14 LIBBIE H. HYMAN AND ALBERT E. GALIGHER 



gether in a wide-mouthed bottle and their rate of oxygen consump- 

 tion was then determined as already described for Lumbriculus. 

 The tests ran from one to two hours. The pieces were then 

 dried on filter-paper and weighed. Six or seven such pieces 

 weigh from 2 to 10 grams; anterior and middle pieces weigh more 

 than posterior pieces. 



It may be pointed out that such pieces from different levels 

 are nearly identical morphologically. The only differentiated 

 part of the digestive tract in Nereis is at the anterior end, and 

 this part of the body was discarded. The posterior end with its 

 terminal cirri was likewise discarded. The only differences 

 among such pieces are differences in the proportions of parts, 

 as in the parapodia, which are largest in the middle regions of 

 the body. The middle pieces probably have the advantage of 

 greater surface of exposure, but, as will be seen, they nevertheless 

 respire the least. It should also be stated that there was very 

 little movement among these pieces. 



4. Experimental results. The experiments on Nereis virens are 

 recorded in table 2; those on Nereis vexillosa in table 3. Only 

 the final calculations of the cubic centimeters of oxygen consumed 

 per gram per hour are given. In the first two experiments on 

 Nereis virens, the posterior pieces were not tested. 



Examination of these tables shows that anterior and posterior 

 pieces of Nereis invariably consume more oxygen per unit weight 

 per unit time than middle pieces. Further, in nearly all cases 

 the posterior pieces have a higher rate of oxygen consumption 

 than the anterior pieces. There is one exception to this, experi- 

 ment 5 in table 3, where the oxygen consumption of the pos- 

 terior pieces is below that of the anterior pieces. In experiments 

 5 and 7, table 2, the posterior pieces are but slightly in advance 

 of the anterior pieces, but in the other experiments the difference 

 is marked. These variations in the experimental results are no 

 doubt due to physiological differences in the individuals employed. 



The existence of a metabolic gradient along the axis of Nereis 

 may therefore be regarded as demonstrated. This gradient is 

 identical with the susceptibility and electrical gradients. 



