24 INVERTEBRATA OF MASSACHUSETTS. 



CLASS 



This division includes all the mollusca which are inclosed in 

 two shells (bivalve), united together at the back by a hinge, like 

 the clam, oyster, mussel, &c. Sometimes the hinge has teeth 

 shutting by the side of each other, and acting much like the com- 

 mon butt-hinge. Sometimes there are small additional bony 

 pieces attached about the hinge, serving to strengthen it ; and 

 in such cases the shell is said to be many-valved (multivalve). 



The animal has no proper head, and its conspicuous parts are, 

 First, its mantle, which lines all the interior of the shell, and incloses 

 the other parts of the animal. Its edges are more or less fringed, 

 and are either free, partly united, or entirely so, excepting a passage 

 for the foot before, and for the siphons behind. Second, a mus- 

 cular mass, usually called the foot, which may be protruded from 

 the shell, and serves as the organ of motion. Third, the respira- 

 tory organs, or branchiae, arranged somewhat like ruffles behind the 

 foot, enveloping the abdominal mass. Fourth, the siphons, which 

 are the openings for the passage of excrement, and for the admis- 

 sion of water to the branchiae ; sometimes they are very long, and 

 their tips are usually fringed. There are also two thin, elongated 

 strips on each side, attached to the mouth and passing back over 

 the sides of the foot, which are supposed to be the principal or- 

 gans for the detection and selection of food, and are called palpL 



This representation of 

 the animal of MACH.ERA 

 nitida may serve to ex- 

 emplify the parts above- 

 mentioned. 



The valves are kept closed by strong muscles, which pass from 

 one valve to the other. When these are relaxed, the valves are 

 thrown open by the elastic nature of the cartilage or ligament at 

 the hinge. The uniting substance is called cartilage when it is 

 placed between the edges of the valves, and ligament when it is 

 situated externally. 



The interior of the shell exhibits the indentations and lines 



