152 SENESCENCE AND REJUVENESCENCE 



dynamic correlate, of the difference in morphological development. 

 In this connection it is also of interest to note that in Pennaria 

 medusa buds appear only upon hydranths which are physiologically 

 relatively old, while the hydranth buds usually arise on the physi- 

 ologically younger regions of the stem. In other species of hydroids, 

 where the growth form is different, the physiological relations may 

 also prove to be more or less widely different, although medusa buds 

 in general arise in connection with a fully developed hydranth, or 

 a highly specialized reproductive zooid or from an apparently 

 specialized region of the stem just proximal to a hydranth, while 

 hydranth buds arise from less highly specialized regions. It is 

 probable that where such complicating factors as presence of the 

 perisarc or differences in motor activity do not obscure the differ- 

 ences in susceptibility associated with physiological age, similar 

 differences between the different forms of agamic reproduction 

 will be found in other species. 



To sum up, the susceptibility method indicates not only that a 

 considerable degree of rejuvenescence is associated with agamic 

 reproduction in Pennaria but also that different stages of rejuvenes- 

 cence are represented in the different forms of agamic reproduction 

 in this species. In the more specialized reproductive process the 

 young stages are apparently somewhat older physiologically than 

 in the less specialized process. 



REFERENCES 

 BAITSELL, G. A. 



1912. "Experiments on the Reproduction of Hypotrichous Infusoria: 

 I, Conjugation between Closely Related Individuals in Stylonychia 

 pustulata" Jour, of Exp. Zool., XIII. 



1914. "Experiments, etc.: II, A Study of the So-called Life Cycle in 

 Oxytricha fallax and Pleurotricha lanceolata" Jour, of Exp. ZooL, 

 XVI. 



CALKINS, G. N. 



19020. "Studies on the Life-History of Protozoa: I, The Life Cycle of 



Paramecium caudatum," Arch. f. Entwickelungsmech., XV. 

 19026. "Studies, etc.: Ill, The Six Hundred and Twentieth Generation 



of Paramecium" Biol. Bull., III. 

 1904. "Studies, etc.: IV, Death of the A-Series: Conclusions," Jour. 



of Exp. Zool., I. 



