CHAPTER XV 



THE SHELL-ANIMALS : MOLLUSCA 



336 Examples of Mollusca. We can best understand 

 what is meant by mollusks by referring to common examples, 

 such as mussels, oysters, clams, snails, sea-snails, garden 

 slugs, cuttle-fishes, and nautilus. At first this appears 

 to be an assemblage of very unlike animals, but closer 

 studies show many points of similarity and lead to the con- 

 clusion that these animals 

 are so closely related as to 

 deserve grouping in one 

 phylum, Mollusca. In 

 this group there are more 

 than 25,000 named 

 species. 



Snails and clams are 

 usually chosen for first 

 study of the molluscan 

 plan of structure. ; 



337. Study of a Clam. 

 The quahog or little-neck 

 clam and the long-neck 

 clam are common in 

 markets in large cities and near sea-coasts; while elsewhere 

 river-clams may be obtained and kept in aquaria. Speci- 

 mens preserved in formalin are best for most work with 

 the fleshy parts. The river-clams are best for demonstration 

 of structure, the quahog is next best. 



405 



FIG. 141. A marine clam. All below 

 the line x-y is usually embedded in the 

 sand, h, hinge of the shell ; u, umbo ; 

 I, parallel lines of growth of the shell ; 

 m, edge of mantle ; /, foot ; e, i, exhalent 

 and inhalent siphons, the arrows in- 

 dicating the direction of currents of 

 water. (From Verrill.) 



