PHYSIOLOGICAL SENESCENCE IN HYDROMEDUS^. 



C. M. CHILD. 



The following observations on senescence in the hydromedusse 

 were made at the Puget Sound Marine Station, Friday Harbor, 

 Wash., during the summer of 1917. I wish to acknowledge my 

 obligation to the director of the laboratory, Dr. Frye, for the use 

 of a room and the privileges of the station, and for his kindness 

 in providing special glassware and other equipment. To other 

 members of the staff I am also deeply indebted for many cour- 

 tesies. 



The great abundance, variety and large size of hydromedusae 

 at Friday Harbor makes this an extremely favorable locality for 

 work on these forms. During the summer of 1917 five species 

 of hydromedusse, Phialidium gregarium, ^Eqiwrea ccerulescens, 

 Sarsia rosaria, Mitrocoma discoidea and Stomatoca atra could 

 readily be obtained in considerable numbers, often by the hun- 

 dred, from the laboratory float, and five other species of other 

 genera were more or less abundant, besides two species of siphono- 

 phores. The first four species named above furnished the ma- 

 terial for the observations and experiments on ageing, and the 

 results are essentially the same in all. 



The work was done entirely on free-swimming medusae and is 

 essentially a comparison of younger and older individuals, no 

 attempt having been made to obtain the medusa-bud stages 

 before detachment from the hydroid. 1 In the selection of indi- 

 viduals of different age for experiment, size of the animal and 

 degree of development of the gonads were used as superficial 

 criteria of age. As might be expected, these two criteria are in 

 general in close agreement, for since the range in variation in 

 size of the medusae-bud at a given stage of development is not 

 very great, a larger medusa must in general have undergone a 

 greater amount of growth than a smaller, and may therefore be 



1 In Pennanria a progressive change in physiological condition has been observed 

 from earlier to later stages of medusa-buds (Child, '15, pp. 150-152). 



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