6 INTRODUCTION [CH. 



this process is carefully to be distinguished from excretion. The 

 accumulation of excreta soon stops metabolism, whereas the 

 intermission of defaecation need only interfere very slightly with 

 metabolism. 



Of the numerous solid particles found in protoplasm some are 

 secretions, others are solid deposits of partly assimilated food, which 

 act as reserve stores, others are indigestible remains or faeces. The 

 fluid drops contained in it consist largely of water some have in 

 solution excreta or secretions ; others contain the results of digestion. 



Animals, as we have seen, possess the power of executing move- 

 ments ; this power is exercised in order to seek their 



Movement. / -i i TT 



food and escape their enemies. However complicated 

 these movements may be, they are all found to be dependent on the 

 capacity of protoplasm to alter its shape, by suddenly contracting 

 and then slowly expanding. By contraction is meant such an altera- 

 tion of shape of the moving part as will tend to diminish its surface 

 but not its bulk; that is, the contracting part tends to assume 

 a spherical shape; by expansion, on the other hand, is meant 

 an alteration of shape leading to increase of surface. A bird 

 flies by contracting the muscles first on one side of the wing, then 

 on the other; a fish swims by alternate contractions of the two 

 sides of the fleshy tail. Any part of an animal fitted to execute 

 movements more quickly in one direction than in another and 

 so to bring about the movement of the whole animal, is called a 

 locomotor organ. Protoplasm in which the power of contraction 

 is highly developed is called muscle. 



A contraction is the result of an explosive decomposition of the 

 living substance; there have been a great many theories as to 

 how the chemical change brings about the change of shape but, 

 since all of them account for some of the facts and none of them for 

 all, there is no need to mention any of them here. 



The sudden chemical change which brings about contraction, 

 although dependent on the unstable character of the biogen 

 molecule, must be precipitated by some change occurring either 

 in the living matter itself or in the surrounding medium, just 

 as an explosion of gunpowder is not brought about without a spark. 

 In either case the change causing the contraction is known as a 

 stimulus, and the capacity of contracting under the influence of 

 stimuli is known as irritability. Thus when a moth flies into a 

 flame it is acting under the stimulus of light; when a hungry lion 

 in the Zoological Gardens rises up and commences running violently 



