CLASSES, ORDERS, AND SUB-ORDERS. 83 



The Amphineura have been divided into two sub-orders Poly- 

 placophora and Aplacophora of which the latter consists entirely 

 of vermiform species not represented in Britain. 



The Prosobranchiata are divided into two sub-orders : 



1. Diotocardia heart usually with 2 auricles; gills 2, bi- 



pectinate. 



2. Monotocardia heart with i auricle ; gill i, monopectinate. 



The Opisthobranchiata have been further divided into four 

 sub -orders 



1. Tectibranchiata right gill present, shell enveloped in folds 



of mantle and foot. 



2. Pteropoda foot modified into fins, animal externally sym- 



metrical and internally asymmetrical, with or without 

 mantle fold or shell. 



3. Ascoglossa gills both absent, shell absent; old teeth not 



lost but preserved in a special askos or sac. 



4. Nudibranchiata gills modified or absent, shell absent, 



mantle fold rudimentary. 



The Pulmonata have been divided into two sub-orders : 



1. Basommatophora eyes at base of tentacles7~~ 



2. Stylommatophora eyes at tip of tentacles. 



The Scaphopoda have a plain tubular shell, occasionally tumid, 

 but generally pointed and slightly curved, as if it were a model 

 elephant's tusk, and they form an interesting but small class 

 containing only three genera. 



The Pelecypoda the bivalves, conchifera, or lainellibrarichiata, 

 as they used to be called are divided into five orders : 



1. Protobranchiata gill filaments divergent. 



2. Filibranchiata gill filaments parallel. 



3. Pseudolamellibranchiata gill filaments loosely connected, 



anterior adductor muscle aborted, so that they have but one 

 muscle instead of two like the others. 



4. Eulamellibranchiata gill filaments bound into plates, mantle 



edges united at one or more places. 



5. Septibranchiata gills modified into a muscular septum with 



symmetrical orifices, mantle edge united in three places. 



