90 MOLLUSCA. 



Foot broad. Gills simple, lateral. (Chiton, 

 Chsetopleura, Trachydermon.) 



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 distinct. 



Order 1. Aspidobranchia. 



Nervous system not concentrated. Gills usually 

 present and paired. Auricles paired. (Acmsea, 

 Patella, Haliotus.) 



Order 2. Pectinibranchia. 



Nervous system somewhat concentrated. Single 

 gill. Single auricle. (Sycotypus, Buccinum, 

 Crepidula.) 

 Subclass 2. Euthyneura. 



Nervous system not twisted into the form of a 

 figure 8. Sexes united. 



Order 1. Opisthobranchia. 



Aquatic respiration. Shell when present rather 

 delicate. (Bulla, ^Eolis.) 



Order 2. Pulmonata. 



Air-breathers. Live on land or in fresh water. 

 Aperture to mantle cavity narrow and contrac- 

 tile. (Limax, Limnasa, Helix.) 

 CLASS 4. Scaphopoda. 



Bilaterally symmetrical. Shell tubular, elon- 

 gated dorso-ventrally and open at both ends. 

 Foot conical. (Dentalium.) 

 CLASS 5. Cephalopoda. 



Bilaterally symmetrical. Shell chambered or re- 

 duced and internal. Distinct head with arms 

 bearing suckers. 

 Subclass 1. Dibranchiata. 



Arms forming a circlet around the mouth. Fun- 

 nel a complete tube. Shell usually internal. 

 Two gills. 



