40 



OLIGOCHAETA 



Fig. 8. 



a symmetry of arrangement in the same segment ; in the latter facts these genera 

 recall the condition so characteristic of certain species of Acanthodrilus. 



HUBRECHT'S special contribution to this matter in his dis- 

 covery that when the nephridium opens in an 'abnormal' position 

 the duct enters the body-wall as if it were going to reach the 

 exterior immediately by the ordinary course ; instead of which 

 the tube bends to the left or to the right, as the case may be, 

 and passing between the two muscular layers of the body-wall 

 reaches its external orifice. The nephridium itself retains the 

 same position in the body-cavity wherever the external pore 

 may be situated ; on a mere dissection it would be impossible 

 to say of any one particular nephridium where the external pore 

 was placed. 



The tube as it passes along the body-wall lies so exactly 

 between the circular and the longitudinal layers of muscle that 

 the appearances presented suggest at first sight merely a break 

 in continuity of the body-wall along this line, due to a defect 

 in the section ; this may possibly, HUBKECHT thinks, have caused 

 other cases similar to this to have escaped the attention of 

 observers ; at any rate there is no doubt that so far as our 

 present knowledge goes a similar state of affairs has not been 



described in any other genera than those mentioned. These facts have led HUBRECHT 

 to suggest, though very tentatively, another theory of the variations of the nephridia 

 in the Oligochaeta. 



It will be remembered that there are certain Cryptodrilids in which three pairs 

 of nephridia are met with. HUBRECHT indicates that the three positions which the 

 nephridiopores occupy in many Lumbricidae may possibly be a reminiscence of the 

 original presence of three pairs just as it has been suggested that the alternation 

 in Acanthodrilus may be a trace of the former existence of two pairs of nephridia 

 in the immediate ancestors of this genus. The same arguments that apply in the one 

 case apply in the other. 



Diffuse nephridia. The nephridia are among those organs of the Oligochaeta which 

 show most variation. For a long time it was thought that they agreed with other 

 segmented worms in possessing a single pair to each segment of the body, a variable 

 number of segments being without nephridia at all. It is now known that this state 

 of affairs is by no means the only way in which the excretory system is developed. 

 A very large number of genera are largely or entirely characterised by possessing 



ACANTHODRILUS 



DISSIMILIS. 

 I. Setae, n. Nephridiopore. 



