METAZOA 155 



The paramoecium is much more highly developed than the 

 amoeba but a -limit to its specialization and growth is soon found 

 and a stage is reached where further specialization in function or 

 increase in size is no longer possible. If further advance is to be 

 obtained, larger and more complicated forms must develop. Sup- 

 pose that when a protozoan divides, the cells did not separate but 

 remained attached, grew, and divided again and again. There 

 would soon be produced a mass of cells much larger than any single 

 one, and with abundant surface exposed for food-getting and 

 breathing. In such an animal the outer cells could best attend to 

 locomotion, sensation, and food-getting, while the inner cells 

 could carry on digestion and reproduction. Pandorina and other 

 simple metazoans represent this stage. 



If a solid mass of cells continued to enlarge, the innermost ones 

 would be so far from contact with food and air that a limit in size 

 of the mass would be reached, just as with the single cell. To 

 meet this condition, the next higher forms consist of hollow spheres 

 of cells, thus giving an inner and outer surface, and permitting 

 much larger and more complicated forms. Volvox is a representa- 

 tive of this condition. It consists of thousands of cells, is large 

 enough to be visible to the eye, and has very highly developed 

 reproductive and locomotor cells. 



A hollow sphere cannot increase indefinitely in size as the single 

 cell layer would not be strong enough, so in the next higher forms 

 an infolding of the wall takes place, much as a hollow rubber ball 

 can be squeezed into a cup-shaped form. Its walls will now be 

 double with a space between them, in which a third cell layer de- 

 velops. This three-layered stage is reached in the simplest sponges, 

 and from the three layers develop all the tissues of higher 

 forms. 



It is important to remember that every plant and animal began 

 life as a single cell, the fertilized egg. This by repeated divisions 

 passed through the stages just described, developed from a mass 

 of unspecialized cells into higher forms with tissues and organs. 

 Finally it reaches its destined stopping place whether in the simple 

 volvox or the complicated insect, bird, or man. 



