CHAPTEE II 



Introduction to the study of the Metazoa — Coelenterata or Zoophyta as the 

 lowest Metazoa constructed essentially of two cell layers — Yolk segmenta- 

 tion and formation of the two primitive germinal layers of the Metazoa 

 (GastrEea theory). 



Introduction. 



In contradistinction to Protista or Protozoa we have real Animals or 

 Metazoa. The bodies of the former consist of one single cell or of 

 several similar cells (with the exception of Volvox), each of which, how- 

 ever, is competent to perform all vital functions (cell colony) ; the 

 bodies of Metazoa, on the contrary, always consist of a number of cells 

 which are not all similar, but have divided among them the different 

 forms of vital activity (cell community). The division of labour may 

 be more or less complete, and according to it the degree of morpho- 

 logical complexity and of physiological perfection is determined. 

 There are animals which are morphologically (according to structure) 

 and physiologically (according to their vital activities) only a little 

 raised above the Protozoan colony, e.g. the Hydra. 



The bodies of these animals consist of only slightly different 

 sorts of cells : digesting cells, neuro- muscular cells, stinging cells, 

 and formative cells of eggs and spermatozoa. All these kinds of cells 

 are, however, indispensable to the existence of the Hydra body ; not 

 one of them can be removed from the body without endangering 

 its existence. The whole body is nevertheless physiologically an 

 individual, but, as opposed to the cell, an individual of a second, 

 i.e. a higher order— a person. Most animals remain at this stage of 

 individuality. A Medusa, a Worm, a Crustacean, or a Mammal is 

 such an individual of the second order. In many animal divisions, 

 however, individuals of the second order multiply by fission or gemma- 

 tion. The new individuals thus arising remain united, and together form 

 individuals of the third order— an animal stoek. The single indi- 

 viduals which collectively form such a stock may remain similar, and 

 they then are related to the stock in just the same way as the cell- 

 individuals of a Protozoan colony are related to the colony ; or division 



