THE AXIAL GRADIENTS IN HYDROZOA. 



C. M. CHILD, 

 FROM THE HULL ZOOLOGICAL LABORATORY, UNIVERSITY OF CHICAGO. 



II. SUSCEPTIBILITY IN RELATION TO PHYSIOLOGICAL AXES, 



REGIONS OF COLONY AND STAGES OF DEVELOPMENT 



IN CERTAIN HYDROIDS. 



The present paper is a study of the susceptibility relations in 

 certain species of colonial hydroids and includes data bearing 

 upon the following points: axial susceptibility in single hy- 

 dranths, medusa buds and stems; susceptibility in relation to 

 stage of development, physiological age and motility; regional 

 differences in susceptibility in colonies and their relation to 

 growth form and general physiological condition; susceptibility 

 in acclimation. These data have been accumulated during some 

 five years past, in part at Woods Hole and in part at Friday 

 Harbor, Wash. The forms chiefly studied are Pennaria tiarella 

 and Tubularia crocea at Woods Hole and Bougainvillea sp.,- 

 Obelia geniculata and O. borealis at Friday Harbor. Some work 

 was also done with a third unidentified species of Obelia and with 

 another campanularian, Gonotliyria clarkii, also at Friday Harbor. 

 I am indebted to Professor Trevor Kincaid and to Professor C. C. 

 Nutting for identification of certain of the Friday Harbor species. 



For the direct determination of susceptibility to slowly lethal 

 concentrations of agents the method is the same as that used for 

 Hydra (Child and Hyman, '19) and many other forms, viz., 

 the determination of the time of death as indicated by the dis- 

 integration or cytolysis of the protoplasm in KNC, HC1, various 

 anesthetics, neutral red, methylene blue, or in some cases by 

 the time before decoloration in KNC or other neutral or alkaline 

 agents after staining with neutral red. This decoloration occurs 

 only when the protoplasmic structure disintegrates: the first 

 stage is a change in color of the neutral red to yellow as the sea 

 water or the alkaline agent penetrates all parts of the protoplasm, 



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