i 4 6 



PRACTICAL ZOOLOGY 



mud or sand, or because both are so well protected by their shells 

 that neither can injure the other. Most of the mussel-like ani- 

 mals live in the sea; one of these, the long-neck or soft-shell clam, 

 is an important article of food and is dug out of the sands or 

 mud between high-tide and low-tide lines in great numbers and 

 sold in fish markets (Fig. 82). The fresh-water mussel and the 



long-neck clam dif- 

 fer in certain re- 

 spects, but their 

 general activities 

 and structure are 

 similar, and either 

 makes good mate- 

 rial for study in 

 the laboratory. 



Locomotion. If 

 a living mussel is 

 placed on the sandy 

 bottom of a body of 

 water, it will not at 

 first show any signs 

 of life, but if we 

 wait long enough, it 

 will slowly open the 

 two valves of its 

 shell and protrude 

 a wedge-shaped, 

 whitish portion of the body, the foot (Fig. 83, p). The foot is 

 gradually extended and forced into the sand, the body is slowly 

 drawn into an upright position, and a large portion of it is 

 soon buried in the bottom. Here the mussel may remain at 

 rest for some time, or it may slowly plow its way through the 

 sand, mud, or gravel by alternately extending its foot and then 

 drawing the rest of the body after it. 

 The Protective Shell. The body of the mussel is exceedingly 



9 



FIG. 83. The external parts of a mussel. Behind 

 is the inner face of an empty shell. 



1, points of insertion of anterior protractor 

 (above) and retractor muscles (below) of the shell ; 

 2, of anterior adductor muscle ; 3, of posterior pro- 

 tractor of the shell ; 4, of posterior adductor mus- 

 cle; 5, lines formed by successive attachment of 

 mantle ; 6, umbo ; 7, dorsal siphon ; 8, ventral 

 siphon; 9, foot protruded; 10, lines of growth. 

 (From Shipley and MacBride.) 



