SUMMARY OF THE CHIEF ELEMENTARY FACTS 



AND PRINCIPLES OF ANIMAL MORPHOLOGY 



STATED IN PART I. 



1. The substance of all animals (as well as of all plants) 

 consists of units, called cells or plastids, of microscopic size. 



i. A cell or plastid consists of a mass of protoplasm, 

 containing (in all but a few exceptional cases) a nucleus. The 

 latter consists of an aggregated network of firmer and 

 thiclcer protoplasm, visible as a darker mass. 



3. Since division of labour results in economy of labour, 

 cells or plastids acquire various additional characters, to fit 

 them to perform various special parts of the work of the 

 animal body ; this is expressed by saying that, in order to 

 perform different fuaetions, they become dififerentiated in 

 structure. A mass of cells of similar structure grouped 

 together is called a Tissue. When a tissue established to 

 perform a special function is greatly developed, and sup- 

 plemented by masses of other tissues specially arranged so as 

 to support it and provide it with nourishment, it becomes an 

 Organ. Animals in which the body is differentiated iuto 

 many different kinds of tissues and organs are spoken of as 

 highly organised. 



-1. The most primitive forms of animal life, spoken of as 

 Protozoa, either are unicellular, i.e. composed of one cell 

 only, or consist of comjfound aggregates in which separate 

 cells cannot be distinguished. The rest of the animal king- 

 dom, spoken of as Metazoa, have multicellular bodies, 

 differentiated into various tissues. 



•5. The body of the Metazoa consists primarily of two 

 layers of cells, ectoderm or outside layer, and endoderm 

 or inside layer. The cells of the ectoderm are originally 

 protective in function, and the cells of the endoderm origin- 

 ally digestive, the two layers thus affording the most 



