788 A RULE OF PROPORTION OBSERVED IN THE SET^E OF CERTAIN NAIDID^. 



has thus a well-marked obliquity. Hence, though the free portion of the inner seta 

 is longer, it will not reach further forwards than the outer one. And, as a matter 

 of fact, the ends of all the setse of a bundle would appear naturally to lie in a 

 straight line approximately transverse to the long axis of the body,* the increasing 

 length of the distal segments of the setse towards the inner end of the bundle com- 

 pensating for the increasing obliquity of their disposition. In any case it is probably 

 the absolute length of the distal segment, rather than its length relative to that of 

 the entire seta, which is of importance to the animal (if indeed the peculiarity is of 

 importance). 



In the case of the anterior setae of Chsetog aster punjabensis, however, the arrange- 

 ment cannot have the effect described. The setae with the shortest free portion are 

 here situated at the inner and outer sides of the bundle, those with the longest in 

 the middle ; and the very considerable difference in the lengths must result in a 

 semicircular arrangement of the points of the setae, when these are put into action 

 against the substratum on which the animal rests. 



* The general disposition of the setal bundles in a number of the Enchytraeidae appears to be the same — the 

 points of the setae being in the same transverse line, and the obliquity of their direction increasing towards the inner 

 side of the bundle, though there is no nodulus (compare figs, of ventral bundles in species of Marionina and 

 Lumbricillus, in a paper by the writer : " On some littoral Oligochaeta of the Clyde," Trans. Roy. Soc. Edin., xlviii, 

 part i, text-figs. 2 and 3). 



