1 8 PHYSIOLOGY OF THE INVERTEBRATA. 



and ketones, aids iis considerably in understanding the origin 

 of various secretory products found in the Invcrtebrata as well! 

 as in the Vertcbrata. 



Living protoplasm is a substance which is constantly 

 undergoing chemical changes. It is the chemical and' 

 physical properties of this complex substance, diversely 

 modified, which underlie all the vital functions — nutritious- 

 secretion, growth, reproduction, motility, &c. Of these" 

 functions the most important is nutrition, the double and 

 perpetual movement of molecular renovation of the living 

 substance. Without nutrition there can be no growth, no- 

 reproduction, no movement, and in fact no physiological 

 function whatsoever. It has been stated that " life can ber 

 conceived of as reduced to its most simple expression, to 

 mere nutrition. A being capable of nourishing itself, and 

 destitute of every other property or function, which, after all^ 

 is only a simple extension of the nutritive property, its life 

 will be only an individual life;" a time will come when the 

 nutritive functions will have less energy — then " the nutritive 

 residue, incompletely expulsed, will impregnate the living' 

 tissues and liquids obstructing them." Such obstruction 

 necessarily interferes with physiological activity, and ulti- 

 mately ends in complete arrest. When this stage arrives^ 

 the organism, no longer capable of adjusting its " internal 

 relations to external relations,"* undergoes those chemico- 

 biological changes which finally result in its molecules (as- 

 new combinations) once more re-entering the mineral king- 

 dom — or the world of inanimation. On the other hand, if 

 the nutritive activity of a living organism " is sufiiciently 

 energetic to rise, as it were, to excess, even to growth and 

 reproduction, the being is sure of living in its offspring ; it 

 fills its place in the innumerable crowd of living beings, and 

 can even, according to the doctrine of evolution, become 

 the source of a superior organised type, can ascend in the 

 hierarchy of life." 



* Mr. Herbert Spencer's definition of " life." 



