324 MOLLUSCA [CH. 



Order 1. Aspidobranchiata. 



Usually two kidneys, two auricles, two ctenidia. The axis 

 of the ctenidium free and both rows of plates present. 

 Ex. Haliotis, Patella. 



Order 2. Pectinibranchiata. 



One kidney, one auricle, one ctenidium of which the axis is 

 adherent to the mantle and only provided with one row of 

 plates. ' 



Ex. Buccinumj the Whelk. 



Division II. EUTHYNEURA. 



The visceral loop is untwisted and the gill is posterior to the 

 heart. 



Order 1. Opisthobranchiata. 



Marine Euthyneura with open mantle cavity in which a 

 ctenidium is often present. 

 Ex. Aplysia. 



Order 2. Pulmonata. 



Land and fresh-water Euthyneura breathing air, having the 

 mantle cavity converted into a lung and the ctenidium aborted. 

 Ex. Helix. 



Class II. LAMELLIBRANCHIATA (PELECYPODA). 



Mollusca with a shell composed of two valves united by a hinge, 

 and a mantle of two lobes. The foot is usually wedge-shaped and 

 the plates of the ctenidia are fused to form gill-plates. No buccal 

 mass. Pleural ganglia fused with the cerebral. 



Sub-class I. PROTOBRANCHIATA. 



Small Lamellibranchiata with a simple ctenidium like that of 

 Gastropoda, and large labial palps. 

 Ex. Nucula. 



Sub-claSS II. FlLIBRANCHIATA. 



Lamellibranchiata in which the filaments of the ctenidium are 

 loosely united with one another and their bent-up ends are not 

 united to the mantle. 

 Ex. Mytilus* 





