METABOLIC GRADIENT IN ANNELIDS O 



ences. This prediction has been verified in the most satisfactory 

 manner. 



The experiments reported in this paper concern the rate of 

 oxygen consumption per unit weight of pieces from anterior, 

 middle and posterior levels of the polychaetes Nereis virens and 

 Nereis vexillosa, and the oligochaete Lumbriculus inconstans. 

 The experiments on Nereis were performed by the junior author at 

 the Puget Sound Biological Station, Friday Harbor, Washington, 

 in the summer of 1920. Those on Lumbriculus were performed by 

 the senior author at the University of Chicago in the fall 

 of 1920. 



EXPERIMENTS ON LUMBRICULUS 



1 . The susceptibility gradient of oligochaetes. The suscepti- 

 bility gradients of a number of oligochaetes including Lumbriculus 

 inconstans were described in detail by the senior author in a 

 former paper (Hyam,'16). It was found that in oligochaetes in 

 general a double gradient is present — that is, the anterior and 

 posterior ends are the most susceptible to toxic solutions and this 

 susceptibility decreases toward the middle. The gradient in these 

 forms may then be described as of the shape of the letter U, 

 except that the two limbs of the U are of unequal length. In 

 some species the anterior end is more susceptible than the posterior 

 end, in which case the left-hand limb of the U would be longer, 

 while in other species the posterior end is more susceptible, making 

 the right-hand limb of the U longer. 



In Lumbriculus inconstans the posterior end of the body is gen- 

 erally the most susceptible part. In some individuals the head is 

 about equally susceptible, but usually is less so. The suscep- 

 tibility decreases from both ends toward the middle of the worm, 

 the least susceptible part being anterior to the middle, generally 

 in about the second quarter of the worm's length. These rela- 

 tions are subject to considerable variation in different individuals. 

 In the posterior part of the body there are further present in 

 many individuals physiological indications of the existence of 

 a second zooid. Lumbriculus reproduces asexually by autotomy. 

 It may break in pieces at any level, but very commonly breaks 



