CHAPTER XVI 



SNAILS AND MUSSELS 



MoUusca 



Clams and mussels, oysters, snails and slugs are all mol- 

 lusks; so are the less known chitons, the squids and the 

 octopus tribe of sea story fame. All these have soft unseg- 

 mented bodies, and shells either covering them over the out- 

 side as in clams, mussels and snails or concealed in a mantle 

 of flesh as in the squid and octopus. Counting those of land 

 and water, approximately fifty thousand species of moUusks 

 are known and one-fifth of them live in fresh water. All of 

 these fresh water inhabitants are either snails, the class 

 Gastropoda (stomach foot), or mussels and clams, the Pelecy- 

 poda ((hatchet foot). 



Habits and dwelling-places. — Snails abound in shallow wa- 

 ters, one to six feet deep, where algse and water plants, their 

 main food supply, are also abundant. Snails and mussels to- 

 gether make up a very large part of pond and stream popula- 

 tions and are also plentiful in lakes and rivers (Figs. 21, 22). 



In general, large mussels (Fig. 265) live ih large bodies of 

 water but there are many exceptions. One known as the 

 white heel-spitter, Lasmigona complanata, which is about four 

 inches long, is equally abundant in large and small streams. 

 Snails and the little mussels or "finger-nail " clams so plentiful 

 in brooks and small ponds are also found in rivers and lakes 

 sometimes at a relatively great depth. 



Mussels live on the bottom, but snails migrate back and 

 forth from top to bottom. Snails are great travelers and there 



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