AGE DIFFERENCES IN SUSCEPTIBILITY 101 



The only possible conclusion is that in this species individuals 

 resulting from the asexual process of fission show age differences 

 similar in character to those in the sexually produced individuals 

 of Planaria maculata. In both cases the rate of metabolism is 

 highest in the young worms and decreases with advancing age. 

 Later chapters will confirm this conclusion (see chaps, v, vii). 



AGE DIFFERENCES IN SUSCEPTIBILITY IN OTHER FORMS 



In order to determine whether age differences in susceptibility 

 are of general occurrence and of the same sort, the susceptibility 

 of young and old individuals of a considerable number of species 

 from different groups has been compared by direct method. The 

 general results of these investigations are briefly stated without 

 the data of experiment. 



The age differences in susceptibility have been determined for 

 various other species of flatworms. In Dendrocoelum lacteum, 

 Phagocata gracilis, and certain unnamed species of the Mesostomidae, 

 all of which reproduce only sexually, the susceptibility by the direct 

 method of the young animals to the cyanides is much greater than 

 that of the old. In Planaria velata, the old worms break up into 

 fragments which encyst and undergo reconstitution into new indi- 

 viduals in the cysts and later emerge as young worms capable of 

 repeating the life cycle. In this species also the susceptibility, as 

 determined by the direct method, is greatest in the young worms 

 after they emerge from the cysts, and decreases from this stage on 

 until the next fragmentation (Child, '13). 



Differences in susceptibility which are undoubtedly connected 

 with physiological age have been found in certain protozoa (see 

 pp. 141-42). Among the coelenterates the fresh-water hydra 

 and two species of hydroids, Pennaria tiarella (see Fig. 50, p. 148) 

 and Corymorpha palma, have been tested. In the two hydroids 

 the sexually produced young at any stage after attaining the form 

 of the adult show a much greater susceptibility than the full-grown 

 mature animals. In hydra, sexually produced young have not as 

 yet been obtained, but the young animals asexually produced show 

 a higher susceptibility than the parent. In the ctenophore, Mnemi- 

 opsis leidyi, the susceptibility decreases with advancing physiological 



