34 STUDIES IN LIFE AND SENSE. 



the impressions which these structures leave on the shell's interior. 

 The latter animal has thus a double holdfast, whereas the oyster has 

 but a single one. If the function of these structures is thus con- 

 cerned 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 constitution. We have seen that 

 the natural and persistent state of oyster life is a condition of un- 

 closure, 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 car- 

 ried 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 in- 

 ternal 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 sur- 

 veillance 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 ren- 



