32 MECHANICAL FUNCTIONS. 



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 to- 

 gether at the will of the animal. 



Thus we see that the Creator has furnished these ani- 

 mals with every comfort and convenience which it 

 would be possible for 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 which 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. 



Several of the bivalve Mollusca have the power of 

 giving themselves a considerable motion, by suddenly 

 closing 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. 



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 \vater in this way. 



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

 What prevents them from opening too widely 1 How are the shells sud- 

 denly brought together 1 Why are bivalve shells generally found only 

 partly open on the shored In what manner do some of these animals 

 give themselves a sudden motion in the water? How is it said the scol- 

 lop contrives to more on the shore ? 



