CORRELATION BETWEEN METABOLIC GRADIENTS. 333 



the same colony; No. 16 of a proximal branch of the same colony; 

 and No. 19 of the distal part of the main axis and No. 21 of the 

 proximal part. Nos. 25, 26, and 28 are old portions of the colony 

 with many medusse buds; such parts show little or no apico-basal 

 potential difference. 



Obclia borcalis. This is a large much-branched colonial hydroid 

 common at Friday Harbor. Apical levels of entire colonies are 

 electronegative (galv.) to basal levels, as in Nos. i to 6 in Table III. ; 

 apical levels are negative to basal levels of branches, as in Nos. 1 1 

 to 17 in Table III.; and the main apical hydranth is negative to 

 the apical hydranth of lateral branches, as in Nos. 7 to 10. 



From these data on several species it is clear that in the colonial 

 hydroids, in general, apical levels of the colony are electronegative 

 (galv.) to basal levels, both as regards hydranths and stems. The 

 electrical gradient is steeper in apical regions and slight or lacking 

 in basal regions. These results correspond with the metabolic 

 gradients, regions or parts of higher metabolic rate being negative 

 to those of lower rate. 



3. Gak'anota.vis. The slight motility of the colonial hydroids 

 precludes the practicability of determining their galvanotactic re- 

 sponse. Experiments were, however, tried on Pcnnaria. Branches 

 were placed at right angles to the direction of the current. It 

 was noted that the larger and, more vigorous hydranths in such 

 branches, usually the main or lateral apical hydranths, turned the 

 manubrium toward the cathode during the passage of the current. 

 When the current was reversed the manubrium was turned to the 

 new cathode. The animals fatigue rapidly, however, and soon fail 

 to respond ; as already stated, only the largest hydranths respond. 

 This behavior, as far as it goes, corresponds with the theory of 

 galvanotaxis outlined above i.e., that part of the animal having 

 the highest metabolic rate and an internal positivity is directed 

 toward the cathode. 



V. HYDRO MEDUS/E. 



i. The Metabolic Gradient. The gradients of these medusae in 

 toxic solutions, dyes, and potassium permanganate have been de- 

 termined by Child ('2i&). In all species tested the manubrium 

 and margin, including the tentacles, are the most susceptible and 



