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 clôture 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 constitution. 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 "ligaments" 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 entrance of disagreeable or noxious elements into the internal arrangements of oyster or mussel life. There is illustrated here a clear saving of life-force, and a persistent system of vital economics in the substitution 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 system, 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
