OYSTERS, CLAMS, MUSSELS 



221 



as to cause irritation, the mussels, as a means of protection, 

 cover them over with the pearly layer. These pearls vary in 

 value from a few cents or dollars up to hundreds of dollars. 

 Perhaps the most famous of all the fresh-water pearls is the 

 "Queen Pearl," which was found in a New Jersey stream in 

 1857. It was sold by the finder for $1500, but is now valued 

 at about $10,000. Occasionally some particularly valuable 

 pearls will be found in a new region, and during the "pearl 

 fever" that follows, thousands of 

 dollars worth of pearls may be 

 found, but the mussel beds of the 

 streams are usually almost or 

 quite depleted. Formerly the 

 shells thus gathered were left on 

 the bank to disintegrate, but 

 they are now used in the impor- 

 tant button industry. 



Fresh-water mussels are some- 

 times used for food. The great 

 shell heaps, or "kitchen mid- 

 dens," found in many places 

 show that they must have 

 formed an important part of the 

 food of the early inhabitants of 

 this and other countries 



Classes of Mollusca. The 

 branch Mollusca (L. mollis, soft) 

 is divided into five classes. The 

 class Pelecypoda (Gr. pdekys, axe, 

 pous, foot) is the largest and most important group, including 

 the mussels, clams and oysters and others that furnish an 

 abundance of cheap, palatable and nutritious food. The name 

 Pelecypoda means "hatchet-foot," and refers to the fleshy foot 

 or organ that enables the clams and mussels to dig or plow 

 their way through the mud. The name Lamellibranchia, re- 

 ferring to the lamella-like gills on each side of the body, was 

 formerly used for this class. As all the members of the class 



FIG. 100. A chiton, Isch- 

 nochiton magdalenen sis . 

 (Reduced.) 



