28 MECHANICAL FUNCTIONS. 



61. Now as the nourishment of the animal requires that 

 the shell should be kept open to a small distance for the 

 admission of the water, and as its safety might require it 

 to be closed suddenly, provision is made for this action, by 

 a strong muscle passing from one of the valves to the other, 

 and by which they are instantly brought together at the 

 will of the animal. 



62. Thus we see that the Creator has furnished these 

 animals with every comfort and convenience which it 

 would be possible fo/ them to enjoy in the situation in 

 which they are placed. A pair of hard shells to protect 

 them a cartilage, answering as a spring to keep these 

 shells a little 'open, to admit the water from w r hich they 

 obtain food and air a ligament, to prevent the shells 

 from opening too widely, in which case the sand and 

 mud would destroy the animal and a muscle, by which 

 he can in an instant close his doors, and become proof 

 against the attack of the most voracious monsters of the 

 deep. When the animal dies, the muscular force ceases, 

 but the cartilage retains for some time its elasticity, and 

 the ligament continues its adhesion to the valves, and 

 from these circumstances it is that we find the shells cast 

 upon the shore, only open to a certain distance, until after 

 the destruction of the ligament, when the cartilage throws 

 them quite open. 



63. Several of the bivalve mollusca have the power of 

 giving themselves a considerable motion, by suddenly clo- 

 sing their shells, and thus forcibly expelling the water from 

 between them. The reaction of the fluid on the temporary 

 current thus produced, may often be seen to throw the shell 

 many inches in the opposite direction. 



64. The common scollop contrives to give itself motion, 

 even on the shore, by suddenly and forcibly closing its 

 valves ; one of which striking against some impediment, 

 as a pebble, acts as a spring, and thus throws it to a little 

 distance. It is said that when left by the tide, they often 

 reach the water in this way. 



In bivalve shells, when the animal is alive how are the shells kept open ? 

 What prevents them from opening too widely? How are the shells sud- 

 denly brought together? Why are bivalve shells generally found only 

 partly open on the shore? In what manner do some of these animals give 

 themselves a sudden motion in the water ? How is it said the scollop 

 contrives to move on the shore ? 



