214 INVERTEBRATE ANIMALS. 
TABULAR VIEW OF THE MAIN DIVISIONS OF THE MOLLUSCA. 
Division A, MOLLUSCOIDA.— Nervous system consisting of a 
single ganglion, or of a principal pair with accessory ganglia; no 
distinct organ of the circulation, or an imperfect heart. 
Crass I. Polyzoa.—Ex, Sea-mats (Flustra). 
Cuass Il. Zunitcata,—LEx. Sea-squirts (Ascidia), 
Crass III. Brachiopoda—Ex. Lamp-shells (Terebratula). 
Division B, MoLLusca PRopER.—Nervous system consisting of 
three principal pairs of ganglia ; heart well developed, composed of at 
least two chambers, 
Cuiass I, Lamellibranchiata.—Ex. Oyster (Ostrea), Mussel (Mytilus), 
Scallop (Pecten). 
Crass Il. Gasteropoda.—Ex, Whelks (Buccinum), Periwinkles (Lit- 
torina), Limpets (Patella), Sea-lemons (Doris), Land-snails (Helix). 
Cuass III. Preropoda.—x, Cleodora. 
Ciass IV. Cephalopoda. 
Order a. Dibranchiata—-Ex, Calamary (Loligo), Poulpe (Octopus), 
Paper Nautilus (Argonauta). 
Order 6. Tetrabranchiata.—Ex. Nautilus, Ammonites, Orthoceras. 
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