REGENERATION 



683 



Transverse sections of hydroid stems must be of minimum length, 

 characteristic for the individual and level from which they were 

 taken. Pieces shorter than this minimum length may regenerate 

 tentacles and other structures characteristic of the oral region at 

 each end of the piece or may fail to regenerate such structures 

 entirely. 



It is possible to split hydroids partially and get individuals with 

 2, 4, 8, or more, complete oral regions with a single foot. 



Platyhelminthes 



The free-living flatworms have served as experimental material 

 for a great many researches on regeneration. Planaria, like the 

 hydroids, may be cut into several transverse segments each of which 



B'V-:/ 



Fig. 365. — Regeneration in Planaria. A, normal animal with transverse line 

 to show level of cut to divide the animal; B, B^, regeneration of anterior half; 

 C, C\ regeneration of posterior half; D. piece cut from middle of body; £)', D". 

 D', D*, regeneration of D; E, anterior piece; E^, E-, E^, regeneration of B; P. P^. 

 regeneration in head of another species. (From Morgan, Regene)-atio7ij published 

 by The Macmillan Company.) 



