536 



EVOLUTION OF ANIMALS 



Part V 



Fig. 27.3. Typical stages in the regeneration of ribbon worm. Linens socialis, 

 from a fragment (/) taken back of the mouth. (From original of figure 4, 

 Wesley R. Coe, /. Exp. Zool., 54:426.) 



go without food for a year pr more during which they live upon their own 

 constantly decreasing bodies. 



Ribbon worms dwarfed by starving are commonly found in nature. Many 

 dwarfs have been produced experimentally. When examined with the micro- 

 scope they reveal a series of sacrifices. Some of the cells of the primitive 

 middle layer, mesenchyme, become wandering phagocytes literally devouring 

 the body cells, especially those of the digestive canal. Loaded with food these 

 cells then disintegrate and their remains furnish food for surviving cells. As 

 starving continues, this process is repeated over and over and the animal be- 

 comes smaller and smaller. In this way digestive tract, reproductive organs, 

 and muscles gradually disappear. 



Circulatory System. Ribbon worms are the simplest animals to have a 

 circulatory system of the closed type with true blood vessels and spaces in the 

 mesenchyme continuous with the vessels. The blood is usually a colorless fluid 

 carrying blood cells but in various species it may be yellow, green, or red — 

 the red color due to hemoglobin contained in the cells as in human blood. This 

 system takes over the distribution of substances that in flatworms were carried 

 by fluid in the gastrovascular cavity. There is no special pumping organ and 

 the blood vessels have few branches. 



