THE AXIAL GRADIENTS IN HYDROZOA. 407 



istics may, however, result from the low temperature of the water, 

 which did not, on the warmest summer days, exceed 15 C. The 

 species is very favorable for experimental work, owing to the 

 long unbranched stems and their freedom from other growths. 



For the oxygen consumption large colonies were collected and 

 from these a number of long straight clean unbranched stems 

 were selected. They were carefully examined under the micro- 

 scope to determine that they were free from diatoms or other 

 organisms. In some cases a few such organisms were seen grow- 

 ing on the perisarc but it was found that these could be removed 

 by gently brushing the stem with a camel's hair brush. The 

 hydranth and upper two or three millimeters and the basal part 

 of the stem were then cut off, leaving a clean stem twenty to 

 thirty millimeters long. This was then cut into two pieces, the 

 basal piece generally taken a little longer than the apical piece 

 to compensate for its smaller diameter. Six to eight such pieces 

 were used in each experiment and an attempt was made to select 

 for each experiment stems of similar diameter. All stems used 

 in any one experiment came from the same colony. 



The method of determining the oxygen consumption was the 

 same as previously employed in a similar study on Grantia 

 (Hyman, '25). For details this paper should be consulted. 

 Briefly small tubes detachable into two sections are employed. 

 At the end of the experiment the pieces of stem are brought by 

 gravity into one section, which is then removed ; the other section 

 is analyzed for oxygen content. In each experiment four tubes 

 are employed: one containing the six to eight apical pieces of 

 stem; one containing the basal pieces of the same stems; and 

 two water blanks. 



To compare the rate of oxygen consumption of two objects, 

 it is necessary to know the quantity of protoplasm in each. 

 Owing to the lack of a balance at Swan's Island, the weights 







of the pieces could not be determined. There would be some 

 difficulty in determining the weight, owing to the presence of 

 the lifeless perisarc around the stems; but some method could 

 probably be devised to eliminate this difficulty. Under the 

 circumstances, however, I was compelled to use the volume of 

 the pieces as a standard of comparison. After each experiment 



