TITBIFEX. 313 



(6) Shorter setae of a curved form, something like 

 an elongated S with a thickening at about the middle — 

 sigmoid setae. 



The uncinate and pectinate setae of Tubifex 

 rivulorum belong to this latter class. Of these, the 

 uncinate form is much more common than the pectinate. 

 They are both of the same general shape, but the 

 thickening referred to by Beddard is not very strongly 

 developed. Neither does it occur at the middle of the 

 seta, but is always nearer the outer free end and approxi- 

 mately divides the seta into a distal third and a proximal 

 two-thirds (PL II, fig. 6, a) In the uncinate setae the 

 distal end is bifid, but the two prongs vary somewhat in 

 size and shape in different setae. The uncinate setae in 

 the dorsal bundles have the two prongs equal in size and 

 of the same shape, that is, somewhat narrow throughout 

 and tapering gradually to a very fine point at the distal 

 end (PI. II, fig. 6, 6). In the ventral bundles, where 

 the uncinate setae only are present, the two prongs are of 

 slightly different size and shape. The dorsal one is 

 somewhat slender, and comes to a sharp point at the tip. 

 The ventral one is shorter than the dorsal, and of a 

 blunter nature, its apex being somewhat more rounded 

 (PI. II, fig. 6, a). There is never, however, such a 

 marked difference in the size of the prongs as in the case 

 of Limnodrilus, where the ventral one is always much 

 shorter than the dorsal. 



The number of uncinate setae which may be found in 

 a bundle differs somewhat in different regions of the body. 

 For instance, in both dorsal and ventral bundles of the 

 first two setae-bearing segments there are never more than 

 two setae, and these are very small compared with those 

 which come close behind. In segments 3 to 9 or 10 they 

 reach their maximum size, and there are often four or 

 five present in the ventral bundles, but rarely more than 



