THE AXIAL GRADIENTS IN HYDROZOA. 369 



perfect condition only were employed. They were placed across 

 the filter paper ends of the electrodes and the reading of the 

 galvanometer recorded. The stem was then reversed on the 

 electrodes and the reading again taken. The difference in the 

 two readings, particularly the difference in the direction of 

 swing of the indicator on the scale gives the desired information 

 about the electrical condition of the stems. Each reading was 

 repeated once, sometimes twice. When dead organisms are 

 tested in this way, the galvanometer gives the same reading 

 regardless of the position of the material on the electrodes. 



The galvanometric readings obtained on Tiibularia are recorded 

 in Table II. In connection with this table it is necessary to 

 explain that the scale of the galvanometer used is printed in 

 red on the right side of the zero point and in black on the left 

 side. When the right electrode, which is connected with the 

 right hand binding post of the galvanometer, is negative, the 

 indicator swings to the right of the zero point and hence reads 

 on the red half of the scale; when the left electrode is negative, 

 the indicator reads on the left hand or black side of the scale. 

 In some cases, both readings may be on the same side of the zero 

 point but one farther to the right or left than the other. Left 

 and right refer, of course, to the hands of the observer as he sits 

 facing the instrument. 



In the table, each number refers to one individual and all of 

 the data given under that number were obtained on one indi- 

 vidual. In Table II., a, are recorded the readings of the gal- 

 vanometer when the hydranth is compared with nearby portions 

 of the stem or with distant portions or with regions where 

 branches are present. Table II., b, gives the readings when 

 distal portions of the stem are compared with more proximal 

 regions or with far proximal regions or with proximal regions 

 bearing branches. In each table, the first column gives the 

 number of the individual, the second describes the material, 

 the third gives the reading in one position between regions not 

 very far separated, the fourth column the readings for the same 

 regions in the reversed position on the electrodes, the fifth and 

 sixth are the same as the third and fourth except that they 

 compare regions more widely separated or distal regions with 

 levels bearing branches. 



