METAZOA 159 



COMPARISON OF PROTOZOA AND METAZOA 



One-celled 



No specialization in simplest except 



the nucleus 

 Some have a cell wall, cilia, " mouth " 



but no regular systems of organs 

 Reproduce by fission or conjugation 

 Excretion by vacuoles 



Minute size 



No "body wall" either in embryo or 

 adult 



Many celled 



Specialized tissues and organs 



Digestive, respiratory and nervous sys- 

 tems 



Reproduce by eggs and sperms 



Excretion by kidneys, or analogous or- 

 gans, skin, and lungs 



Much larger size 



Three layers in embryonic body wall 

 which develop as follows: 



1. Ectoderm, forms outer skin and its 



appendages : Nervous system 

 and sense organs 



2. Mesoderm, forms inner skin, fat, 



bone, muscle, connective tissue, 

 serous membranes 



3. Endoderm, forms mucous mem- 



branes and all organs that it 

 lines, gills, lungs, glands 



