OLIGOCH^TA. 258 



pair of gills, already mentioned, here fuse along one of their sides 

 with the dorsal wall of the fossa, and thus constitute longitudinal 

 ridges of the wall. Here also another gill arises, ventral to the 

 first pair, from the lateral wall of the funnel ; its fellow however 

 springs (in this particular specimen at least) from the right gill of 

 the first pair. These two, constituting the second pair, lie at first 

 free in the cavity of the fossa ; but on proceeding backwards they 

 soon fuse with its walls, and appear, like the first pair, as ridges 

 (fig. 2). 



The third pair of gills now appear ; in these, unlike the first two 

 pairs, the anterior ends, which are first encountered, are free. The 

 branchial chamber now opens out dorsally ; the first pair of gills 

 appear in a transverse section as projections just within the margin ; 

 the second pair are similar, and more ventrally situated ; che third 

 pair are free in the cavity at a still lower level. 



The first pair of gills now flatten out and disappear ; the third 

 pair fuse with the wall of the funnel ; and a foiu-th pan- appear 

 ventrally, at first free within the cavity (fig. 3), but soon fusing 

 with the wall of the funnel. A section of the funnel therefore now 

 shows three projections of the wall on each side, corresponding to 

 three ridges. 



The second pan of gills now flatten out and disappear ; the third 

 does the same ; and lastly the fourth also. All have disappeared 

 some little distance in front of the posterior end of the funnel. 



The free gills and gill ridges are cihated ; they are covered in part 

 by a characteristic pyriform epithelium (fig. 3) ; the diameter of 

 the gills, or of the ridges, is from 50 to 70 \l. 



The relative lengths of the several portions of the branchial fossa 

 may be estimated from the number of sections which go to each. 

 The sections being of a thickness of 8 ^.. it is fomid that the anteriorly 

 pointing diverticulum dorsal to the end of the gut is 40 [l in length ; 

 from the mouth of the diverticulum (Avhich coincides with the end 

 of the gut) to where the branchial fossa opens out dorsally is 96 jx ; 

 and from this point to the posterior end of the animal is 272 [k. The 

 whole length of the fossa is therefore less than half a milhmetre. 



The dorsal setae begin in segment VI., and are of two kinds, 

 capillary and needle setae. In the anterior segments there are three 

 capillary and three needle setse per bundle, arranged in pairs of one 

 of each kind ; further back the bundles consist of two capillary and 

 two needle setss ; and behmd this again of a single pair onl5^ The 

 capillary setse vary in length from 240 to 320 \). ; thus they do not 

 exceed the diameter of the body. The needle seta? (fig. 4) are 

 nearly straight, with however a slight sickle -shaped curve ; the 

 point is bifid, but the forking is so fine as to be only just visible with 

 the ordinary high power of the microscope. Theii' length varies 

 considerably, from 82 to 102 ^jl. The very slight nodulus is distal 

 to the middle of the shaft. 



