1908.] J. Stephenson: The Anatomy oj some Aquatic Oligochœta. 265 



far as the seventeenth segment, behind which three smaller bundles were visible; as 

 before, the first six setal bundles of the posterior animal were new formations ; the 

 number of definable segments was thus 20 in the anterior animal, while in the 

 posterior there were 21 In an animal which divided under observation there were 

 in the anterior portion 22 segments plus several newly forming at its hinder end, 

 and in the posterior portion 24 plus similarly several newly forming posteriorly. 



The zone of budding may therefore form as far forward as the twelfth segment ; 

 and in the posterior animal it appears to be the rule that the first seven (prostomium, 

 peristomium, and six seta-bearing segments) are new formations. 



Chains of three animals are occasionally met with. 



Setce. — The ventral setae (fig. 27) are of an elongated shape, straight for the 

 greater part of their length, with curved ends; they are unequally forked at their 

 free extremity, and possess a slight nodulus; their length is from a half to two- 

 thirds the diameter of the body; they do not project far outside the body-wall; 

 they occur in bundles of 3 to 9, 5 being the commonest number. 



The dorsal setse begin like the ventral setae on the second segment, and are 

 capilliform. Those of the second segment are as a rule somewhat shorter than those 

 of other segments. Those of the third segment, two or occasionally three on each 

 side, are much longer, about three times the diameter of the body ; frequently where 

 there are two in the bundle, however, while one is of the full length just stated, the 

 other is only about two-thirds as long, that is, about equal to twice the diameter 

 of the animal's body. When turned forwards the long setse reach about as far as 

 the tip of the prostomium. These long setae, however, appear to be liable to damage 

 on account of their length ; they may be found on one side only ; or on the posterior 

 of two not yet separated animals only, or even only on one side of this. They may 

 break off while under examination; or a specimen which had them when first 

 examined may, on re-examination after an interval, be found to have lost them. The 

 distinction between this species and the next is thus sometimes rendered difficult. 

 On the remaining segments the dorsal setae attain a length equal to or somewhat 

 greater than the diameter of the body; in the posterior part of the body, where 

 the diameter is somewhat less, they may be nearly twice as long as the diameter of 

 the body. 



The dorsal setse of this species were formerly supposed to be smooth ; of late 

 years, however, it has been recognised that they present on one side a fine notching, 

 or series of saw-teeth (Michaelsen [5], [6J; Piguet [7]). These saw-teeth vary in size 

 and visibility in specimens from different localities, and are absent from the specially 

 elongated setse of the third segment. 



I have detected this notching in all specimens where I have specially looked for 

 it ; it is visible, though it cannot be called obvious, with a -^^ inch oil-immersion lens • 

 it is absent towards the base of the setse, and does not occur on the setse of the third 

 segment; the setse themselves are slightly bowed, and the notching is present on the 

 convex side only; the teeth are set apart at a distance of about 3 fx from each 

 other. 



