364 LIBBIE H. HYMAN. 



sorts of irregular results are obtained and it is not surprising 

 that under such circumstances, the experimenter would be lead 

 to question the existence of axial differences along the stem of 

 Tubularia. When the correct method of cutting the apical 

 pieces is observed, namely, when the hydranth, its stalk, and 

 the first millimeter or two of the stem are discarded, then there is 

 not the slightest question that the apical halves of stems prepared 

 in this manner give rise to oral hydranths very much in advance 

 of the basal halves. 



The experiments presented in this paper were performed at 

 Woods Hole in June and in December, 1919. I am greatly 

 indebted to Professor F. R. Lillie for a research room at the 

 Marine Biological Laboratory on both of these occasions and 

 for a grant from the departmental funds covering the expenses 

 of the December trip. I am also indebted to Professor C. M. 

 Child for advice and suggestions throughout the course of the 

 work. 



B. SUSCEPTIBILITY GRADIENTS IN TUBULARIA. 



The death gradients in lethal concentrations of various sub- 

 stances have already been described by Child ('19^) for a number 

 of hydroids including Tubularia. I have repeated and confirmed 

 these observations for a number of forms. In Tubularia, the 

 disintegration (i/ioo to 1/400 mol.KNC) begins at the tips of 

 the proximal tentacles and proceeds down these tentacles to 

 their bases; soon after the proximal tentacles have begun to 

 disintegrate, the process is initiated in the distal tentacles and 

 proceeds to their bases. At about the same time as the tips of 

 distal tentacles begin to die, the mouth region disintegrates and 

 the disintegration extends along the body of the hydranth to its 

 base. In many cases, it was observed that the outer surface of 

 the proximal tentacles (i.e., the surface that contains the most 

 nematocysts) preceded in disintegration the inner surface. In 

 the stalk of the hydranth there is a specialized region bearing a 

 ring of nematocysts; in many cases it was noted that this region 

 disintegrated early and without any definite relation to the 

 progress of disintegration in other parts of the hydranths, as was 

 also noted by Child. Such specialized regions, owing to func- 

 tional activity, are commonly highly susceptible to toxic agents, 



