BIVALVES WITH SIPHONS. 



57 



the winter and early spring. They live for ten or fifteen 

 years. 



VALUE. Unios produce pearls, and in St. Clair County, Illinois, 

 and Rutherford County, Tennessee, their collection is a profitable 

 business. In Scotland, $50,000 worth of fresh-water pearls have been 

 taken from unios during the summer. A pearl was taken from a 

 unio near Salem, New Jersey, a few years ago, that sold in Paris for 

 $2,000. 



BIVALVES WITH SIPHONS. 



Tridacna ( Tridatmda). In the Tridacna gigas (Fig. 

 59), the largest living bivalve, the shells are often five feet 



SCALE IN FEET. 



-^ 



FIG. 59. Giant clam (Tridacna gigas). 



long ; each valve weighing over 250 pounds, the animal 

 itself frequently 30 pounds, one serving as a meal for fifty 



men. The shell is trigonal, 

 with deep radiations. They 

 are common in the Torres 

 Straits, where they are sunk 

 into the coral rock, present- 

 ing the appearance of huge 



FIG. 60. Bivalve, with siphons, #, 

 excurrent ; b, incurrent ; c, foot. 

 (After Morse.) 



elongated sea- anemones, the 

 mantle being of brilliant blue and green. So securely are 

 they imbedded that they have to be quarried out at low 



