1908.] 



J. Stephenson : The Anatomy of some Aquatic Oligochœta. 



257 



The dorsal setce (text-fig. 2) begin in the sixth segment ; in one specimen the 

 seventh was the first to bear them, though there seemed to be a small 

 lobulated setal sac in the sixth ; like the ventral setae, they diminish 

 in size towards the posterior end, and usually cease altogether a few 

 segments in front of the ending of the ventral setae. They may be 

 divided into long and short, or hair-setae and needle setae. 



The length of the hair-setae varies \ they may be equal or nearly 

 equal to the diameter of the body, or may be only a half or two- 

 thirds cf the diameter ; in some cases they have obviously been 

 broken off short ; in others they have either fallen out altogether 

 or have not been developed. They are frequently smooth ; but 

 frequently also they may have few (fig. 8) or many extremely fine 

 thorn-like projections, which may even be branched ; to these 

 ''thorns" minute foreign particles frequently adhere; an extreme 

 example of this condition is shown in plate xvi, fig. 9. The thorn- 

 like processes are, when present, not arranged in definite lines or at 

 regular distances. They seem to be due to the fraying out from 

 the side of the seta of some of its component fibrils and to the 

 forcible breaking back, without actual detachment, of the frayed 

 ends. Their presence would thus be an indication of age or hard 

 wear ; and as a matter of fact they appear to be most numerous in 

 those cases where age or hard wear are also evidenced by the irregular 

 lengths of the setae, by their broken condition, or even their entire absence from 

 certain segments. It has seemed to me that the earlier specimens of a batch 

 of material examined soon after being taken from their natural surroundings, do not 

 show these thorn-like processes so often nor to so extreme a degree as those examined 

 subsequently, after having been kept, perhaps, a month in the laboratory. Here, 

 again, we have perhap-^ an indication that the processes are due to disintegration 

 brought about by relatively unfavourable conditions. 



In the first draft of this paper, I described the needle-setae of the dorsal 

 bundles as having usually a single point, and only very occasionally being bifid at 

 the free end. During the last month or two, however, I have examined a consi- 

 derable number of specimens from a different source (the river Ravi), and in these 

 I have usually found some, at least, of the needle-setae to be bifid ; indeed I have 

 thought it possible that all are bifid, and that the failure actually to see this depends 

 on the fact that in many cases the prongs of the forked end are so placed under the 

 microscope as to overlie one another and hence are not seen separately. It is possible 

 that my earlier failure to note the bifid ends of these setae may have been due to a 

 lack of sufficiently close observation ; on the other hand, both doubly- and singly- 

 pointed needle-setae may perhaps occur, as is stated to be the case in an allied form 

 (AT. variabilis, var simplex, Piguet [7J). 



Describing, however, the bifid form as the typical one, these needle-setae of the 

 dorsal bundles are about -06 (058 — -07) mm. long, slightly sickle- shaped, with an 



Fig. 2. — Dorsal 

 needle-seta of Nais 

 variabilis, var. ptm- 

 jabtnsis. 



