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XXL — On a Rule of Proportion observed in the Setae of certain Naididae. 

 By J. Stephenson, M.B., D.Sc, Professor of Zoology, Government College, Lahore. 



(MS. received January 14, 1915. Eead February 15, 1915. Issued separately May 11, 1915.) 



The common forked setae which constitute the ventral setal bundles in the 

 Naididae possess, like the simple setae of many of the earthworms, a slight swelling 

 on the shaft, the so-called nodulus. The setae themselves are the principal organs of 

 locomotion, being used as levers, which obtain a hold of the substratum by means 

 of their hooked and forked distal ends ; and besides being movable in an antero- 

 posterior direction they are capable of some degree of protrusion and retraction. 

 The position of the nodulus on the shaft is held to regulate the distance to which the 

 seta can be protruded ; though it can be retracted so that the nodulus is below the 

 level of the epidermis, it cannot be protruded beyond that degree which brings the 

 nodulus level with the surface.* 



To the systematist, the setae are useful as providing criteria for the distinction of 

 genera and species ; and are, indeed, in the Naididae by far the most important organs 

 for this purpose, since the reproductive apparatus, which would probably furnish 

 equally valuable data, is known only in a minority of cases. An accurate description 

 of the setae forms, therefore, a chief part in any account of a member of this group. 



Among the characters of the setae which are used by systematists is the position 

 of the nodulus on the shaft, whether nearer the distal or the proximal end of the 

 shaft ; and also, descending to greater detail, the proportions between the segments 

 of the shaft respectively distal and proximal to the nodulus. Comparative observa- 

 tions on the position of the nodulus, therefore, may be of value both from a physio- 

 logical and a systematic point of view. 



Differences in the position of the nodulus in the ventral bundles of the anterior 

 and posterior segments respectively of the same animal are common, and have been 

 known and described for some time; e.g., to quote at random: — " Ceux (the setae) 

 des premiers segments ont le nodule un peu proximal, tandis qua partir du 6 me 

 segment il est franchement distal " (Nais communis, described by Piguet). But 

 differences in the position of the nodulus among the setae of a single bundle had not 

 attracted any attention till, in a paper published a few years ago,f I called attention 



* Of. Vejdovsky, System und Morphologie der Oligochaeten, 1884 :— "Es (the nodulus) ist offenbar eine Regulative 

 fur die Hervorstreckung eines bestimmtes Borstentheiles aus dem Leibesscblauche. Der nodulus steckt niimlich 

 bestandig in dem Integument, kann auch tief in die Leibeshdhle eingezogen werden, kommt aber niemals, oder nur 

 ausnabmsweise iiber die Korperoberflache heraus " (p. 74). 



t "Studies on the Aquatic Oligochseta of the Punjab," Bee. Indian Museum, vol. v, pt. i, 1910. 

 TRANS. ROY. SOC. EDIN., VOL. L, PART IV (NO. 21). 109 



