52 C. M. CHILD. 



ference in pulsation rate in itself indicates a very marked decrease 

 in metabolic rate with advancing age. Mayer ('06) has noted 

 that in the scyphomedusa Cassiopeae the rate of pulsation de- 

 creased with advancing age. 



Young and old animals also differ in their behavior in another 

 way. Under the usual conditions periods consisting of a larger 

 or smaller number of pulsations alternate with periods of rest. 

 The total length of pulsation periods as compared with rest 

 periods decreases with advancing age; in other words the younger 

 animal spends a larger portion of the time than the older in 

 rhythmic pulsation. Pulsation periods may either be longer or 

 more frequent or both in the younger animal than in the older. 

 In JEqiiorea this difference is perhaps even more strongly marked 

 than in other forms observed. In the large, sexually mature 

 animals 60-80 mm. in diameter, when under natural and so far 

 as possible constant conditions, the periods of quiescence are 

 often one or two minutes long, sometimes even longer, the animal 

 drifting passively during this time without a single pulsation. 

 Following such a quiescent period a single pulsation or a 

 pulsation period may occur. Under the same conditions the 

 quiescent periods in young animals 15-20 mm. in diameter are 

 usually very much shorter, commonly only a few seconds and 

 the pulsation periods are usually longer than in the old animal. 

 The young animal is then more continuously active and gives 

 the impression of a much greater degree of "spontaneity," since 

 it is usually impossible to distinguish any external exciting factor 

 responsible for the beginning of pulsation after a period of quies- 

 cence. In the other species observed similar differences between 

 young and old exist, but periods of quiescence are usually shorter 

 than in JEquorea. 



The irritability of the young animal as indicated by the effec- 

 tiveness of direct mechanical stimulation in inducing pulsation 

 during a quiescent period is distinctly greater than that of the 

 old. Usually the quiescent animal in the earliest stages of 

 gonad-development responds by pulsation to the slightest touch 

 in the marginal region, while the sexually mature animals com- 

 monly respond only to much more intense mechanical action. 



These differences between younger and older medusae show of 



