142 PEARLS 



Haemoglobin. A red pigment found in blood, which plays an 

 important part in the processes of respiration. Its place is 

 usually taken by other non-red pigments in invertebrate 

 animals. 



Half pearls. Pearls sawn into halves. 



Hinge pearls. Pearls of irregular elongated shapes, found near 

 the hinge of shellfish from fresh waters. 



Hypostracum. The most internal layer in a bivalve shell at the 

 places where the adductor muscles are attached. It is com- 

 posed of minute columns set at right angles to the plane of 

 the shell (see Fig. 2). 



Lamellibranchiata. The group of molluscs known popularly as 

 bivalves and characterised by the possession of lamella-like 

 gills. 



Lingah pearls and shells. Shells and pearls from the Persian 

 Gulf, often the same kind as the Bombay pearls. 



Mantle. Two flaps arising one on either side (right and left) of 

 the body of a bivalve. Each flap has the same shape as the 

 valve of the shell against which it always lies. It is composed 

 of connective tissue with an outer bounding layer of epi- 

 thelium, and this epithelium on the surface in contact with 

 the shell is responsible for both shell and pearl formation. 



Mollusc. A soft-bodied, non-segmented invertebrate animal 

 which typically possesses a hard shell. This shell may be 

 univalve as in the snails, or bivalve as in the oyster, cockle 

 and mussel. Sometimes it is reduced and internal, as in 

 the slugs and cuttlefishes. 



Muscle pearls. Small pearls found in the muscular tissue near 

 its attachment to the shell. 



Mytilus pearls. Pearls found occurring in the common edible 

 mussel (Mytilus). 



Nacre. The mother-of-pearl layer which lines most bivalve shells 

 and forms all or only the outer layers of nacreous pearls. 



Naked diving. Diving without special dress, etc. 



