354 



GENERAL BIOLOGY 



contains the fundamental tissues, in such relation that food 



can be taken, may, under favorable con- 

 ditions, develop into a perfect hydra. A 

 single arm broken from a starfish will 

 regenerate the body and all the other 

 arms. But as we ascend the scale of 

 animal life, the power of regeneration 

 becomes more limited as organization 

 becomes more complex and the adjust- 

 ment between the organs, more delicate. 

 Herein lies one of the limitations of 

 specialization already mentioned (page 

 251). Blood vessels, for example, are 

 excellent agents of circulation when 

 intact, but when cut, they wonderfully 

 facilitate bleeding to death. Planarians 

 (fig. 204) are classical subjects for 

 regeneration experiments. They may 

 be cut to pieces apparently without very 

 serious inconvenience, and regenerate 

 missing parts with great readiness. 

 They have no parts that can be put 

 entirely out of commission by being 

 severed. They have no blood vessel 

 system, nor organs of respiration. They 

 have a sort of brain, but it is of so little 

 consequence that when the head containing it is cut off, 

 another one is promptly grown. The other organs all 

 appear equally well adapted to withstand mishaps. The 

 extraordinary food canals, branching and ramifying all 

 through the tissues, supply with a food receptacle even the 

 smallest piece of the body that may readily be severed. 

 Figure 206 shows the regeneration of the two halves of a 

 planarian that was cut in two in the median plane of the 



Fig. 204. Diagram of 

 a planarian, showing 

 food cavity in gray 

 and central nervous 

 system in black, tn, 

 mouth at the end of 

 a cylindric pharynx 

 that is directed 

 downward. 



