PROPORTION OBSERVED IN THE SET^E OF CERTAIN NAIDID^}. 787 



The above observations represent all that I have hitherto made on this subject, 

 with the exception of a number of specimens of Chsetogaster punjabensis, to be 

 recounted below. They show — 



(1) That in a number of the Naididae the position of the nodulus varies in a 

 regular manner in the setae of each bundle ; so that, both relatively and absolutely, 

 the segment of the seta distal to the nodulus is longest in the innermost seta, and 

 becomes progressively shorter towards the outermost seta of the bundle (Aulophorus 

 Stephensoni, A.furcatus, Dero limosa, Nais raviensis, Haemonais laurentii) ; 



(2) That in another group of species this relation, though distinguishable, is less 

 well marked (Nais communis, Naidium minutum) ; while 



(3) In a third group it is scarcely recognisable at all (Pristina aequiseta). 

 Though the observations are not very numerous, they show that the peculiarity 



is not a generic character. They indicate, moreover, that in describing the setae of 

 the Naididae it is not sufficient to indicate the position of the nodulus in general 

 terms, or from an examination of one or two setae only ; since even in the same 

 bundle its position may be proximal to the middle of the length of the shaft in one 

 and markedly distal in another seta. 



The exception above referred to, Chsetogaster punjabensis, is characterised by a 

 different type of relation. Here the distal segment of the seta is longest in the 

 middle setae of the bundle, or at any rate not in the end setae ; and the length of 

 the distal segment decreases from the setae where it is longest towards both the 

 inner and outer sides of the bundle. Thus in the bundles of segment ii the lengths 

 of the distal segments, from inner to outer side, were, in m : — 40, 54, 59, 75, 77, 69, 

 59 ; and on the other side :— 40, 56, 60, 75, 75, 65, 51. 



In the posterior segments this peculiarity, though present, was less marked : — 

 48, 50, 50, 50, 44, 37 ; and in another segment :— 48, 50, 50, 44, 42, 34. And it may 

 even happen that the condition becomes that previously described for Dero, etc. : — 

 50, 50, 48, 46, 37. 



Have these peculiarities any meaning ? With regard to the one first described, 

 and illustrated by Dero, etc. : supposing all the setae of a bundle to be extended to 

 their maximum, so that the nodulus in each case comes level with the surface ; the 

 setae being all directed forwards, the hook of the innermost seta would reach a little 

 further forwards than that of the next, and so on to the seta at the outer side, which 

 reaches forwards the shortest distance. The effect would be that in locomotion the 

 forwardly extended setae would grip the substratum at different levels. 



This does not actually seem to happen ; and a reference to text-figure 1 will 

 show the reason. The setae are not arranged parallel to each other in a bundle, but 

 diverge like the ribs of a fan, at least when in action. From successful preparations 

 by the method previously described it appears that in such a fan-shaped bundle the 

 outermost seta, laid out flat, points approximately straight forwards, while the 

 others incline more and more towards the middle line ; so that the innermost seta 

 TRANS. ROY. SOC. EDIN., VOL. L, PART IV* (NO. 21). 110 



