SUSCEPTIBILITY IN IIYUKOIDS. Ill 



the development of motor activity, except the very earliest. 

 In Pennaria the earliest visible stages show no distinct difference 

 in susceptibility from the hydranth body on which they arise, 

 but somewhat later stages (Fig. 4) are slightly more susceptible 

 than the hydranth -body, though distinctly less susceptible than 

 young hydranths of the stage of Fig. 7. Half grown buds are 

 almost always slightly less susceptible than the hydranth body 

 to which they are attached, and the still later stages are much 

 less susceptible than the hydranth and are the least susceptible 

 structures in the colony except the older parts of the stems. 



In Tubularia the gonophores, w r hich are modified medusae, 

 show a decrease in susceptibility from a certain early stage on, 

 and the later stages are less susceptible than the hydranth from 

 w r hich they arise. In Bougainmllea also, w^here the medusae 

 arise singly from special buds instead of from the hydranth body, 

 the differences in susceptibility with stage of development are 

 the same. 



The gonozooid of the campanularian species examined is in 

 general less susceptible than the fully developed hydranth, and 

 even the earlier stages of gonozooid development are less sus- 

 ceptible than early hydranth stages, though more susceptible 

 than late hydranths, in spite of the presence of perisarc. The 

 medusa buds on the blastostyle show differences in suscepti- 

 bility with developmental stage similar to those observed in the 

 medusa bud of Pennaria, the advanced stages being much less 

 susceptible than corresponding hydranth stages. 



The results with- medusa buds of different species are then 

 consistent. The medusa bud represents a reproductive pro- 

 cess with a certain amount of dedifferentiation in the earliest 

 stages, and in these stages some increase in susceptibility ap- 

 pears, as compared with the region from which the bud arises, 

 but as differentiation progresses, the susceptibility again de- 

 creases. The changes are similar to those in the hydranth, 

 except as the latter arc masked by the development of motor 

 activity in relatively early stages. The susceptibility of the 

 medusa bud, however, is in general lower than that of corre- 

 sponding stages of hydranth development, a fact which agrees 

 well with the greater differentiation of the medusa. In short, 



