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; pons, 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; pons, 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. 



1. 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. 



