THE AXIAL GRADIENTS IN HYDROZOA. 185 



sion and contraction than those of H. viridissima and H. vulgaris, 

 which are much alike. The chief differences, however, are in the 

 column. In all three species the basal region of the column in- 

 cluding a third to a half the body length in ordinary degrees of 

 extension is more extensile and contractile than the distal half 

 or two thirds. In other words, all three species show at least a 

 functional specialization of a more highly contractile basal re- 

 gion and a less highly contractile digestive region. Since this 

 specialization is particularly significant in relation to suscepti- 

 bility it will be convenient for present purposes to distinguish 

 the basal region as the "stalk" from the digestive region or 

 "body" in the stricter sense. 



In H. viridissima this specialization is not usually accompanied 

 by any marked degree of structural differentiation. The column 

 of the fully extended animal is cylindrical and of practically 

 equal diameter throughout its length (Fig. i), 1 although there is 

 usually a graded decrease in both the amount of nutritive sub- 

 stances and food reserves in the entodermal cells and of the sym- 

 biotic algae toward the base of the column and consequently an 

 increase in transparency in the same direction. In the various 

 stages of contraction this basal region usually undergoes a greater 

 degree of shortening than the more apical regions but is not 

 marked off by any characteristic difference in form from the rest 

 of the column (Figs. 2, 3). In extreme contraction such as is 

 induced for example by strong chemical stimuli, the column may 

 become spherical, or nearly so (Fig. 3). 



In H. vulgaris a somewhat greater degree of differentiation of 

 the stalk usually appears. In fully extended animals in good nu- 

 tritive condition, the basal third more or less of the column is 

 distinguishable not only by its thinner wall and greater trans- 

 parency, the transition from the body to stalk being more ab- 

 rupt than in H. viridissima, but the stalk is somewhat more 

 slender than the body which often shows a slight increase in 

 diameter basipetally to the beginning of the stalk region (Fig. 4). 

 In a moderately contracted condition the stalk is likewise usually 



1 H. viridissima is of course much smaller than H. vulgaris and H. oligactis, 

 but for conveneince in comparison all figures are drawn to such scale that corre- 

 sponding stages of development of the three species are approximately of the same 

 size. 



