MOLLUSKS 319 



heaps (" castings") at the opening of the burrow. In this way the 

 earthworm makes the soil porous, and brings subsoil to the surface 

 (cf. 296). 



337. Mollusks. Clams and oysters are mollusks that 

 live in salt water. A very similar form, the mussel, 

 lives in fresh-water lakes and in streams. "Mollusk" 

 comes from a word meaning "soft," and is given to this 

 class of animals because of their soft bodies. 



The mussel (Fig. 261) is found at the water bottom, 

 slowly plowing its way through mud or sand. It has a shell 

 made of two similar Kidney Ventric 



parts (valves) ; these Posterior Adduc ^ 

 are joined at the top 

 (the dorsal side of 

 the animal) by a 

 hinge. Inside the 

 shell is the mantle, 



a SOft, niUSCUlar FIG. 261. Freshwater Clam, or Mussel; its Principal 



Organs. 



structure. This lines 



each of the valves. The "foot" of the animal projects 

 from the anterior (head) end of the body. At the oppo- 

 site (posterior) end there are two openings called siphons. 

 The lower of these has organs like cilia, for forcing a 

 current of water through the body. The other siphon is 

 for outgoing water. The mussel's food, as well as its air, 

 comes to it in the water taken in at the lower siphon. 

 Between the mantle and the foot, one on each side, are 

 the gills, through which respiration takes place. 



The halves of the shell are held together at the hinge by a tough, 

 horny ligament. The outer layer of the shell is made of the same horny 

 material. Inside this there is a layer of limestone, and on the inner 



