12 ZOOLOGY 



In these latter instances the protoplasm eliminates a large per 

 cent, of its water, forms a thick wall, and thereby becomes 

 enabled to resist drouth and heat and cold as it could not possibly 

 do otherwise. Under ordinary circumstances protoplasm ap- 

 pears as a semi-fluid or gelatinous material. 



Concerning the architecture of protoplasm there is much 

 diversity of opinion. It seems probable that this, like the 

 chemical composition, is subject to considerable variation. It 

 is certainly very complicated and represents several physic- 

 ally distinct substances mingled in a very effectual and 

 wonderful way. In some cases at least these take on the ap- 

 pearance of a foam structure such as is obtained in an emul- 

 sion of oil in water, or of air and water in a soapy lather. 

 Whatever its form may be, it seems that there must be a close 

 relation between the architecture and the great activity which 

 protoplasm shows. It is physically as well as chemically 

 unstable. 



19. Physiology of Protoplasm. The mass of protoplasm 

 which we have called a cell, or unit, performs practically all 

 the functions shown by the more complex organism. It has 

 the power of feeding, of growth, of reproduction, of motion in 

 response to stimuli, and of waste and repair. Even in the higher 

 animals, made up of many of these units, the processes are 

 performed, on last analysis, by the individual protoplasmic 

 units of which the body is composed. 



20. Irritability. Owing to its chemical and physical in- 

 stability, living protoplasm is constantly changing. These 

 changes may be the direct result of internal or external con- 

 ditions to whose influence the protoplasm may respond by a 

 manifestation of energy greater than that involved' in the 

 stimulus. This quality is called irritability. It further seems 

 that changes may originate within the protoplasm itself, 

 though this is much more difficult to demonstrate and may 

 merely represent our ignorance of the processes occurring in 

 the protoplasm. This power is called automatism. These are 

 the most fundamental qualities belonging to protoplasm, and 

 serve to make possible those which follow: viz., motion, assimi- 



