PHYLUM MOLLUSCA 243 



The mollusks are divided into five classes according to their 

 symmetry and the characters of the foot, shell, mantle, gills, and 

 nervous system. 



Definition. Phylum MOLLUSCA. CLAMS, SNAILS, SQUIDS, 

 OCTOPI. Triploblastic, bilaterally symmetrical animals; anus 

 and ccelom present; no segmentation; shell usually present; 

 the characteristic organ is a ventral muscular foot. 



CLASS I. AMPHINEURA (Gr. amphi, on both sides; neuron, 

 a nerve), the chitones (Fig. 179), with bilateral symmetry, often 

 a shell of eight transverse calcareous plates, and many pairs of 

 gill filaments; 



CLASS II. GASTROPODA (Gr. gaster, the belly; pous, a foot), 

 the snails (Fig. 180), slugs (Fig. 184), whelks, etc., with a 

 symmetry and usually a spirally coiled shell; 



CLASS III. SCAPHOPODA (Gr. skaphe, a boot; pous, a foot), the 

 elephants'-tusk shells (Fig. 1 88), with tubular shell and mantle; 



CLASS IV. PELECYPODA (Gr. pelekos, hatchet; pous, a foot), 

 the clams, mussels (Fig. 174), oysters, and scallops, usually with 

 bilateral symmetry, a shell of two valves, and a mantle of two 

 lobes; 



CLASS V. CEPHALOPODA (Gr. kephale, head; pous, a foot), 

 the squids (Fig. 191), cuttlefishes, octopods (Fig. 196), and 

 nautili (Fig. 194), with bilateral symmetry, a foot divided into 

 arms provided with suckers, and a well-developed nervous system 

 concentrated in the head. 



i. THE PEARLY FRESH-WATER MUSSEL ANODONTA AND 

 THE UNIONES 



The fresh-water mussel is a mollusk belonging, together with 

 the oyster, the long-neck clam, the scallop, and other similar 

 animals, to the class PELECYPODA. Mussels inhabit the lakes 

 and streams of this country wherever the water contains car- 

 bonate of lime and does not entirely evaporate during any part 

 of the year. Anodontq and the Uniones are similar except for 

 minor details. 



