MOLLUSCA. 



159 



the separate pieces, in cither case, being termed valves; so 

 that shells may be either univalve, bivalve, or miiltivalve, 

 according as they consist of one, two, or more pieces. Uni- 

 valve shells have generally more or less of a spiral form, 

 and are then called turbinated shells. In a few, the cavity 

 of the shell is divided by transverse partitions into nume- 

 rous compartments. Some Mollusca have internal shells for 

 the defence and support of particular organs; and others 

 have shells which are partly external, and partly internal. 

 As respects their shape, colour, and appearance, shells ad- 

 mit of infinite diversity; yet, as will presently be shown, 

 all are composed of the same kind of material; and their 

 production and increase are regulated by the same uniform 

 laws. 



§ 2. Jlcejihala. 



The Mollusca which inhabit bivalve shells, such as the 

 Oyster, the Muscle, and the Cockle, are all acephalous. 

 The two valves of the shell are united at the back by a 

 hinge joint, often very artificially constructed, having teeth 

 that lock into each other: and the mechanism of this arti- 

 culation varies much in different species. The hino-e is se- 

 cured by a substance of great strength. It is seen in VW. 



101, which shows the valves 

 of the Unio batava, with the 

 connecting ligament. This 

 ligament is composed of two 

 kinds of texture: the one, 

 which is always external, is 

 strictly ligamentous; that is, 

 perfectly inelastic: the other 

 has more of the properties of 

 cartilage, being highly elastic, 

 and formed of parallel series 

 of condensed transverse fibres, 

 101 directed from the hinire of one 

 valve to the similar part of the other, and having generally 



