67S THE POPULAR SCIENCE MONTHLY. 



ures leave on the shell's interior. The latter animal has thus a double 

 hold-fast, whereas the oyster has but a single one. If the function of 

 these structures is thus concerned with the cloture aspect of bivalve 

 life, how, it may be asked, is the opening of the shell provided for ? 

 This is exactly the point to which Nature directs her energies in 

 arranging her economical disposition of the oyster or mussel constitu- 

 tion. We have seen that the natural and persistent state of oyster- 

 life is a condition of unclosure, while the opposite action of shutting 

 the shell is only a transitory and infrequent phase of bivalve existence 

 at the best. There is afforded a chance for the exercise of mechanical 

 expediency in making the open state of the shell a matter of ease, and 

 one carried out without effort or exercise of energy. And so is it 

 contrived. 



Suppose that, placing two oyster-shells in their natural position, 

 we insert a piece of India-rubber between the valves at the point 

 where they are hinged together. If we now forcibly close the shells 

 by pressure, the India-rubber is compressed. When we release the 

 pressure of our fingers, the elasticity and recoil of the India-rubber 

 forces the valves apart. In such a fashion, then, does Nature provide 

 for the constant maintenance of the unclosed condition. The " liga- 

 ments " of the shell are natural elastic pads existing at the hinge-line. 

 By their elasticity they keep the valves unclosed. There is no strain 

 involved in the action, which is a merely mechanical one, after all. 

 But when the more infrequent act of closure has to be performed, 

 then muscular energy requires to be displayed. The quick snap of 

 the valves reminds us that muscular exertion, even if necessitating vital 

 wear and tear, has its corresponding advantage in the rapidity and 

 effectiveness with which it provides for protection against the en- 

 trance of disagreeable or noxious elements into the internal arrange- 

 ments of oyster or mussel life. There is illustrated here a clear sav- 

 ing of life-force, and a persistent system of vital economics in the sub- 

 stitution of a mechanical for a muscular strain where the maintenance 

 of the open state of the shell is concerned. 



Returning to the human domain for a final glance at our subject, 

 there are found in the spheres of digestive nervous actions many facts 

 and examples proving the exercise of a constant economic surveillance 

 of our life. The digestive duty may be defined as that whereby our 

 food is converted into a fluid capable, when added to the blood, of 

 repairing and replenishing that fluid. To this end, as is well known, 

 the nutriment has to pass along the tube known as the digestive sys- 

 tem, and to be subjected to the chemical action of the various fluids 

 or secretions which are poured upon it in the course of its transit. In 

 the stomach, for example, certain important food-principles those of 

 nitrogenous kind are first selected as it were from the nutriment, 

 chemically altered by the gastric juice, and rendered capable of being 

 absorbed into the system. Instead of waiting for a lengthened period 



