GENERAL STRUCTURE OF ANIMALS. n 



plain the physiology, and often something more when 

 the morphology has an important bearing on classifica- 

 tion, and especially when it has an important bearing 

 on the question of evolution. Embryology we will touch 

 on only when it bears in an important way on the same 

 two subjects. Classification we shall not touch at all 

 except in the indirect way explained above. Some 

 scheme of classification, however, we must, of course, 

 assume as the necessary condition of study, for we can 

 deal with animals only in groups. But this we put off 

 until we must have it. Lastly, if we find time we will 

 devote some pages to the laws of geographical distri- 

 bution of animals, as this has a most important bearing 

 on the question of evolution. 



So much to define the scope and limits of our course. 

 It is limited (i) to the science of life, biology ; (2) to 

 the science of animal life, zoology ; (3) in zoology it is 

 limited to comparative physiology mainly, but not entirely, 

 for function is indissolubly united with structure and 

 form. It may therefore be called comparative physiology 

 and morphology. 



SECTION III. 



GENERAL STRUCTURE OF ANIMALS. 



We have already said that all or- 

 ganisms are composed of living cells. 

 A living cell consists of three parts, 

 viz., (1) a mass of semiliquid proto- 

 plasm, usually granulated, (2) a nu- 

 cleus of more solid matter, and (3) fJ^*"' -^ceiiT 

 a thin delimiting membrane (Fig. 3). 

 Cellular structure is coextensive with life, but the cells 

 of animals differ considerably from those of plants, and 

 are far less distinct for the following reasons: 



