316 G. H. PARKER 



hydranth instead of through the base. In this experiment 

 transverse transmission through the hydranth was as easily 

 demonstrated as it had been through the base. 



In a third experiment the stalk alone was used, the hydranth 

 and base having been cut off. When one arm of such a prepara- 

 tion was stimulated the other arm almost invariably failed to 

 respond thus showing that though transverse transmission is 

 easily accomplished in the base or in the hydranth, it is not so 

 easily accomplished in the stalk, where longitudinal transmission 

 is the rule. 



The predominance in the stalk of longitudinal as compared 

 with diffuse transmission makes localization a significant feature 

 in the responses of this part. If a faradic stimulus is applied 

 to one side of the stalk next the hydranth or next the base, the 

 stalk simply shortens as a whole. If, however, the stimulus 

 is applied on one side of the stalk midway its length the stalk 

 bends to that side and usually presses the hydranth with great 

 accuracy against the stimulated spot. This response is not 

 only appropriate for the particular side stimulated but also in 

 most cases for the given level of the stimulated point on that 

 side. The significance of these responses to localized stimula- 

 tion was often observed in the stock aquarium. This contained 

 by accident a number of small nudibranch gastropods, which 

 were found to feed on the substance of Corymorpha. When one 

 attacked a Corymorpha, it began at the base of the stalk where 

 the hydroid rose from the mud and as soon as it started to nib- 

 ble the stalk on a given side the Corymorpha responded- by 

 applying to the point of attack the hydranth, the tentacles of 

 which were extremely stimulating to the nudibranch and usually 

 drove off the intruder. The success of this form of protec- 

 tive response naturally depended upon the accuracy of the 

 localization. 



To ascertain whether transverse transmission is at all signifi- 

 cant in the stalk of Corymorpha, the following experiment 

 was tried. The stalk of a polyp was cut transversely halfway 

 through at a point midway its length and the polyp was then 

 allowed to come to rest in a vertical position. On stimula- 



