I STRUCTURE AND PHYSIOLOGY OF ANIMALS 15 



it from without. In the case of such an extremely simple 

 organism as Amceba, these changes are also, necessarily, extremely 

 simple ; but they are of a quite definite character. In addition 

 to the effects produced on its actions by mechanical obstacles and 

 the presence of food-particles, Amceba responds by definite changes 

 in itself to such external influences as changes in the amount of 

 oxygen supplied, in the quantities of various salts present, in the 

 temperature, and in the electric conditions of the water in which it 

 lives. The power of locomotion, the capacity for assimilating organic 

 substances, and the absence of two special compounds chlorophyll 

 and cellulose are specially characteristic of the animal as distin- 

 guished from the plant. 



In connection with these vital processes of Amceba the nucleus 

 plays an important role. If an Amceba be cut in two, the part 

 containing the nucleus continues to comport itself like a normal 

 complete animal ; it continues to take in and digest food, and 

 eventually recovers its original size. The non-nucleated part, on 

 the other hand, though it may continue to live for a good many 

 days, is unable to absorb nourishment ; its movements lack 

 co-ordination and do not respond to stimuli in the normal way. 

 Eventually death takes place in about ten to thirty days as a result, 

 apparently, of the exhaustion of the store of potential energy the 

 processes of destructive metabolism having continued without the 

 compensating processes of nutrition and assimilation. 



It has already been stated that an Amceba may be killed by being 

 subjected to excess of carbon dioxide. If this action takes place 

 gradually the Amceba is able to withdraw its pseud opods and 

 assume the defensive, motionless, encysted state, in which it is able 

 better to resist unfavourable conditions. But there are many 

 poisons which cause more rapid, even instantaneous, disintegration. 

 Others cause immediate death without much alteration and by their 

 use it is possible to fix the Amceba in such a way as to retain its 

 structure not greatly altered and preserve it permanently. 



2. THE ANIMAL CELL. 



In all but the lowest animals the various functions just enume- 

 rated are carried on by means of a more or less complex machinery 

 of organs muscles, alimentary or enteric canal, glands, heart and 

 blood-vessels, gills or lungs, nervous system, organs of excretion, and 

 organs of reproduction But in all animals, however complex, the 

 same substance, protoplasm, which in Amoeba constitutes the 

 bulk of the body, is the essential and active part. Wherever in 

 the body active functions are being discharged and active changes 

 are going on, there we find protoplasm present ; where there is 

 no protoplasm there is no vital activity. In the earliest stages of 



