CHAPTER XIX 



METAZOA 



Vocabulary 



Metazoa, " animals further along," that is, in development and 



specialization, many-celled animals. 

 Specialization, development of separate organs for different 



functions, division of labor. 

 Respective, separate or individual. 

 Stimuli, any outside forces that affect plant or animal, such as 



light, heat, contact, sound, etc. 



All one-celled animals are called protozoa (first animals); all 

 those consisting of more than one cell are called metazoa (animals 

 further along), meaning that they are more complex in structure 

 and more specialized in function than a single-celled animal can be. 



Development. No matter how complicated a plant or animal 

 may eventually become, it started in a one-celled stage, the fertil- 

 ized egg. This in turn was the product of the union of the single 

 sperm cell with the single ovule cell. To trace the development 

 from this one-celled stage to the highly complicated forms is too 

 difficult at present, and forms the basis for the whole science of 

 embryology. However, some of the steps in the process can be 

 briefly mentioned. 



A one-celled animal (protozoan) takes in food and oxygen, and 

 excretes waste only by means of its. exposed surface. If the di- 

 ameter of a solid be doubled its surface area is squared, but its 

 bulk is cubed. Hence if a protozoan increased much in size, it 

 would reach a point where the surface was too small to provide for 

 the bulk, and it would die. Before this point is reached, division 

 takes place and growth begins again, up to limit of size set by 

 the ratio between surface and bulk. This is why protozoa are so 

 small and why they divide so frequently. The size which a cell 

 may reach is therefore limited by the extent of its surface. 



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