208 



C. M. CHILD AND L. H. HYMAN. 



more or less specific agent for nervous tissue in higher animals 

 and the extreme irritation produced by Janus green indicates 

 that it also affects this mechanism in some way. If this sugges- 

 tion is correct, the basipetal course, first of excitation and second 

 of paralysis, may indicate regional gradation in susceptibility in 

 the receptive or nervous apparatus of hydra. Moreover, on this 

 basis the differences between the three species suggest that this 

 apparatus shows a higher degree of regional specialization in 

 H. oligactis than in the other two species, and that H. viridissima 

 is at least somewhat more sensitive or excitable than H. vulgaris. 



SUSCEPTIBILITY IN RELATION TO UNSYMMETRICAL 



CONTRACTION. 



Usually all the tentacles of an individual behave in much the 

 same manner in a given agent, but sometimes one or more ten- 

 tacles contract more strongly than the rest. In such cases the 

 more strongly contracted tentacle is usually more susceptible and 

 disintegrates earlier than those which do not contract. 



In cases also where the body becomes strongly bent and retains 



H 



'- ."V ' '-J-' V 

 f :' .'"''. 



^"^:^ 

 .-'f-^v-- 

 ('&?& 



77 



IjM 



; ? r.4 



^^ 



\ 



80 



8l 



that position, disintegration occurs earlier on the more contracted 

 than on the less contracted side. Fig. 75 shows a case of this 



