IO8 C. M. CHILD. 



i KREXCES ix SUSCEPTIBILITY WITH STAGE OF DEVELOP- 

 MENT. 



The appearance of motor activity during hydranth develop- 

 ment affects susceptibility to some extent as in the case of 

 Hydra (Child and Hyman, '19). In general motor activity in- 

 creases susceptibility, as numerous experiments, not only with 

 Hydra and hydroids, but with other forms have shown. The 

 effect of motor activity on susceptibility in Planaria has long 

 been used as an experiment in laboratory class work. The 

 earlier stages of hydranth development are non-motile, the later 

 stages motile, e.g., in Pennaria the stage of Fig. 7 shows no motor 

 activity, the stage of Fig. 8 is slightly motile and the stages 

 a and c in Fig. 9 are highly motile. In spite of this difference 

 the early stage of Fig. 7 is distinctly more susceptible than Fig. 

 9, a, but betw r een the stages of Fig. 7, Fig. 8 and Fig. 9, c the dif- 

 ferences in susceptibility are not very great. In stages after 

 motility is fully developed the susceptibility shows in general 

 a decrease with increasing size of the hydranths on a particular 

 branch or in a particular region of the colony, except that in 

 monopodial forms such as Pennaria and Bougainvillea certain 

 peculiar conditions exist in the terminal hydranths of the colony 

 and main branches (see p. 114 below). 



The effect of motor activity on susceptibility is also evident 

 in the susceptibility of the tentacles, as compared with that of 

 the hydranth body in different developmental stages. In the 

 early stages, e.g., Fig. 7, the tentacles are non-motile and disin- 

 tegrate at about the same time as the region of the body to which 

 they are attached, but after they develop motility, they are tin- 

 most susceptible portions of tin- hydranth, as noted above, and 

 in the forms with two series the proximal u-ntacles are more 

 susceptible than the distal. In Pennaria tin- proximal tentacles 

 of the advanced stage, Fig. 9, a, begin to disintegrate at about 

 the same time or a little later than those of the early stage, Fig. 

 7, but at this time the body of the early stage is disintegrated 

 down to the level of the proximal tentacles, while the body of 

 stage a, Fig. 9 does not begin to disintegrate until later, the actual 

 limes depending on concentration of the agent used. 



In all other forms examined, Tnbitlaria, Bougainvillea, the 



