500 



ANNELIDA. 



a 



FIG. 409. Nephridium of Pontodrilus (after 

 E. Perrier). pv nephrostome ; o dilated 

 part (vesicle) near the external opening; 

 tv the tube of the nephridiura passing 

 through the nucleated protoplasmic mass u. 



different places. This 

 difference in position 

 of the opening may 

 be accompanied by a 

 difference in struc- 

 ture, e.g. in AcantJw- 

 drilus novae-zealan- 

 diae the nephridia 

 which open in front 

 of the ventral setae 

 have a caecal appen- 

 dage, while those 

 which open in front 

 of the dorsal setae 

 are without this 

 appendage. 



The plectonephric 

 or diffuse nephridia 

 are networks of fine 

 tubes lying on the 

 body-wall and septa 

 in each segment 

 (Figs. 410, 411). 

 They have several 

 openings to the ex- 

 terior, and they some- 



segment, but in Brachydrilus 

 there are two pairs, and in 

 Trineplirus three. The neph- 

 ridia always extend over two 

 segments, the internal opening 

 being in the segment anterior to 

 that in which the main part of 

 the nephridium lies. 



The external opening is usually 

 near the setae of the ventral row, 

 but sometimes it is near the setae 

 of the dorsal row, and even dorsal 

 to these ; and nephridia in suc- 

 cessive segments may open in 



FIG. 410. The diffuse or plectonephric nephridia of OUgcchacta. 

 1, portion of a transverse section at the level of the setae of 

 Octochaetus multiporvs. S setae; a, a 1 nephridial pores; n 

 nerve cord ; p peritoneum. 2, section across a nephridial tube 

 of Perichacta aspergillum passing through the wall of the body. 

 ep epidermis ; m layer of circular, m' of longitudinal muscles 

 (the lines mark the inner limits of these layers) ; a nephridial 

 tube with its vesicle in the layer of circular muscles; b blood 

 vessel. (From Perrier, after Beddard.) 



