THE AXIAL GRADIENTS IN HYDROZOA. 187 



distinguishable as a region of smaller diameter than the body 

 (Fig. 5) and in extreme contraction the body is not spherical 

 but more or less urn-shaped with a somewhat flattened base, 

 which represents the contracted stalk. Buds in H. vulgaris arise 

 at or near the base of the body proper, and in the state of extreme 

 contraction the position of the bud is at or near the border of the 

 flattened base as indicated in Fig. 6. Since the bud serves as a 

 landmark indicating approximately the boundary between body 

 and stalk its position in Fig. 6 shows that there is in this con- 

 tracted state a relatively greater decrease in length of stalk than 

 of body. 



Hydra oligactis shows a greater degree of differentiation of stalk 

 and body than the other two species. In well extended animals 

 in good nutritive condition the column is very distinctly marked 

 off into two regions, the body proper, including the apical half 

 or less of the column, with a larger diameter and a thick opaque 

 entoderm, and the stalk, comprising the basal half or more of 

 the column, with a smaller diameter very commonly only about 

 half that of the body and a thin, highly transparent entoderm 

 (Fig. 7). Moreover, the transition from body to stalk is in this 

 form much more abrupt than in the other species. Fig. 8 shows 

 a condition of moderate contraction, Figs. 9 and 10 greater de- 

 grees of contraction and Fig. n a state of extreme contraction 

 induced by chemical stimulation. Since in H. oligactis as in H. 

 vulgaris buds arise at or near the base of the body, the position 

 of the bud in Fig. 1 1 indicates approximately the boundary be- 

 tween body and stalk and shows the greater contractility of the 

 stalk. 



The differences in diameter, general appearance and opacity 

 between body and stalk become less marked with lack of food 

 and in advanced starvation may almost or entirely disappear 

 even in H. oligactis, and with less extreme starvation in H. vul- 

 garis. On the other hand, individuals of H. vulgaris are not in- 

 frequently seen in which the entoderm of the digestive region is 

 so thickened, apparently by nutritive reserves, that the form 

 approaches that of H. oligactis, and occasionally H. viridissima 

 shows some modification in this direction. But while the shape 

 of the animal may vary widely with nutritive conditions, the 



