126 Descriptive Zoology. 



however, is the only part eaten. It has a sweetish taste. The scallop 

 shell was worn as a badge by the crusaders, as evidence of having 

 visited the Holy Land. 



Pearls. These are formed of nacre, the material which constitutes 

 the inner layer of the shell. They begin as deposits around grains of 

 sand or other foreign objects that have gained entrance within the shell. 

 They are usually found between the mantle and the shell, but may be 

 in almost any of the soft parts. It is said that the Chinese introduce 

 little images into the cavity of the pearl oyster, leaving them to become 

 coated over with nacre. The most valuable pearls are usually obtained 

 from the pearl oyster, but they are often found in certain species of 

 fresh -water clams. The most celebrated pearl fisheries are in the 

 Persian Gulf. 



Characteristics of the Bivalve Mollusks. The clam and 

 most of the other bivalve mollusks have the following 

 characteristics, and have received various names, accord- 

 ing as any given writer places special emphasis on one 

 characteristic or another : 



1. There is no head; hence some designate the group 

 Acephala. 



2. There are large, leaflike gills, from which comes the 

 name Lamellibranchiata. 



3. There is usually a muscular, tongue-shaped or hatchet- 

 shaped foot, giving rise to the term Pelecypoda. 



4. The mantle consists of two. lobes, each lobe lining 

 a valve ; hence they are called Bivalve Mollusks. 



