CONCHIFEEA. 257 



previously in a state of compression, forces them 

 asunder. It is the contractile power of this muscle 

 which renders it so difficult to open an oyster ; the 

 inserted knife cuts through the muscle, and it opens 

 immediately. In a great majority of the Conchifera, 

 there are two of these muscles placed far apart, as in 

 the common Mussel. The hinge also, in many, is 

 much more complicated, presenting a curious array of 

 notches, and teeth, depressions and elevations that 

 lock into each other. 



FlG. 195. SPINED VENtJS 



In genera so constructed the ligament is placed 

 upon the outside of the hinge, and opens the shell by 

 its contraction, not by its expansion. 



Whoever for a moment reflects upon the arrange- 

 ment of the branchial apparatus, and the position of 

 the mouth, consisting, as it does, of a simple aperture 

 unprovided with any prehensile organs, must perceive 

 that there are two circumstances connected with the 

 economy of a conchiferous Mollusk, and those not of 

 secondary importance, by no means easily accounted 

 for. It is, in the first place, absolutely essential to 

 the existence of these animals that the element in 

 immediate contact with the respiratory surfaces 

 should be renewed as rapidly as it becomes de- 

 teriorated, or suffocation would inevitablv be the 



