432 



BIOLOGICAL EFFECTS OF RADIATION 



CHAETOPODA 



Among the annulata regeneration has been so widely observed that 

 some degree of this potency is presumably universal in these animals. 

 In the oligochaetes the gross aspects of the process and its histological 

 details have become well known in genera that are presumably repre- 

 sentative. Thus in Limnodrilus and Tubifex (33, 34, 58, 59), posterior 

 regeneration occurs at any level behind the tenth somite, and a full com- 

 plement of somites can be eventually restored (Fig. 11). Anterior 



Fig. 12.- — Effect of X-rays upon posterior regeneration of Tubifex tubifex as shown by 

 microphotographs of sagittal sections. A, Control 9 days after removal of posterior 

 segments; X145. B, Same of X-rayed worm 12 days after a similar removal; X170. 

 Plane of removal is shown by a line in each figure. (From Stone, 58.) 



regeneration occurs in Tubifex when not more than 10 or 12 of the ante- 

 rior somites are removed, but more than 3 new somites are never formed 

 in the restoration of a normal head region (Fig. 13). In recent studies 

 of Lumbriculus, Zhinkin (72, 73) and Turner (63, 64) have confirmed the 

 earlier observations that in this species posterior regeneration occurs to 

 about the same extent as in Tubifex, but that anterior regeneration occurs 

 from any level except the extreme posterior end. The histological 

 changes during posterior regeneration are essentially the same in the 

 two genera. Among the peritoneal cells on the posterior faces of the 

 septa, except in the most anterior region, there are what have been called 

 the "reserve cells" or "quiescent neoblasts." When the posterior end 

 of the worm is removed, the reserve cells in neighboring somites are 

 activated to a growth or metamorphosis in the course of which they are 

 transformed into much larger cells the cytoplasm of which is crowded 



