ON H^EMONAIS LAURENTII. 77 \ 



to four. They are very variable, even in the same bundle, but may be divided into 

 two groups, anterior and posterior. A similar distinction can be made in certain other 

 genera, e.g. Nais, where the ventral setse of segments ii-v differ in character from 

 those behind ; in this case, however, the chaage occurs much further back, and is 

 gradual rather than sudden. 



In the anterior part of the body the setae (text-fig. 1, a) vary considerably in 

 length — 80, 98, or 104 m, but are all comparatively slender, 3 v- ; the distal prong is 

 slightly longer than the proximal, and about half as thick as the latter at its base ; 

 the angle between the prongs is neither markedly wide nor narrow. The nodulus is 

 usually proximal to the middle of the shaft, but its exact position is variable even in 

 the same bundle ; thus along with setse in which the proximal segment of the shaft 

 bears to the distal the ratio 3 : 4, there may be one with the ratio 1 : 2, while in 

 some cases the nodulus is only slightly proximal, or is actually at the middle. 



There is no sudden change in the character of the setse in passing backwards. In 

 segment viii, in one instance, the length was 104 m, the nodulus was at the middle of 

 the shaft, and the prongs had the above relations. In segment x the length was 

 105—1 10 m, prongs as above, nodulus at the middle or slightly proximal. In segment 

 xv, however, one seta of a bundle of four had different characters : it was shorter, 

 78 m ; noticeably stouter, 4 m ; the proximal prong was both longer and thicker, l£ as 

 long and twice as thick as the distal, and the nodulus was distal to the middle of the 

 shaft (proximal segment : distal segment : : 7 : 5). Another seta in this segment 

 presented intermediate characters — length 90 m, shaft intermediate in thickness, 

 prongs of equal length, proximal prong twice as thick as distal, nodulus distal to 

 middle (proximal : distal : : 4 : 3). 



Behind this the setae were all of the stout type (text-fig. 1, 6), e.g. in segment xvii 

 — length 80-96 m, breadth 4-4*5 m, prongs equal in length or proximal slightly longer, 

 proximal prong more than twice as thick as the distal at its base, nodulus distal, but 

 its position variable (proximal segment : distal segment : : 5 : 4 or 3 : 2) ; there may 

 be a slight " webbing " between the bases of the prongs. 



A marked variation — the absence of the distal prong — occurs sometimes in the 

 ventral as in the dorsal setae. In some cases a slight " shoulder " on the shaft 

 represents, as in the dorsal setae, its normal place of origin ; in other cases there is no 

 indication of a second prong, and the seta ends distally in a simple stout hook 

 (text-fig. 1, c). 



Attention may be drawn to the fact that the variations in the position of the 

 nodulus are not haphazard, but follow a definite rule. Careful preparations will 

 show the setal bundles in most cases laid out flat, with the setae in their natural order ; 

 and in such it can be seen that the nodulus is most distally situated in the outermost 

 seta, most proximally in the innermost, and that there is a progressive change in the 

 ratio between proximal and distal segments of the shaft as we pass from the outer to 

 the inner seta ; so that while this ratio may be 2 : 1 in the outer, it is perhaps 4 : 3 



