NUTRITIOX IX SENESCENCE AND REJUVENESCENCE 157 



and highly irritable, reacting strongly and rapidly to various kinds 

 of stimuli. Slight movements of the water or a slight jarring uf 

 the aquarium, to which well-fed, old worms do not respond at all, 

 will bring them into active movement, and when wounded or when 

 the body is cut in two they react much more strongly than old 

 worms. In all these respects they resemble young rather than old 

 animals. In fact, their general behavior indicates very clearly 

 that they have become physiologically young during the course ol 

 reduction. But with the aid of the susceptibility method it is 

 possible to obtain more positive knowledge upon this point. 



The comparative susceptibility of starving animals may be 

 determined in two ways: when temperature and other external 

 conditions are controlled, the susceptibilities of a uniform stock at 

 different stages of starvation may be directly compared with each 

 other. This method of procedure will show directly whether the 

 susceptibiUty increases, decreases, or remains constant during 

 starvation. On the other hand, the susceptibility of animals at 

 any stage of starvation may be compared with that of fed animals 

 of the same size or of animals of the original size and condition of 

 the stock before starvation. In this way also changes in suscepti- 

 bility may be determined. Records of experiments of both sorts 

 are given below. 



In Table II the decrease in length and increase in susceptibilit)-, 

 determined at intervals of about two weeks during three months of 



TABLE II 



starvation, are recorded. The first two columns of the table are 

 self-explanatory; in the third column the times given are the times 

 of complete disintegration of the first and last of the worms of each 



