Class =AMPHINEURA= (/Bilaterally symmetrical mollusks, having shell composed of eight plates, or devoid of shell. Including chitons and their allies/) Order =POLYPLACOPHORA= (/Shell composed of eight plates/) Order =APLACOPHORA= (/Devoid of shell/) Class =GASTEROPODA= (/Unsymmetrical univalve mollusks, usually spirally coiled. Periwinkles, whelks, snails, etc./) Order =OPISTHOBRANCHIATA= (/Branchiæ, when present, behind the heart/) Suborder =TECTIBRANCHIATA= (/Branchiæ more or less covered by mantle; shell delicate and often rudimentary/) Suborder =NUDIBRANCHIATA= (/Devoid of shell and mantle, and without true branchiæ/) Order =PROSOBRANCHIATA= (/Branchiæ situated in front of the heart/) Suborder =DIATOCARDIA= (/Usually two auricles, two branchiæ, and two nephridia/) Suborder =MONOTOCARDIA= (/One auricle, one branchia, one nephridium/) Order =PULMONATA= (/Air-breathing mollusks/) Class =SCAPHOPODA= (/Mollusks without head, eyes, or heart, with trilobed foot and tubular shell open at both ends. The tooth-shells/) Class =PELECYPODA= (/Bilaterally symmetrical, headless, bivalved mollusks. Mussels, cockles, oysters, etc./) Order =PROTOBRANCHIATA= (/Gills form a single pair of plume-like organs, each with two rows of flattened gill-filaments/) Order =FILIBRANCHIATA= (/Two pairs of plate-like gills formed of V-shaped filaments/) Order =PSEUDOLAMELLIBRANCHIATA= (/Gills plaited so as to form vertical folds/) Order =EULAMELLIBRANCHIATA= (/Firm, basket-work gills, the filaments united by vascular connections/) Class =CEPHALOPODA= (/Mollusks of high organization, having a definitely formed head surrounded by tentacles; shell, when present, usually internal; shell, external in nautili. Cuttlefishes, squids, octopi, and nautili/) Subclass =Tetrabranchiata= Subclass =Dibranchiata= Order =OCTOPODA= Order =DECAPODA= [pg302] TERMS USED IN DESCRIBING MOLLUSKS ^Abductor muscles^: Muscles which move parts away from the axis. ^Adductor muscles^: Muscles which draw parts together; opposite of /abductor/. ^Aperture^: Opening of the spiral shell. ^Bilateral symmetry^: Having two equal sides, divided by a central axis. ^Branchiæ^: Gills, or breathing-organs; organs subservient to respiration through water. Same as /ctenidia/. ^Buccal mass^: The mouth-parts as a whole; the organ of prehension and mastication of food; the pharynx: present in all mollusks except lamellibranchs. ^Canal^: A narrow prolongation of the aperture of a spiral shell. ^Carinated^: Ridged as if keeled; having a keel.