METABOLIC GRADIENT IN ANNELIDS 13 



2. The electrical gradient. The electrical gradient of freshly 

 collected Nereis and its galvanotactic reaction are the same as 

 those already described for oligochaetes. The anterior and pos- 

 terior ends are electronegative (galvanometrically) to the middle 

 region. When placed in an electric current, the worms bend into 

 a U-form, anterior and posterior ends directed toward the cathode 

 and middle toward the anode. 



From these lines of evidence it may be expected that the 

 oxygen consumption of different levels will vary in the manner 

 already described for Lumbriculus. Posterior levels should 

 consume the most oxygen, anterior less, and middle pieces the 

 least. Experiment proves that such is the case. 



3. Material and method. Two species were employed. Nereis 

 virens Sars and Nereis vexillosa Grube. The former species 

 was collected from the docks at the town of Friday Harbor; in 

 such locations the worms are found among the Mytilus, tuni- 

 cates, and similar forms which cover the piles. A few individuals 

 of this species were also collected along the shores. The worms 

 used were about 150 mm. long. Nereis vexillosa was obtained by 

 digging in gravelly and sandy shores at low tide. The individuals 

 used in the experiments were from 160 to 180 mm. in length. 

 Sexually mature individuals were discarded. The worms were 

 used as soon as possible after collection — from half an hour to 

 three or four hour sin nearly all cases. In two cases, experi- 

 ments 6 and 7, table 3, the worms were kept about twenty hours 

 before being utilized. 



In preparing the worms for the experiments, the first 25 mm. 

 and the last 10 to 15 mm. of the body were cut off and discarded. 

 Three pieces were then cut from the body of the worm, short 

 pieces being discarded between the first and second and second 

 and third pieces. The way in which the pieces were taken from 

 the worm is shown in figure 3. The pieces were 30 to 40 mm. in 

 length. In each experiment several worms were employed, 

 generally six or seven-, once five, and twice twelve. After cutting, 

 the pieces were placed in finger-bowls and washed in several 

 changes of sea-water until bleeding had ceased and the cut 

 ends contracted. All of the pieces of each level were placed to- 



