MOLLUSCA LAMELLIfcRANCHIATA. 107 



separate shell, formed of carbonate of lime. These shells 

 are really cones with their apices turned to one side - 

 that in which the mouth of the animal is situated. The 

 apex, where the growth of the shell commences, is termed 

 the umbo, or beak. The beaks are situated at the dorsal 

 part of the shell. The opposite part, where the shell 

 opens, is the base. The side of the shell, towards which 

 the beaks point, is called the anterior side, because the 

 mouth of the animal is placed near it. The opposite is 

 the posterior side. The length of the shell is measured 

 from tKe anterior to the posterior side ; the breadth from 

 the beaks to the base. The anterior side is usually 

 shorter than the posterior ; the shell is therefore said to 

 be inequilateral. The valves are generally equal in size; 

 that is, they are equivalve. The shells of the scallop and 

 oyster, however, are inequivalve. As the mouth is 

 situated near the anterior margin, the valves are conse- 

 quently placed on the sides of the animal, and are there- 

 fore termed right and left. In the Brachiopoda, as 

 already noticed, the valves are placed on the dorsal and 

 ventral surfaces of the body, and are always equilateral 

 and inequivalve. 



Near the beaks is a line called the hinge line, where 

 the valves are united together by a series of projections 

 called teeth, which fit into corresponding cavities in the 

 opposite valves. The valves are bound together by a 

 structure termed the ligament, of which there are usually 

 two parts one external, the other internal. This liga- 

 ment is placed immediately behind the beaks. The ex- 

 ternal ligament is a fibrous horny substance, which is 

 always stretched when the valves are shut. The internal 

 ligament, or cartilage, consists of elastic fibres, which 

 are placed perpendicularly between the two valves. 

 When the valves are shut, these fibres are compressed 

 and shortened. When the pressure is removed, the liga- 

 ments open the valves by their elasticity. 



The valves are shut by the adductor (Lat. ad. to ; 

 duco, I lead) muscles. There are sometimes two of these 



