspidobranchia. — Nervous system little concentrated; 

 one or two gills; two auricles. Haliotus, Trochus, Fissurelln, 

 Patella, Acmaea. 



2. Order Ctenobranchia. — Usually one gill; one auricle. Cari- 

 naria, Atlanta, Littorina, Crepidula, Natica, Strornbus, Pleurocera, 

 Campeloma, Murex, Terebra, Buccinum, ,Busycon. 



b. Subclass Euthyneura. — Nervous system not twisted; hermaphro- 

 ditic. 



1. Order Opisthobranchia. — Gills. Bulla, Aeolis, Navanax, Tethys, 

 Triopha. 



2. Order Pulmonata. — Lungs. Physa, Planorbis, Lymnaea, Fer- 

 rissia, Limax, Arion, Agriolimax, Polygyra, Helix, Pupa, Gastro- 

 copta. 



V. Class Cephalopoda. — Distinct head with arms bearing suckers; shell 

 frequently reduced, internal. 



a. Subclass Tetrabranchia. — Four gills; chambered shell. Nautilus;. 



b. Subclass Dibranchia. — Two gills; arms in circle around mouth; 

 tubular funnel; shell internal. 



1. Order Decapoda. — Ten arms, two of which are tentaciilar; 

 suckers stalked. Loligo, Sepia, Spirula, Rossia. 



2. Order Octopoda. — Eight arms; suckers sessile. Argonaula, 

 Polypus. 



