MOLLUSCA 123 



Order 2. Aplacophora. 



Body elongated, covered by a mantle. Adult 

 without shell but with spicules. No true foot. 

 Gills posterior. (Neomenia, Dondersia.) 

 Class 3. Gastropoda. 



Body unsymmetrical, usually covered by a 

 spiral shell. Foot usually flattened and 

 adapted for creeping. Radula usually present. 

 Subclass 1. Streptoneura. 



Nervous system twisted into the form of a 

 figure 8. Sexes separate. 



Order 1. Aspidobranchia. 



Nervous system not concentrated. Gills usu- 

 ally present and paired. Auricles paired. 

 (Acmaea, Patella, Haliotus.) 



Order 2. Pectinibranchia. 



Nervous system somewhat concentrated. 

 Single gill. Single auricle. (Buccinum, Busy- 

 con, Crepidula.) 

 Subclass 2. Euthyneura. 



Nervous system not twisted into the form of a 

 figure 8. Hermaphroditic. 



Order 1. Opisthobranchia. 



Aquatic respiration. Shell when present 

 rather delicate. (Bulla, Aeolis.) 



Order 2. Pulmonata. 



Air breathers. Live on land or in fresh water. 

 Aperture to mantle cavity narrow and con- 

 tractile. (Limax, Limnaea, Helix.) 

 Class 4. Scaphopoda. 



Bilaterally symmetrical. Shell tubular, elon- 

 gated dorsoventrally and open at both ends. 

 Foot conical. (Dentalium.) 

 Class 5. Cephalopoda. 



Bilaterally symmetrical. Shell chambered or 

 reduced and internal. Distinct head with 

 arms bearing suckers. 

 Subclass 1. Dibranchiata. 



Arms forming a circlet around the mouth. 

 Funnel a complete tube. Shell usually inter- 

 nal. Two gills. 



Order 1. Decapoda. 



Ten arms, two of which are elongated, suckers 

 on stalks. (Loligo, Sepia, Spirula.) 



