The class of molluscs that contains the most species is the class _______ . A: Aplacophora (无板类) B: Cephalopoda C: Gastropoda D: Polyplacophora (多板纲) E: Bivalia

下列软体动物中没有开放式循环系统的是 ( )。 A: Bivalvia (双壳纲) B: Gastropoda (腹足纲) C: Cephalopoda (头足纲) D: Polyplacophora (多板纲)