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. 188), 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. Anodonta and the Uniones are similar except for 

 minor details. 



