MOLLUSCA ACEPHALA. 



195 



generally more or less of a spiral form, and are 

 then called turbmated shells. In a few, the cavity 

 of the shell is divided by transverse partitions into 

 numerous compartments. Some MoUusca have in- 

 ternal shells for the defence and support of par- 

 ticular organs ; and others have shells which are 

 partly external, and partly internal. As respects 

 their shape, colour, and appearance, shells admit 

 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. Acephala. 



The Mollusca which inhabit bivalve shells, such 

 as the Oi/sler, 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 articulation varies much 

 in diiFerent species. The hinge is secured by a sub- 

 101 __^ _ stance of great strength. 



It is seen in Fig. 101, 

 which shows the valves 

 of the Unio batava, with 

 the connecting ligament. 

 This ligament is com- 

 posed of two kinds of 

 texture : the one, which 

 is always external, is 

 strictlyligamentous; that 

 is, perfectly inelastic : the 

 other has more of the 



