MOLLUSCA. 113 



Phylum V. MOLLUSCA. 



Unsegmented animals, with a soft body, mantle and dorsal shell 

 (usually), muscular ventral foot, large digestive gland (liver), and open 

 vascular system. Enteroco3le represented by pericardial cavity, which 

 typically communicates with exterior by a pair of nephridia. Cerebral, 

 pleural, and pedal ganglia. Larva (Veliger) usually with a velum. 



Subphylum A. Lipocephala (Acephala). 



Head-region rudimentary and devoid of eyes. No odontophore. 



Class I. Lamellibranchiata. 



With bilobed mantle and shell (valves equal but unsymmetrical), 

 large gill-plates (in most); two pairs of labial palps; paired nephridia 

 (organs of Bojanus), auricles, digestive gland and gonads; sexes generally 

 separate. 



Order 1. Asiphonida, no siphons. With one adductor muscle : Pecten 

 Jacobseus, clam, Ostrea edulis, oyster, both hermaphrodite. With 

 two adductors : Meleagrina margaritifera, pearl-mussel. Mytilus 

 edulis, edible mussel. Lithodomus dactylus. Trigonia. Unio pic- 

 torum, Anodonta cygnsea, freshwater mussels (the free larva, "glochi- 

 dium," parasitic on fishes). 



Order 2. Siphonida, siphons ; two adductor muscles. Cardium edule, 

 cockle. Cyclas cornea in fresh water. Venus. Solen Aspergillum, 

 with tubular shell. Teredo navalis, ship-worm. 



