HYDROIDS 189 



plished in the stalk, where longitudinal transmission is 

 the rule. 



The predominance in the stalk of longitudinal as com- 

 pared with diffuse transmission makes localization a sig- 

 nificant feature in the responses of this part. If a faradic 

 stimulus is applied to one side of the stalk next the hy- 

 dranth or next the base, the stalk simply shortens as a 

 whole. If, however, the stimulus is applied on one side 

 of the stalk nearer the middle of 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 stimu- 

 lated point on that side. The significance of these re- 

 sponses to localized stimulation was often observed in 

 the stock aquarium. This contained by accident a num- 

 ber of small nudibranch gastropods, which were found 

 to feed on the substance of Corymorpha. When one at- 

 tacked a Corymorpha, it began near the base of the stalk 

 where the hydroid rose from the mud and as soon as it 

 started to nibble the stalk on a given side the Cory- 

 morpha responded by applying to the point of attack 

 the hydranth, the tentacles of which were extremely stim- 

 ulating to the nudibranch and usually drove off the in- 

 truder. The success of this form of protective response 

 naturally depended upon the accuracy of the localization. 



To ascertain whether transverse transmission is at 

 all significant in the stalk of Corymorpha, the following 

 experiment was tried. The stalk of a polyp was cut trans- 

 versely halfway through at a point midway its length and 

 the polyp was then allowed to come to rest in a vertical 

 position. On stimulating locally below the wound and on 

 the side away from it, the hydranth, as might have been 



