36 C. M. CHILD 



posterior to the third or fourth segment the susceptibility in- 

 creases again posteriorly to the gro\Ying region (p. 14). Here, 

 then, two opposed gradients are present in the body. Hyman 

 ('16) has shown that a similar condition exists in most oligochetes 

 from an early stage, and there is little doubt that the same is 

 true for other polychetes besides Arenicola. 



As Hyman has suggested, these two gradients are probably 

 somewhat different in character, but the susceptibility method 

 fails to show the difference. The gradient in the three-segmented 

 larva and in the region posterior to the third or fourth segment 

 in the young worm is undoubtedly a secondary developmental 

 or age gradient. Each segment arises earlier than the segment 

 next posterior to it and is therefore, at least for a time, more 

 advanced in development, more differentiated, physiologically 

 older and possesses a lower metabolic rate than the latter. Con- 

 sequently the metabolic rate decreases from the posterior grow- 

 ing region to the first body segment. Essentially the same re- 

 lations appear in the posterior zooids of Planaria (Child, '11, 

 '13). In the annelid, however, the development of the nervous 

 system and the extension of functional dominance from the head 

 posteriorly bring about integration of the segments into a com- 

 posite individual. With the differentiation of the cephalic 

 ganglia and the development of the sense organs the head-region 

 receives stimuli from its environment which increase its meta- 

 bolic activity, and, as nervous connection with the segments is 

 established, transmission to these becomes more and more effec- 

 tive, and this, together with the differentiation of the segmental 

 ganglia, brings about increased metabolic activity in the seg- 

 ments, but at the same time determines their subordination 

 to the head or to more anterior segments. Under these con- 

 ditions the most anterior segment being subject to the earliest, 

 most frequent and most intense excitation from the head-region 

 acquires a higher metabolic rate than the second, this a higher 

 rate than the third, and so on. As development progresses, 

 the length of body thus brought under nervous control undoubt- 

 edly increases, but a longer or shorter posterior region may still 

 retain the developmental gradient in at least some tissues of 



