MOLLUSCA ACEPHALA. 



217 



2. Acephala. 



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 

 hinge is secured by a substance of great strength. 



It is seen in Fig. 101, 

 which shows the valves 

 of the Unio batava, with 

 the connecting liga- 

 ment. This ligament is 

 composed of two kinds 

 of texture : the one, 

 which is always exter- 

 nal, is strictly liga- 

 mentous ; that is, per- 

 fectly inelastic : the 

 other has more of the 

 properties of cartilage, being highly elastic, and 

 formed of parallel series of condensed transverse 

 fibres, directed from the hinge of one valve to the 

 similar part of the other, and having generally 

 a deep black colour, and a pearly lustre. The 

 cartilage is always situated within the ligament. 



