INTERNAL ORGANS. 1 59 



This spring is called the ligament, and is composed of a 

 dark, tough, elastic substance resembling india rubber. 



In this species, the long ligament, /, is external. 

 It becomes brittle when it is dried, but when the ani- 

 mal is alive it is strong and firm; besides acting as a 

 spring to open the valves, it also helps to bind them 

 firmly together. 



In the living mollnsk the valves are lined with a 

 soft skin or mantle, called the pallium. This pallium 

 secretes the substance of the shell, building on new 

 matter at the edge and thickening its interior. It is 

 firmly attached along a curved line which runs from 

 one muscle scar to the other. In some species this 

 pallial line, marked p. /., makes a simple curve, but 

 in others, as shown in the engraving, it bends back 

 more or less deeply, making a bay or sinus, marked 

 p. s n which stands for pa Ilia I sinus. 



Having thus briefly noticed the parts of the shell, 

 let us now examine the structure and habits of its 

 occupant. 



Inside the mantle are the principal organs of the 

 mollusk, which consist of the gills, liver, digestive 

 and circulatory organs. There is no head, and the 

 mouth is only a slit surrounded by four triangular 

 lips, and opening directly into the gullet, which in 

 turn leads directly to the stomach and the intestine. 

 The heart is a simple sack-like organ; in the oyster it 

 may easily be seen lying in a little opening just below 

 the great central muscle. The liver is large, of a 

 dark color, and is separated into granules. The gills 

 are four in number, two on each side of the body, 

 lying just beneath the mantle. They resemble thin, 

 delicate ribbons, and are attached by their edges. 

 They are crossed by very many tubes through which 



